Multistage damming of the Rhine River by tephra fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming I

Journal Pre-proof Multistage damming of the Rhine River by tephra fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming I Cornelia Park, Hans-Ulrich Schmincke

PII:

S0377-0273(19)30050-2

DOI:

https://doi.org/10.1016/j.jvolgeores.2019.106688

Reference:

VOLGEO 106688

To appear in: Received Date:

26 January 2019

Revised Date:

5 September 2019

Accepted Date:

14 October 2019

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1 Journal of Volcanology and Geothermal Research - Research Article

Multistage damming of the Rhine River by tephra fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming I

Cornelia Parka,1, Hans-Ulrich Schminckea,2 Both authors will be corresponding authors aGEOMAR

Helmholtz Centre for Ocean Research Wischhofstr. 1-3 24148 Kiel Germany

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Present Address: 1C. Park Stäudach 81 72074 Tübingen Germany e-mail: [email protected] 2H-U.

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Schmincke Lisch 8 24326 Ascheberg Germany e-mail: [email protected]

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We did not provide a revised manuscript with changes marked because changes were in part too extensive. It would have been difficult to keep track. However, we have provided a very detailed response to every point made by the reviewers.

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The crossed out sentence was just to make the upload system happy, because it insisted on a manuscript with changes marked.

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Highlights

First recorded example of tephra fallout damming a major river

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Repeated massive syn-eruptive damming

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

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Floods triggered by multiple breaches of tephra dam caused widespread mass erosion

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Striking large-scale upper flow regime deposits

Abstract The Rhine - the largest river in Western Europe – was dammed during the Plinian Laacher See Eruption (LSE; 12,900 BP).

2 Damming during the climactic Plinian episode of LSE occurred both upstream and downstream of the broad tectonic Lower Neuwied Basin (LNB) that interrupts the narrow Rhine canyon. We here document details of the upstream damming at the bottleneck entrance to the LNB near the present city of Koblenz. Our reconstruction is based on a high-resolution analysis and correlation of the complex intercalation of primary fallout tephra relics with fluvially reworked Laacher See Tephra in the LNB. Tephra units representing complete eruptive cycles repeatedly fell on drained ground in between one minor and 4 major flooding events - even at the base of side channels that had been active prior to the LSE and that had been flooded by a preceding flooding event. This demonstrates that flooding occurred generally during breaks and not during fallout events. The repeated formation and breach of a dam at the upstream entrance of the LNB (Koblenz Dam) consisting of fallout components and driftwood washed together convincingly explains the multiple repetition of the drainage of the channels in the LNB followed by large-magnitude flooding in rapid succession. The strongly pulsating nature of the LSE reflected in multiple interruptions of eruptive activity fundamentally controlled the damming and flooding dynamics. The Rhine became completely blocked during distinct fallout phases due to overloading with pumice that had fallen into the river and its major tributaries. The temporary dam collapsed during eruptive breaks. This is the first recorded example of tephra fallout damming a major watercourse. The extremely low gradient of the Rhine River allowed the repeated accumulation of large volumes of water in

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a long, multi-phase dammed-up lake (Lake Koblenz) that extended along the upstream course of the river for up to c. 30 km despite the low height (<10 m) of the dams. Each breach of Koblenz Dam caused extensive and wide-spread erosion and reworking of freshly deposited tephra throughout the entire LNB up to 3.5 km perpendicular to the major axial Rhine channel. The floods deposited striking, large-scale upper flow regime structures interpreted as in-phase wave draping, antidunes and chute-and-pool structures consisting largely of gravel-sized tephra components. Primary tephra sheets - several meters thick

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- became detached by undercurrents above impermeable boundaries and floated potentially along the full length of both active and abandoned channels. Large tephra bodies with the dimensions of a garage were lifted off by the flood waves and transported downstream for at least tens of meters.

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Damming of a major river while a large Plinian eruption is in full progress represents an extraordinary challenge for hazard mitigation. This is especially pertinent for an area close to, and downwind from, the vent and therefore simultaneously affected

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by massive Plinian fall such as in the LNB.

Abbreviations

Lower Neuwied tectonic Basin

LSE

Laacher See Eruption

LST

Laacher See Tephra

LLST

Lower-LST

MLST

Middle-LST

ULST

Upper-LST

RR

Rhine River

SM

Supplementary material

i.dr.st.

In dry state

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LNB

Key words

River damming; Plinian eruption; Syn-eruptive; Fallout; Pumice; Antidune

1. Introduction Volcano-hydrologic events in populated areas can lead to disasters (Macias et al. 2004). Here, we document repeated damming of a high-discharge river solely by fallout during a major Plinian eruption – differing, as far as we know, from all

3 previously described scenarios of volcanically-controlled river damming. Our interpretation that the ephemeral tephra dam was episodically breached to cause wide-spread flooding downstream, while the overall eruption was still in full progress, is another important but rare type of volcanic hazard. In most cases, major flooding due to release of volcanically dammed lakes takes place subsequent to the eruption (e.g. White et al. 1997, Manville and Hodgson 2011). Flooding associated with Plinian eruptions resulting from various direct (except for Plinian fall) or indirect eruptive effects, such as the explosive ejection or breakout of lakes developed in craters and calderas, has been described repeatedly (e.g. Cronin et al. 1997, Manville et al. 1999, Manville and Hodgson 2011). There are also numerous accounts of lakes blocked by debris-avalanche deposits/reworked volcanic material (e. g. Glicken et al. 1989, Hodgson and Nairn 2005, Capra 2011), or lakes blocked by pyroclastic flows (e.g. Smith 1991a/b, Macias et al. 2004, Kataoka et al. 2008, Andrews et al. 2014). Flooding during a major Plinian eruption was triggered by the failure of a temporary ignimbrite dam during the 1982 eruption of El Chichon, Mexico (Macias et al. 2004) and by the c. AD 1315 breakout flood from Lake Tarawera, New Zealand (Hodgson and Nairn 2005). A preliminary model of the Rhine River damming was presented by Park and Schmincke (1997) and Schmincke et al. (1999). New critical exposures with the quality of “missing links” allowed a much more precise and high-resolution reconstruction of

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the flooding events. Short summary accounts have been published in Park and Schmincke (2009) and Schmincke (2014). We reconstructed the whole scenario from a puzzle of c. 60 small tephra pits temporarily opened up between c. 1990 and 2015.

The present account is the first in a series of four papers in which we attempt to reconstruct the complex proximal and farreaching impact of Laacher See Eruption on the Rhine River from the tectonic Neuwied Basin as far as the Netherlands. The

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Rhine River was impacted by two contrasting types of eruptive activity during the LSE - fallout and pyroclastic flows - resulting in damming at two opposite locations at different stages. An overview of the complete syn-eruptive damming scenario is given in Ch. 4 and figures 1 and 2.

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Here, we focus on a high-resolution analysis and correlation of the complex intercalations of primary fallout tephra relics with fluvially reworked Laacher See Tephra within the tectonic Lower Neuwied Basin downstream of Koblenz. We present evidence for one minor and four major highly energetic flooding events affecting all types of channels in the LNB during breaks

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in eruptive activity and discuss the sedimentology of the upper flow regime flood deposits. We present arguments that these flooding events are strong indicators for five stages of dam formation (Koblenz Dam) at the upstream entrance of the basin close to the city of Koblenz (Koblenz lakes) triggered solely by fallout directly into the Rhine River during active Plinian fall and for the repeated collapse of the dam during breaks between fallout phases.

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In a companion paper (Park and Schmincke 2019b in press), we discuss topo- and hydrographic preconditions and address eruption-induced factors that explain why major dam formation occurred upstream of the main tephra loading area, and also discuss inferred mechanisms for dam formation.

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2. Geological setting

Laacher See Volcano is located on the elevated western shoulder of the tectonic Neuwied Basin that interrupts the deeply incised Middle Rhine Canyon (Fig. 1). The Rhine traverses the lowest part of the basin (here called Lower Neuwied Basin,

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LNB) from its eastern entrance at Koblenz (located at the mouth of the River Moselle - the major tributary of the Rhine within the basin, 23 km east of the Laacher See Volcano) to its western outlet at Andernach (10 km east of the volcano) over a distance of c. 24 km. The basin is up to c. 7 km wide in a north-south direction perpendicular to the river. The Older Lower Gravel Terrace NT2 (Weichselian, pre-Bølling interstadial; Schirmer 1990) formed the floor of the LNB at the time of Laacher See Eruption (Figs. 3A, 4). The present Rhine has an extremely low gradient within the LNB (0.15- 0.46 ‰; www.bafg.de, Park and Schmincke 2019b in press), the elevation difference over the distance of 22 km between Koblenz and Andernach being c. 5.5 m. Our fieldwork shows that the NT2 (throughout the LNB) was dissected by a network of active Rhine River channels (differing in depth of incision) at the time of the Laacher See Eruption (Figs. 1, 3A) – quite different from the single-thread course of the

4 present Rhine (see reconstruction in Park and Schmincke 2019b in press). The pre-eruptive Rhine River in the LNB was degradational, cutting into the NT2-terrace. It would be best classified as a transitional form between its glacial braided-riverheritage and a post-glacial meandering river, corresponding to the model of a gravel-bed wandering river (cf Miall 1996, Miall pers. com. 2019). The major (locally possibly branching) channel of the pre-eruptive Rhine must have been located within the central, up to 2.5 km wide part of the Lower Neuwied Basin (Figs. 1, 3A). Following the LSE, this axial zone was scoured during the Allerød and subsequently refilled by NT3 gravel deposits during the Younger Dryas. It became scoured again during the Holocene (Thoste 1974, Ikinger and Weidenfeller 2000; NT3 = Younger Lower Terrace containing reworked LST components; Younger Dryas = final short stadial at the end of the Weichsel glacial, starting c. 200 years after the LSE). These events destroyed any evidence of the major channel. A major, broad meandering active side channel was located south of the major channel and another - but narrow - active side channel to its north. The side channels were connected to the main branch of the preeruptive Rhine by several minor channels. The southern side channel was incised c. 6 m into the flood plain, the main branch of the Rhine perhaps as much as c. 8 m. The water depth in the multiple and in part wide channels of the pre-eruptive Rhine was significantly shallower than in the present waterway. The maximum water depth was possibly c. 1-2 m within the pre-

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eruptive side channels and c. 3-4 m within the main channel at mean discharge (Park and Schmincke 2019b in press). The pre-eruptive discharge of the Rhine was between 1800 and 2000 m³/s (Park and Schmincke 2019b in press).

The NT2 flood plain was also crossed by a network of abandoned channels of contrasting elevation, hierarchy and age (Figs. 3A, 4; Park and Schmincke 2019b in press).

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For location of exposures see SM-1

3. Evolution of the Laacher See Eruption

The Laacher See Eruption (c. 6.3 km3 DRE, VEI=6) 12,900 BP was characterized by repeated and pronounced changes in

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eruptive and transport mechanisms and by major chemical and mineralogical zonation of the magma reservoir (e.g. Bogaard and Schmincke 1984, Wörner and Schmincke 1984a; Schmincke et al. 1990, 1999; Harms and Schmincke 2000; Ginibre et al. 2004; Schmincke 2008).

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Two major climactic Plinian fallout stages (LLST and MLST-B/C/D) were separated from each other by a complex intermediate phase (MLST-A) characterized by repeated vent collapse and vent migration associated with longer eruptive breaks and thus only low, short-lived Plinian eruption columns and the generation of voluminous pyroclastic flows at the end of MLST-A phase (Bogaard and Schmincke 1984; Freundt and Schmincke 1986; Schmincke et al. 1990, 1999; Schmincke 2008; Fig 2). The

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climactic fallout stages were synchronous with, and responsible for, upstream river damming at Koblenz and are described in more detail in Ch. 6.2 and SM-5.

Our revised event-based stratigraphic subdivision (unpubl.) of the Laacher See Tephra (LST) more realistically reflects the pronounced fluctuation of the eruption (Figs. 2, 7; Park and Schmincke 2019b in press; SM-2). We maintain the basic

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subdivision of the Laacher See Tephra (LST) into a Lower (LLST), Middle (MLST) and Upper Laacher See Tephra (ULST) for practical purposes. It is simple and easily-recognized at most localities and conforms to most previous publications on LSE. We have partly changed and supplemented the previous detailed stratigraphic subdivision of LST (Bogaard and

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Schmincke 1984) to better agree with the actual course of eruptive activities. We have reassigned the basal part of the former MLST-B terminating with major pyroclastic flow generation to subunit MLST-A. We have newly introduced subunit MLST-B1 and MLST-B2. We have modified the stratigraphic assignment of the upper part of subunit MLST-C by newly introducing subunit MLST-D. We have also subdivided the ULST into subunits -A, -B and –C. For the nomenclature of the stratigraphic subdivision see SM-2. We estimate that the main climactic episode of high discharge fallout and pyroclastic flow generation (LLST and MLSTA/B/C/D) that was synchronous to upstream river damming lasted less than 5 days interrupted by multiple, in part long-lasting breaks.

5 4. Overview of the overall syn-eruptive Rhine damming scenario We here present an overview of the overall flooding scenario because this is important for the perception of the partial aspects discussed in this paper. The story is so complex that we will treat it in four separate papers. The evolution of the interaction between the LSE and the Rhine River is summarized in figures 1 and 2. Dammed-up lakes formed at two pivotal locations and at different times during LSE. Tephra fall events during climactic eruptive activity led to substantial overloading of the Rhine and repeated damming events (Koblenz lakes) upstream at its entrance into the Lower Neuwied Basin. Repeated collapse of the unstable Koblenz dams during breaks in eruptive activity caused five regional flooding events FE1 to FE5. A major stable dam (Brohl Dam) was formed by pyroclastic flows that had entered the narrow Rhine canyon c. 7 km downstream of the LNB at the end of eruptive phase MLST-A – about halfway through the climactic eruption. From that stage onward, flooding events FE3, FE4 and FE5 triggered by the renewed breach of Koblenz Dam contributed to a rapid incremental growth of Lake Brohl. Re-establishment of Koblenz Dam and flooding in the LNB triggered by its breach terminated when the backwater of Lake Brohl within the Rhine channels reached upstream behind the mouth of the Moselle river at Koblenz after the MLST-D phase. Brohl Dam remained stable for more than two weeks. The level of Lake Brohl rose and the backwater increasingly reached farther upstream, possibly as far as the Upper Rhine Graben. The breach of the ignimbrite dam (Brohl

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Dam) synchronous with the powerful resumption of eruptive activity (ULST-B) – following a longer break during the terminal stage of LSE - triggered a major flood wave that raced downstream the Lower Middle Rhine valley (at least as far as Cologne). The Koblenz Dam and the flood waves resulting from its breach are subject of this and companion paper Park and Schmincke 2019b. Brohl Dam and Lake Brohl will be subject to two further papers (in prep.).

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5. Methods

Tephra deposits reworked by the syn-eruptive flooding events were preserved in large areas because the many active and abandoned channels that had been flooded during Laacher See Eruption were not, or only partially, scoured subsequently

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due to the very wide flood plain within the LNB and the generally degradational character of the Rhine River during the Allerød – an exceptionally favorable condition. However, our high-resolution reconstruction of the Rhine damming was only possible by field work spanning more than two decades because the primary and fluvially reworked LST deposits were exposed in

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less than a handful small ephemeral tephra pits per year owing to the fact that the LST had been almost completely exploited close to the Rhine River already at the beginning of last century.

5.1. Precise stratigraphic assignment of primary fallout tephra relics

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The pronounced lithostratigraphic and compositional changes of LST from base to top were the fundamental asset to precisely stratigraphically assign primary fallout relics within the complicated multistage intercalations with fluvially reworked LST deposits and thus for the precise temporal, spatial and causal reconstruction of the repeated damming and flooding events. a. The lithostratigraphy of the deposits of both major Plinian stages of the LSE (LLST and MLST-B/C/D) varies moderately to

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strongly along the 24 km course of the Rhine River in the LNB, (a) because the directions of the fallout fans of successive phases diverged in part significantly (especially during the LLST stage; Fig. 6; new unpubl. data), (b) because the width of many fans perpendicular to their axes was small and (c) because the LNB is located at a distance to the vent (10-23 km)

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where this divergence of fan axes is pronounced (Fig. 6). Moreover, the stratification of the tephra deposits resulting from the unsteady eruption is less pronounced within the LNB compared to proximal areas because the fraction of lithoclasts had already diminished significantly at this distance from vent (Fig. 6). Correlations of primary LST deposits within the LNB on the scale of subunits were only possible on the basis of widespread and detailed stratigraphic correlations with the undisturbed LST deposits at c. 150 localities closer to vent. b. The pronounced compositional zonation of the Laacher See magma reservoir is expressed by systematic changes in color and vesicularity of the pumice clasts from base to top of the LST deposits as well as in the type and abundance of phenocrysts (Figs. 2, 7; SM-3; Wörner 1984a, Bogaard and Schmincke 1985). Highly evolved phonolitic, phenocryst-poor, white to offwhite pumice was erupted during the LLST stage (inferred top of the magma reservoir). The pumice clasts became darker,

6 more mafic and more crystal-rich as the eruption progressed. Dense, gray, crystal-rich lapilli were erupted during phase MLST-D and gray to black, very crystal-rich (>30 Vol.%) lapilli during the ULST stage. The density of the pumice clasts increased accordingly due mainly to the higher phenocryst abundance but it was enhanced by an inferred increasing hydroclastic fragmentation of the magma starting with phase MLST-C (Fig. 7; Bogaard and Schmincke 1984, 1985; Schmincke et al. 1990, 1999; Park and Schmincke 2019b in press).

5.2. Compositional verification of the stratigraphic assignments An unequivocal verification of the stratigraphic assignment of pumice clasts incorporated within a primary tephra relic or a flood deposit in the field – based on the macroscopic criteria described above – is based on comparing their bulk rock XRFcompositions with closely sampled primary tephra sections within the LNB (Locs. 689, 758; SM-3; Fig. 7) as well as to our database (unpubl.). It comprises c. 400 bulk rock XRF-analyses of almost all stratigraphic subunits of LST sampled at c. 75 localities around the Laacher See Volcano since 1998. For example, Zr-, Sr-, Rb- and Nb-concentrations of pumice change

5.3. Correlation of the fluvially reworked sequences

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significantly from base to top within the LST (Fig. 7; SM-3). For analytical methods see Harms and Schmincke (2000).

Correlation of the fluvially reworked sequences throughout the Lower Neuwied Basin was achieved primarily by considering: (a) their morphologic setting within the pre-eruptive channels (base, lower point bar, upper margin, etc.); (b) the hierarchy (active or abandoned, chute channel, etc.) and vertical spacing of these channels (Figs. 3, 4); (c) the precise stratigraphic assignment of under- and or overlying primary LST deposits (Fig. 7); and (d) the specific sedimentary fingerprint of each

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fluvially reworked sequence that was laterally consistent within the entire LNB.

5.4. Criteria to distinguish deposits resulting from erosion and deposition at the top of a primary tephra sequence

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by an overriding flooding event from flood deposits caused by undercutting at the base of a tephra sequence by a later flood

Undercutting is a common mechanism of river bank erosion (Thorne 1982). Here, undercutting and flotation of overlying

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tephra deposits took place on a large scale over large areas thus complicating interpretation of the already complex intercalations of primary fallout and flood deposits. The flotation not only affected extensive flood plain areas bordering the active channels, but also the entire tephra fill within the abandoned channels over their full length throughout the LNB. Undercutting took place very selectively above impermeable boundary layers - predominantly above the paleosol horizon

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below LST, above the thin MLST-A unit (consisting either entirely of ash or of an alternation of ash layers with thin fallout lapilli layers) overlain by mud drapes of FE3 or in between the ash layers of MLST-A (Figs. 7, 13A, 13B, 14A, 14B; Figs 2A, 2B, 3Ab in SM-5). These boundaries were in part used repeatedly for undercutting during several successive flooding events. Unequivocal indicators of later undercutting include: (a) Erosion of a primary tephra section - starting from its base upward -

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is the major criterion. This implies the absence of varying proportions of the lowermost tephra layers and leads to an erosive boundary (above the fluvial deposits) that quite often discordantly cuts the overlying fallout layers. This documents that erosion must have taken place after deposition of these layers. (b) Local injections of supplementary tephra in between primary tephra

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deposits (Fig. 14Ab; Figs. 2Ab1, 2Ab2, 2B, 3Ab (right part of the tephra section), 3B in SM-5). This is especially evident when the chemical composition of the injected tephra is not consistent with that of the primary deposits directly below and above (Fig. 7). (c) Horizontal movements and overthrusting of large primary tephra sections (Figs. 10, 11, 14Aa). (d) Absence of a mud drape above fluvially reworked deposits. We assume that a mud drape completely free of coarser components is most plausibly a primary flood effect and could not form below an undercut section of primary LST from which coarser lithoclasts would have dropped out during deposition of the mud. We have recognized additional indicators for undercutting not further discussed here.

5.5. Estimating flood levels

7 Flood levels of the syn-eruptive flooding events were estimated in the southern active side channel within the central section of the LNB - c. 12 km downstream of the postulated upstream end of the Koblenz dams (blue dot in Figs. 3A, 3B). This was the only area for which reliable estimates were possible because here a continuous transition from the channel base to the flood plain was exposed over a sufficient length along the channel (Locs. 652, 653 654, 655) – evidently not displaced by post-eruptive tectonic activities. The flood levels were measured versus the mean bed level. These values are not equivalent to the maximum flow depth, however, because the riverbed was overlain by the fallout tephra of the preceding eruptive phase and possibly partly by the deposits of the preceding floods when each flood arrived (Fig. 10). The flood levels varied along the Rhine River within LNB depending on the width of the flood plain, the number of flooded channels and other factors. In contrast to FE1, FE2 and FE3, the major axial Rhine channel was already partly filled with the backwater of Lake Brohl (see Ch. 4. and Park and Schmincke 2019b in press) when FE4 took place. In addition, the higher elevated active side channels were partly filled by the backwater of Lake Brohl when FE5 took place (Fig. 10).

6. Results 6.1. Characteristics of primary and river-transported LST components

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a. The LLST and MLST tephra components that fell into the Rhine River channels either directly or were incorporated via fluvial erosion of primary tephra deposits consisted of components of strongly contrasting densities and did not correspond to the non-volcanic clast assemblages normally transported by the Rhine River (Fig. 8). Buoyant pumice clasts with densities of 0.41-0.48 g/cm3 - mainly derived from LLST and MLST-B - dominated volumetrically (78 Vol.% in LLST, 75 Vol.% in MLSTB1, 68 Vol.% in MLST-B2; see Fig. 6 Park and Schmincke 2019b in press). Non-buoyant dense MLST-C/D pumice (c. 1.2-

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1.5 g/cm3) and lithoclasts of various lithologies (slate c. 2.8 g/cm3, sandstone c. 2.4 g/cm3, basalt c. 3 g/cm3; densities after Telford et al. 1990) were less abundant. Pumice clasts generally behave in non-standard ways, due to variable and possibly positive buoyancy, depending on the degree of water saturation (Whitham and Sparks 1986; Manville et al. 1998, 2002; White

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et al. 2001; Manville at al. 2009). Once water-saturated, the depositional behavior of pumice much more closely resembles that of non-volcanic components (White et al. 2001; Manville et al. 2002). For methods of density measurements see SM-3 in Park and Schmincke 2019b in press.

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A terminological discrimination of buoyant from non-buoyant (prior to immersion into water) pumice was needed, because “buoyant” is not a fixed particle property. Buoyant pumice started to soak with water when it fell into or was eroded by the Rhine River and was no longer buoyant when it was deposited. Primarily buoyant pumice = buoyant (in dry state = i.dr.st.) pumice, primarily non-buoyant pumice = non-buoyant (i.dr.st.) pumice.

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b. In the Lower Neuwied Basin, the LLST and MLST-B/C tephra consisted mainly of gravel-sized components (54 wt% in LLST, 65 wt% in MLST-B and 88% in MLST-C; mean of three exposures close to the Rhine). The pumice clasts were generally one or two size classes larger than the lithoclasts due to aerodynamic sorting during transport within the eruption clouds (Fig. 6 in Park and Schmincke 2019b in press). Silt and clay-sized components >4 phi were largely lacking - except for ash coatings

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around pumice lapilli. MLST-A tephra consisting mainly of a sequence of ash-layers (>4 phi) - with intercalated coarser tephra layers south of the major Rhine channel - was only between 3 and 25 cm thick in the central LNB (>40 cm with thicker coarser layers upstream at Koblenz and along the Moselle River). This is why the flood deposits are almost exclusively gravel- to

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coarse sand-sized, moderately to well-sorted and clast-supported. This strongly contrasts with the mostly more poorly sorted and fines-rich flood deposits resulting from the breach of dams formed by block-and-ash flows, ignimbrites or landslide deposits (e.g. Manville et al. 1999, Segschneider et al. 2002, Hodgson and Nairn 2005, Kataoka et al. 2008). c. The late Weichselian gravel deposits forming the base of the pre-eruptive active Rhine channels are dominantly composed of subround to round gravels up to cobbles (different lithologies derived chiefly from the Alps) in a matrix of largely quartz sand. The standard Rhine components are thus easily distinguished from the generally smaller and entirely angular lithoclasts derived from the LST consisting mainly of fragments of Devonian sandstone and slate and diagnostic, but less abundant, basanite and tephrite fragments. Criteria that distinguish syn-eruptive from post-eruptive fluvial deposits are illustrated in figure 8 and discussed in SM-4.

8 6.2. Evidence for five syn-eruptive flooding events in the LNB We distinguish five syn-eruptive flooding events within the LNB from each other (FE1 to FE5; Figs. 2, 10). We here focus on Flooding Event FE4 following the MLST-C phase because the FE4 floods were the most voluminous and powerful floods and deposited high-contrast, large-scale upper flow regime structures. Evidence for Flooding Event FE1 (following LLST-A1), Flooding Event FE2 (following LLST), Flooding Event FE3 (following MLST-A) and Flooding Event FE5 (following MLST-D) is summarized in Fig. 10. Detailed evidence is given in SM-5. We use the term flooding event instead of flood because the scenario following the eruptive phases MLST-A (FE3), MLST-C (FE4) and MLST-D (FE5) comprised several successive flood waves.

Flooding Event FE4 (post MLST-C) Prior to flooding: Non-eroded relics of primary and completely undisturbed MLST-B/C tephra testify that the tephra fell on drained ground and

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remained dry up to Flooding Event FE4 - even at the base of active side channels (Locs. 638, 650, 655, 657, 664) where shortly before the LLST and the MLST-A tephra had been completely eroded by FE2 and/or by FE3 (Figs. 9, 10, 12a, 13Ac; Figs. 1A, 2Ab, 2B in SM-5; Fig. 8 in Park and Schmincke 2019b in press).

The MLST-B1, -B2 and -C phases (together with the transitional phase MLST-D not addressed here) represent the second major Plinian stage of the Laacher See Eruption (see SM-2). They were characterized by high mass eruption rates, by coarse grain size (mainly gravel-sized; Park and Schmincke 2019b in press) and large tephra thickness (up to 2.6 m in the LNB).

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The buoyant pumice clasts of the MLST-C tephra (0.59 g/cm3) have a much higher density compared to those of the LLST (0.47 g/cm3 in the LNB area; see Fig. 7 and SM-6), MLST-B1 (0.48 g/cm3) and MLST-B2 (0.47 g/cm3) because of more abundant phenocrysts, more mafic composition and lower vesicularity due to a significant phreatomagmatic impact (see Ch.

lasted c. 6 h and was followed by a longer break.

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General characteristics of Flooding Event FE4:

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5.1). About 34 Vol.% of the MLST-C pumice clasts are non-buoyant. We estimate that the eruption of the MLST-B/C tephra

Water level:

The maximum water level of FE4 overtopped the normal flood plain level raised by up to c. 4.5 m of fallout tephra (LLST + MLST-A/B/C), i.e. it reached ≥ 11 m above the base of the southern active side channel at the measuring area in the center

Major, wide-spread erosion:

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of the LNB (Figs. 3A, 3B, 10) and ≥ 13 m above the base of the major axial Rhine channel.

Flooding Event FE4 caused major wide-spread erosion and reworking of freshly deposited MLST-B/C tephra (c. 17 to 42

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Vol.% non-buoyant components in bulk samples of MLST-B and c. 63 to 72 Vol.% in MLST-C; Fig. 6 in Park and Schmincke 2019b in press). Erosion took place both within active and abandoned channels as well as on the adjacent broad flood plain areas that had been dry land for some time prior to the LSE (as indicated by soil formation, trees and bushes) and had not

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been affected by the preceding syn-eruptive floods (Figs. 4, 10, 12b, 13A, 13B, 14A). The impermeable MLST-A deposits (see Ch. 5.4) - overlain by two mud drapes deposited by FE3 - protected the underlying LLST tephra and/or the deposits of the preceding flooding events from being eroded by FE4. We have found evidence of an uncommon mechanism of en masse erosion that contributed to the extensive removal of primary MLST-B/C. Flooding Event FE4 undercut the up to 2.6 m thick MLST-B/C tephra cover above the impermeable FE3 mud drapes on the flood plain bordering the channels (Figs. 13Aa, 13B, 14Abc, 14Ba; Fig. 8 in Park and Schmincke 2019b in press). The thus detached MLST-B/C tephra started to float as an entire sheet. Subsequently, the sheets were fragmented into smaller sections that were transported downstream as large intact floating rafts (Locs. 636, 642, 703, 715, 742). Different stages of detachment, transport and redeposition of these rafts are documented in the field (Locs. 631, 636, 642, 703, 715, 742; Figs. 14A, 14B). This way, Flooding Event FE4 effectively reamed the MLST-B/C tephra from the flood plain for tens of meters sideways from

9 the channel margins (depending on the hierarchy and vertical spacing of a channel) and potentially along the full length of the channels within the LNB (up to c. 24 km). The deposits: The FE4 flood deposits are largely gravel-sized, matrix-free and clast-supported and display large-scale sedimentary structures. They show a striking, black-and-white striped stratification and lamination (“zebra-deposits”) that results from the strongly contrasting colors and densities of the fluvially reworked tephra components (dark-gray/black = lithoclasts and vesicle-poor non-buoyant (i.dr.st.) pumice, off-white/tan = buoyant (i.dr.st.) pumice, light-gray = vesicular non-buoyant (i.dr.st.) pumice; Figs. 8a, 12, 13A, 14Bb). Non-buoyant (i.dr.st.) tephra components clearly dominate in the FE4 deposits. The FE4 flood deposits form a sequence in part >2 m thick in the channels and <1,5 m thick on the adjacent flood plain. Three individual superimposed bedsets, each capped by a laterally consistent layer of silt to fine gravel-sized, largely pumiceous components can be correlated throughout the entire central LNB studied (8 km parallel and up to 7 km perpendicular to flow directionThey do not show any major facies variation within the LNB - irrespective of the type of channel (active or abandoned). The deposits of the third bedset are enriched in buoyant (i.dr.st.) pumice clasts.

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Interpretation: It seems very likely that erosion by floatation of entire tephra bodies contributed considerably to bulk erosion of MLST-B/C also inside the active channels where no evidence is left. Non-buoyant tephra components clearly dominate in the FE4 deposits because the bulk of the buoyant (i.dr.st.) pumice clasts (comprising 58 – 82 Vol.% in primary LLST and MLST-B deposits; Park and Schmincke 2019b) had been selectively rafted away during flooding. Three individual successive FE4 flood waves are inferred from the three superimposed bedsets. The matrix-rich layer overlying each bedset was

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deposited during the waning stage of the flood waves. Interbedded mud drapes were one major piece of evidence for multiple Lake Missoula flood events (Waitt 1980, Clague et al. 2003). The decrease in horizontal and vertical dimension of the sedimentary structures and an increase of the volume percentage of buoyant (i.dr.st.) pumice within the third bedset indicate

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a decrease of flow velocity towards the end of FE4.

Effects of the FE4 flood waves in active channels and on the adjacent flood plain:

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In the channels:

The primary MLST-B/C tephra was reworked and eroded except for rare preserved relics within the channels. The FE4 deposits consist of a succession of black and white striped, even to undulating bedsets that are laterally consistent throughout an outcrop (at least 20 m parallel to flow direction; e.g. Fig. 12a). The dark appearing basal and thicker part (10-20 cm thick)

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of the bedsets represents a complex succession of bipartite, bicolored laminae, in which the dark basal part of the bipartite laminae dominates in thickness. The upper thinner part (5-10 cm) of the bedsets represents a succession of bipartite laminae, in which the lighter-colored upper part of the laminae dominates. The dark basal part of the bipartite laminae consists of lithoclasts and non-buoyant, vesicle-poor dark-gray MLST-C pumice. The thinner, lighter-colored upper part of the laminae

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consists predominantly of light-gray, vesicular but non-buoyant (i.dr.st.) MLST-C pumice clasts (Fig. 8a). Quite commonly, large grounded rafts of primary/pseudo-primary MLST-B (up to 5 m long, up to 1 m high) are intercalated within the black basal parts of the bedsets. The grounded tephra rafts show different degrees of hydrodynamic erosion, shearing and

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disintegration ranging from prominent dome-like or wedge-shaped hummocks – some showing traces of remnant stratification - to extensive plane-parallel lensoid bodies (Figs. 12, 13Ac, 14A, 14B). The degree of shearing of the grounded rafts decreases towards the top of the FE4 sequence. The large grounded tephra rafts formed fixed obstacles triggering in-phase wave draping (cf. Cheel 1990), for instance at the base of the narrow northern active side channel (Loc. 703; Figs. 12a, 13A, 14Bb). At the stoss-side of many rafts, the drape laminae are partly eroded and the lighter-colored top parts of the overlying bicolored bedsets thicken and coarsen to form sets of lenticular convex-upward bodies that dip upstream at low angle (less than 20°). Locally, the light-colored top parts of the bipartite laminae within the dark base of the bedsets thicken and coarsen to form sets of lenticular convex-upward bodies that dip upstream even where grounded rafts are absent.

10 At the end of FE4, the bedsets became enriched in buoyant (i.dr.st.) pumice. They thickened at the lee side of the grounded rafts. The grain size especially of the non-buoyant components decreased. The matrix-rich unit overlying the deposits of the third FE4 flood wave is thicker compared to those capping the deposits of both preceding flood waves. It thickens in depressions. The bedding planes of the FE4 flood deposits are in part diffuse, especially within the dark basal part of the bedsets. However, the boundary between two successive bedsets is better defined. The boundary to grounded rafts or layers originating from sheared-out rafts is commonly distinct.

On the floodplain: On the floodplain, lensoid, upstream dipping laminasets (concave base and convex top) defining the shape of antidunes (Loc. 702; Figs. 12b, 13Aa, 13B) were accreted at the stoss-side of hydrodynamically truncated relics of primary MLST-B tephra, which had withstood complete en masse-detachment because they were anchored by bushes and trees buried in the tephra (Figs. 12b, 13Aa, 13B, 14Ac). Erosion occurred in the lee of one tephra relic and deposition at the stoss side of the next relic

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following upstream. Effects of the FE4 floods within the abandoned channels:

We found evidence at many places (e.g. Locs. 742, 743, 744), that the entire MLST-B/C fill was undercut by FE4 analogous to the processes described above (Fig. 14Ab) and then lifted – most prominently within the slightly lower abandoned channels north of the major channel (Fig. 3; Park and Schmincke 2019b in press) and potentially along their full length. Several parallel and narrow (up to 10 m wide) channels were newly cut into the lifted MLST-B/C tephra fill of practically all abandoned channels

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(Figs. 3A, 3B in SM-5), even where evidence for the previous Flooding events FE1, FE2 and FE3 was lacking. These secondary channels were refilled with upper flow regime structures resembling those described above. MLST-B/C was also

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reamed from the margins of the abandoned channels (Fig. 8 in Park and Schmincke 2019b in press).

7. Discussion

7.1. Hydrodynamic conditions forming the FE4 flood deposits

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The bedforms deposited by the FE4 floods resemble those described from laboratory flume studies (Cheel 1990, Cheel and Udri 1996, Alexander et al. 2001), those described from river deposits (e.g. Harms and Fahnestock 1965, Langford and Bracken 1987, Fielding 2006) and those deposited by pumiceous lahars and breakout floods (Manville et al. 1999, Segschneider et al. 2002, Manville et al. 2002) formed under upper flow regime conditions. However, the sedimentary

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structures deposited by the FE4 floods are largely gravel-sized and large-scale. Gravel antidunes have been described by Alexander and Fielding (1997) and Mack et al. (1996). The FE4 deposits also resemble deposits formed by volcanic base surges such as the large-scale chute and pool structures and antidunes formed during the terminal stage of the Laacher See

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Eruption (Schmincke et al. 1973).

Transitional upper flow regime and upper flow regime sedimentary structures are inadequately described from the rock record (Fielding 2006). Antidune cross-bedded pumiceous sands are common in the post-Taupo sequence although rare in

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sediments of normal density material (Manville et al. 2002). A detailed sedimentologic and hydrodynamic analysis of the FE4 deposits is not the main focus of this paper. However, in view of the rarity of records, the large-scale and the unusually striking stratification of the FE4 structures and in view of the fact that all exposures have been quarried or filled up (status August 2019), we will provide a short analysis. We suspect that the in-phase wave drape laminae and antidune and chute and pool structures within the FE4 deposits result from stationary water-surface waves. The grounded tephra rafts in the channels and the primary tephra relics on the flood plain likely enhanced the formation of stationary waves. Each grounded tephra raft posed an obstacle triggering the grounding of the next raft at its upstream side (Fig. 12). Accretion at the stoss-side of the grounded tephra rafts and primary tephra relics (Fig. 12) was probably caused by upstream migration and breaking of the waves (compare to flume experiments by Cheel 1990, Cheel and Udri 1996 and Alexander et al. 2001). Stationary water-surface waves are observed in rivers and floods

11 when flow velocity is greater than or equal to the celerity of water-surface waves (Froude et al. 2017). In these flow conditions, over non-cohesive mobile bed, feedback between the flow and the bed will form sediment waves (antidunes) that are inphase, or nearly in-phase, with the water-surface waves. Trains of stationary and upstream migrating water-surface waves were prevalent during a flash flood at Monserrat (West Indies) (Froude et al. 2017). Observations of lahars following the Pinatubo eruption (Philippines) (Rodolfo et al. 1996, Hayes et al. 2002) and Mt. St Helens eruption (U.S.A.) (Pierson and Scott 1985) prove that stationary waves and waves breaking in upstream direction can form despite high concentrations of suspended particles. The higher volume percentage of buoyant (i.dr.st.) pumice and the smaller grain size especially of the non-buoyant components within the terminal bedsets of FE4 indicates a decrease in flow velocity. Thickening of the laminae at the lee side of the grounded rafts suggests that the surface waves migrated downstream. A concentration of silt to fine gravel-sized, largely pumiceous components at the top of the deposits of each of the three FE4 flood waves represents sediment that had settled from suspension in the tail of the rapidly decelerating, sediment-charged flow. Flow conditions were possibly quite chaotic when the sediments were deposited. Locally, Froude numbers may have significantly exceeded F>1 (Cheel pers. com. 2018). Upper flow regime conditions were reached not only in the active

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channels but also in abandoned channels at higher elevation and on the flood plain.

The striking, zebra-like lamination and stratification of the flood deposits may have been caused by unsteady and non-uniform flow. Unsteadiness (e.g. variations of discharge, flow velocity and flow depth) occurs in a wide range of scales within a flash flood (Froude et al. 2017). The sediment concentration and thus bulk density of parts of the flow may change rapidly in time or space because of the potential for rapid sediment entrainment and deposition. During a flood, inherent instability in the

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flow may lead to the development of a surge or multiple surges.

It is highly plausible that the prominent off-white layers intercalated between two bicolored, bipartite bedsets (Figs. 12, 13Ac, 14Bb) originate from grounded MLST-B tephra rafts that subsequently became sheared by the current into extremely long-

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stretched horizontal lenses. Their quasi-primary composition (tan buoyant (i.dr.st.) MLST-B pumice clasts and less abundant dense lithoclasts), the poor separation of buoyant and non-buoyant components, the coarser grain size of the pumice clasts within these layers - compared to the black and white striped bedsets - and the lack of admixed non-buoyant (i.dr.st.) MLST-

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C pumice support this interpretation. Local load casting initiated by the basal dark unit (rich in lithoclasts and non-buoyant (i.dr.st.) pumice) of the overlying bedset suggests that the off-white pumice clasts within these lenses still could float. The majority of single pumice clasts derived from LLST and MLST-B tephra floated below the water surface in our lab experiments (unpubl.) and took weeks to months until they completely saturated with water. This suggests that the shearing of the

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grounded rafts and the deposition of the overlying unit took place in rapid succession or even quasi-simultaneously. This would also explain why only partially water-logged pumice clasts could be trapped in the deposit – a very specific mode of pumice deposition in addition to the mechanisms discussed in Manville et al. 2002. The in part crude stratification, vague grading and diffuse bedding planes especially within the dark basal part of the bedsets

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(consisting predominantly of non-buoyant components) suggest rapid deposition and in part quasi-simultaneous erosion and deposition (see below) from a flow highly charged with sediment – potentially from some kind of traction carpet at the base of a turbulent flow (cf. Todd 1989, Manville et al. 2002, 2003). Poor physical size sorting of buoyant (i.dr.st.) pumice clasts may

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arise from a wide range of primary densities of the pumice clasts and different degrees of saturation with water (cf. Manville et al. 2002). Size-sorting of non-buoyant (i.dr.st.) components is much better developed within the antidune structures accreted at the stoss-side of the primary tephra relics on the flood plain (Fig. 12b) compared to the FE4 deposits within the channel fill suggesting that these structures were formed within the turbulent, more dilute upper part of the flood. These antidune structures were formed more than 8 m above the pre-eruptive base the active side channels that were deeply incised into the flood plain. The FE4 deposits argue for a large-magnitude flood highly charged with sediment.

7.2. Extensive undercutting and flotation of primary tephra deposits and the transport of large primary tephra bodies as rafts

12 Undercutting of the channel banks is a common mechanism of river bank erosion (Thorne 1982). Here, however, undercutting of thick primary tephra sections on the floodplain extended for up to several tens of meters sideways from the channel margins (see Ch. 6.2). Within the northern abandoned channels, the entire LLST tephra fill plus the overlying impermeable MLST-A ash deposits were floated by subsurface run-off at the base of LLST during FE3, most probably along the full length of the channels (up to 6 km). In addition, this assemblage was undercut starting from secondary channels newly scoured into the channel fill by FE3 (Fig. 14Aa; Fig. 3A in SM-5). The theoretical mean bulk density of dry LLST tephra is roughly 0.75 g/cm3 (mean of four exposures close to the Rhine) and that of dry MLST-B/C tephra 0.93 g/cm3 (mean of two exposures; for calculations based on the relative proportions and densities of incorporated components see SM-6). The higher bulk density of MLST-B/C tephra compared to LLST is mainly due to the fact that the MLST-C tephra contains 59 Vol.% of non-buoyant components (SM-6). The LST deposits had probably been dry prior to en masse removal - even at the base of the channels (see Ch. 6.2, 7.3 and SM-5). The fact that the bulk density of dry LLST and MLST-B/C tephra sheets was largely <1 g/cm3 and the occurrence of impermeable boundary layers below and/or above these tephra units contributed to this unusually extreme dimension of undercutting. Field evidence shows that large primary tephra sections up to 2.6 m thick were lifted off and rafted downriver for tens (possibly

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hundreds) of meters and became incorporated either as quasi-intact bodies within the flood deposits (Figs. 6c, 9) or as bodies showing all transitional stages of hydrodynamic shearing (see Ch. 6.2; Figs. 12, 13Ac, 14Bb). During a flash flood, various parameters control the entrainment and transport of large boulders: gravity, buoyancy, drag force, impulsive force, lift and frictional resistance of the boulder to motion (Alexander and Cooker 2016). Here, rafting of the large tephra bodies by the syn-eruptive floods was probably facilitated by their low bulk density - even when water-logging of the pumice clasts and

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water filling of the interstices between the particles had already started.

The majority of single pumice clasts derived from LLST and MLST-B tephra floated below the water surface in our lab experiments (unpubl.) – despite the extremely low densities measured in a dry state (Fig. 7). This implies that their density

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rapidly increased to around 1 g/cm3 due to an instantaneous absorption of water. An initial rapid phase of absorption - while large pores are flooded - followed by a slow steady absorption phase over a period of weeks to months until the pumice clasts become sufficiently water-logged to sink is a common mechanism when cold pumice lapilli are immersed in water in lab

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experiments independent of their specific properties (Whitham and Sparks 1986, Manville et al. 1998, 2002). There are several reasons for the high absorption rate of LST pumice lapilli. First, many of the LST lapilli have tubular vesicles permeating an entire clast. Secondly, the vesicles were wide open at the clast surface because the majority of pumice lapilli are fragments of originally larger ones. Moreover, many lapilli have a thin absorptive ash cover. The fact that the density of

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single LST pumice clasts rapidly increases to around 1 g/cm 3 after immersion in water implies that the density of entire rafts would have rapidly exceeded 1 g/cm3. This suggests that the tephra bodies most probably “floated” completely immersed below the water surface. The highly particle-laden suspension surrounding the rafts had a higher density and transporting capacity compared to plain water (cf Manville et al. 2002) that possibly enhanced the transport of the rafts.

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What held together these virtually clay-free tephra rafts during transport? A related question is how the impressive, seemingly self-supporting erosional structures - that could not have been formed, and would not have been stable, on land - were generated. Examples are remains of completely unconsolidated primary LST eroded by eddies into coil-shaped structures

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seemingly hanging in “midair” (Loc. 702; Figs. 13Aa, 13B) or tephra sections (up to 2 m thick) overhanging deeply incised (several meters parallel to the ground), wedge-shaped cavities caused by undercutting (Figs. 14Ab, 14Ac). Moreover, field evidence shows that entire tephra bodies were able to stay intact when toppling upside-down into the channels (Fig. 14Ba). A combination of several factors may be responsible. A more in-depth analysis is beyond the focus of this study but two major factors will be addressed: a. Close mechanical interlocking of tephra particles within primary LST must have played a role. The majority of the volumetrically dominant (see Ch. 6.1) pumice clasts have angular shapes (due to fragmentation during transport in the eruption cloud and/or impact on the ground; Figs. 5-2, 5-27 in Fisher and Schmincke 1984, unpubl. data) and a high surface roughness due to their vesicles. The lithoclasts derived from the conduit walls also had angular shapes. Manville et al. (2002)

13 showed that frictional interlocking becomes more effective as particle size approaches the dimensions of the surface roughness elements (shard cusps and vesicle openings) starting at -1 phi. Thus, the sand-sized particles filling the interstices between larger pumice clasts may have contributed in holding the tephra rafts together. However, mechanical interlocking cannot be the only factor. On land, undercutting of primary tephra deposits at the base deeper than c. 10 cm commonly triggers the partial collapse of the overhanging tephra section. b. The seemingly self-supporting structures potentially temporarily floated because of their low bulk density and the high bulk density of the surrounding fluid (traction carpet), and were preserved because erosion of the primary deposits and resedimentation of mainly non-buoyant particles (predominantly lithoclasts and non-buoyant MLST-C pumice) probably took place near-instantaneously.

7.3. Arguments that exclude a conventional flooding scenario a. Flooding events occurring during breaks in eruptive activity At first sight, repeated flooding synchronous with active Plinian fallout pulses would be the most plausible scenario. Tephra falling into the Rhine River would have caused its level to rise. The Rhine then would have started to overtop its banks,

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flooding abandoned channels and the adjacent flood plain. However, the arguments listed below exclude flooding during Plinian fallout deposition and are strong evidence that the flooding events occurred during breaks in eruptive activity: (1) The systematic interbedding of fluvially reworked tephra with incremental primary LST units related to successive eruptive phases allows us to infer an unequivocal causal relationship between repeated distinct flooding events and eruptive activity. Rare, in situ relics of primary LST that had withstood erosion by a successive flood indicate that tephra sequences

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representing complete eruptive cycles repeatedly fell on drained ground (mud drapes) in between the documented flooding events - even at the base of side channels that had been active prior to the Laacher See Eruption and that had been flooded by a preceding flooding event (Figs. 9, 10, 13Aa, 13B, 14A; Figs. 1, 2A, 2B, 3 in SM-5; Fig. 8 in Park and Schmincke 2019b

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in press). Mud drapes deposited during the waning stage of each flooding event also indicate that the channels had been temporarily abandoned (see Ch. 7.4). (2) In case of syn-Plinian fluvial reworking, lithoclasts that rained directly into the river - and could not be floated away like the pumice clasts - should have become incorporated within the flood deposits in the

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LNB. These lithoclasts (especially large ones) should predominantly show up within the fine-grained, almost entirely pumiceous flood deposits and within the mud drapes deposited during the waning stage of the flooding events. Field evidence shows, however, that none of the fluvial sequences in the LNB was disturbed by simultaneous Plinian fall (Ch. 6.2, SM-5; Fig. 8 in Park and Schmincke 2019b in press). (3) The dense, gray, easily identifiable MLST-C pumice lapilli (Figs. 7, 8; SM-5)

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were incorporated already into the very base of the up to 2 m thick aggrading deposits of FE4 (consisting of reworked MLSTB/-C tephra) even within the active side channels. This indicates that flooding must have occurred after the deposition of the complete MLST-B/C unit and prior to deposition of MLST-D. (4) Locally, the lowermost unit of the FE4 channel sequence can be followed towards the flood plain up to tens of meters away from the channel margin. There, it overlies the complete MLST-

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B/C tephra sequence and underlies MLST-D reworked by FE5. This also argues for the same order of events. The observation that flooding occurred after, and not during, the deposition of a complete tephra unit and prior to tephra deposition by the successive eruptive phase was also documented for the other flooding events (see SM-5). (5) Stationary water-surface waves

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reduced in height or disappeared when rain fell directly into a flash flood at Montserrat because flow depth increased (Froude et al. 2017). When tephra fell directly into the Rhine River, turbulence of the flow may have also been reduced by the sinking or floated particles (cf. Manville et al. 2002). This suggests that the antidune and chute and pool structures deposited by FE4 could not have been formed during Plinian fallout deposition and is an additional argument that flooding must have occurred during a break in eruptive activity. b. Extensive and extremely widespread erosion and reworking of freshly deposited tephra sequences Today, the Rhine River erodes its bed between Koblenz and Andernach by about 3 mm annually (Frings and Vollmer 2009). The syn-eruptive floods described here were, however, extremely erosive. They eroded and reworked large volumes of fresh tephra deposits - up to 1.7 m thick LLST was eroded by FE2 and FE3 and up to 2.6 m thick MLST-B/C by FE4 - from all active

14 side and subsidiary channels and even from abandoned channels and vast flood plain areas (Figs. 3A, 3B, 4, 13A, 13B, 14A; Fig. 3A in SM-5). Approximately 75-80% of the tephra-covered area below c. 70.5 m asl within the central Lower Neuwied Basin shown in figure 3Ab – an area c. 40 km2 in size - was affected by syn-eruptive erosion and reworking. Evidence for the degree of erosion is lacking within the up to 2.5 km wide axial zone of the LNB bordering the major channel due to post-eruptive scouring (see Ch. 2; Figs. 3A, 3B, 4). Following LSE, both side channels were abandoned as indicated by a plug of slack water deposits up to several meters thick overlying the syn-eruptive flood deposits (Figs. 12a, 13Ac, 14Bb; Fig. 3Aa in SM-5). The northern side channel was never reoccupied. The southern side channel was not reoccupied prior to the onset of increased precipitation caused by the climate impact of LSE (Fig. 127 in Schmincke 2014, Park and Schmincke 2019b in press and unpubl. data). This might indicate considerable downcutting into the NT2 gravel terrace and deepening of the pre-eruptive major Rhine channel especially by FE4. Erosion of such dimensions by any type of extreme meteorologically-induced flooding in the Rhine River (caused by high intensity rainfall and/or snowmelt) is clearly unrealistic (see Ch. 7.4). c. Identical facies over large areas The deposits of the various flooding events are distinguishable from each other by their specific fingerprint and by interbedded

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primary tephra layers, and were correlated throughout the entire LNB. The floods affected all channels on the very broad flood plain bordering the major pre-eruptive Rhine channel (up to 7 km wide  to the Rhine, Figs. 3A, 4) simultaneously and in a similar manner. The deposits are similar along the total studied length of the channels (c. 10 km) irrespective of the hierarchy and vertical spacing of these channels and of the distance of a specific abandoned channel from an active channel (Figs. 3A, 4; Fig. 3A in SM-5). In case of an extreme “conventional” flooding event, the Rhine River would have left different

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deposits in different types of channels and on the flood plain depending on its local flow properties (cf. Miall 1996, Bridge 2003; see Ch. 7.4). d. Upper flow regime conditions

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The sedimentary structures of all flooding events and the extensive and wide-spread erosion and reworking indicate high flow velocities (for evidence for FE4 see Ch. 6.2, for evidence for FE1, FE2, FE3 and FE5 see Fig. 10 and SM-5). FE4 was characterized by even higher flow velocities than the previous flooding events. Erosion and reworking of the entire MLST-B/C

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sequence and the large-scale, gravel- and coarse sand-sized antidunes and chute and pool structures indicate that upper flow regime conditions (cf. Manville et al. 1999, Alexander et al. 2001, Manville et al. 2002, Segschneider et al. 2002, Fielding 2006, Froude et al. 2017) had been reached throughout the entire flooded area – in all channels (even in the highest elevated abandoned channel) and on the adjacent flood plain.

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e. Extremely high peak flood levels and extremely broad flooded area The maximum water levels were high despite the extremely broad flooded area (Figs. 3A, 3B, 4, 10). The peak flood level of FE4 (≥ 11 m above the base of the southern active side channel at the measuring area within the center of LNB) significantly overtopped flood plain areas raised by up to c. 4.5 m of fallout tephra (LLST + MLST-A/B/C) even within the central LNB

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where the division of the water volume by inundating numerous active and abandoned channels (as shown by field evidence) was at a maximum. The flood levels of each of the syn-eruptive flooding events were possibly at least 2 m higher within the major axial channel of the Rhine River (field data are lacking due to post-eruptive scouring) because it was more deeply

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incised (by c. 2 m) compared to the southern side channel (Figs. 1, 3A, 4; Park and Schmincke 2019b in press). An extremely large cross-section area (up to 7 km  to flow direction) was flooded during peak discharge of FE4 (Figs. 3A, 3B, 4). Approximately 75-80% of the central Lower Neuwied Basin shown in figure 3A were flooded. f. The rapidity of water level increase and the multiple repetition of syn-eruptive flooding events Peak flood levels associated with major erosion of freshly deposited tephra - even high on the flood plain (elevated by the primary tephra deposits) - were reached during the initial stage of the syn-eruptive flooding events as shown by field evidence indicating a sudden inflow of large volumes of water. After that, flood water levels fell and flow was diverted away from raised areas into the river channels. The rapid water level increase and the short duration (during breaks in climactic Plinian eruptive activity as shown by field evidence) as well as the multiple repetition of the syn-eruptive flooding events contrasts with the

15 discharge hydrograph and duration (days to weeks) of “conventional” meteorologically-induced floods in the Rhine River area at gauging station Andernach (www.undine.bafg.de). However, the timing of the syn-eruptive floods resembles that of meteorologically-induced flash floods (cf Sene 2013) or that of breakout floods of impounded lakes that are characterized by a rapid rising limb (cf Manville and Hodgson 2011). 7.4. Trigger mechanism of the highly energetic floods In the preceding sections, we have provided clear evidence that large water masses must have been released from upstream of the LNB during five times, called by us flooding events. What were the reasons? a. Lahars or flash floods caused by regional rainfalls Heavy rainfalls are common triggers of lahars and flash floods in volcanic areas during and - more commonly - after large eruptions (e.g. Mt. Pinatubo, Rodolfo et al. 1996; Soufriére Hills Volcano, Montserrat, Froude et al. 2017). We did not find any indicators of effective surface run-off (contemporaneous with the LLST and MLST-B/C/D phases) within the area (37 km x 31 km, c. 1200 km2) surrounding the LNB and LSV shown by Fig. 1a - not even on very steep terrain or at the base of major preeruptive drainage gullies or creeks. The coarse-grained, matrix-free and thus highly permeable Plinian fallout tephra deposits

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would have suppressed surface run-off (cf Leavesley et al. 1989). We also did not find indicators of sub-surface run-off. Therefore, the initiation of lahars from the flanks of the LNB by massive regional rainfalls (e.g. supported by high concentrations of ash in air) can be excluded as a cause of the syn-eruptive flooding events in the LNB. Flooding Event FE3 (see SM-5) took place after the transitional phase MLST-A that covered the landscape surrounding the Laacher See Volcano with a sealing cover of cohesive ash layers extending over an extremely large area. In this case, we think that surface run-off

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could have contributed to flooding because we found minor gully erosion into the top of MLST-A on higher ground (unpubl. data). In contrast, we have documented excessive reworking of LST (sequences >10 m thick deposited by a very large number of individual lahar events) on higher ground during the terminal phase of LSE (ULST-C) and much more frequently

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following LSE at many localities, where we have not found any evidence of reworking during the eruptive phases synchronous to syn-eruptive flooding (Schmincke 2008, 2014; Schmincke et al. 1990, 1999; unpubl. data). We have also documented numerous post-eruptive lahar events in the LNB following LSE (unpubl. data) in the southern active side channel and its

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subsidiary channels (Locs. 630, 632, 653, 654, 660, 663, 664, 665), in the northernmost abandoned channel and at the northern rim of the LNB (Locs. 715, 716, 744, 756, 758, 759). b. Flooding due to excessive precipitation in the headwaters

Could rainfalls (promoted by ash fall) in the headwaters of the Rhine River or the Moselle River have caused syn-eruptive

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flooding? The active and abandoned channels in the LNB that had been flooded during the LSE, were not flooded by recent events due to considerable deepening of the Rhine channel during the Holocene (see Fig. 4c), so there is no opportunity for comparison. The largest flood during the past ten years (2007/2016) reached 8.37 m at gauging station Andernach (www. dgj.de). A flood of comparable volume would have been able to shallowly inundate the flood plain (raised by primary LST)

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surrounding the active channels at the time of LSE - providing that the base of the channels was elevated by fluvially reworked LST by c. 2-4 m (major pre-eruptive channel incised c. 8 m into NT2 gravel terrace, active side channels c. 6 m; Park and Schmincke 2019b in press). However, even an extreme weather-induced flood and thus a peak inflow into the LNB would not

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have been able to reach the erosive power, transporting capacity and flow velocities simultaneously in all active and abandoned channels on the very wide flood plain of the LNB (Figs. 3A/B and 4) as documented for the syn-eruptive flooding. Large historic millennium floods – among them the largest flood event by volume (in 1374 BP) ever observed at Cologne (flood level 13.3 m, peak discharge 23,800 m3/s after Herget and Meurs 2010) - caused only minor erosion on the flood plain (pers. com. H.-R. Bork). The famous Magdalenenhochwasser - triggered by extremely heavy rainfalls mainly affecting the River Main (a major tributary of the Rhine River) in July 1342 AD - is recognized as the largest historic flood in Central Europe (www.unidine.bafg.de). Rainfalls started on July 19th. However, the flood wave did not reach the mouth of the River Main into the Rhine River – located 100 km downstream of the LNB – prior to July 21th (no data for the LNB). A very extraordinary flooding event with record-breaking flood levels in the Rhine area in spring 1784 AD was caused by the climate impact of the

16 Laki eruption (Iceland). A sudden intrusion of warm air associated with excessive rainfalls starting over large areas after an extremely cold period during February 23th caused abrupt snowmelt and ice breaking-up and ice jamming events in all rivers almost simultaneously five days later (Glaser and Stangl 2004, Glaser 2013). These examples show that heavy rainfalls in the headwaters of the Rhine River can be excluded as a cause of syn-eruptive flooding because the flood waves would have taken too long to reach the LNB. Moreover, the multiple and short-term repetition of flooding during LSE could not be explained by this type of trigger. c. Lake breakout floods Field evidence indicates that primary LST repeatedly fell on drained ground (mud drapes) in between the documented flooding events - even at the base of side channels that had been active prior to LSE and that had been flooded by a preceding flood event (see Ch. 7.3; Figs. 9, 10, 13Aa, 13B, 14A; see Figs. 1, 2A, 2B, 3 in SM-5; Fig. 8 in Park and Schmicnke 2019 b in press). Mud drapes deposited during the waning stage of each flooding event imply that the channels had been abandoned. Sedimentation of fine-grained clastic material from suspension is impossible in an active channel (Miall, 1996). Moreover, the en masse removal of voluminous, freshly deposited tephra bodies, not only from the channel margins but also from the channel bases and their transport as floating rafts by a successive syn-eruptive flood as shown by field evidence (Ch. 6.2,

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7.2, SM-5), suggests that the tephra deposits had been dry when flooding occurred.

The repeated formation and breach of a dam at the upstream entrance of the Lower Neuwied Basin near Koblenz (Koblenz Dam) consisting of fallout components and driftwood washed together (cf Umazano et al. 2014) is the most convincing explanation for the multiple repetition of the drainage of the channels in the LNB followed by the powerful flooding events in rapid succession during the climactic Plinian episode of Laacher See Eruption. There are no exposures within the area where

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the postulated ephemeral Koblenz Dam had been located due to later erosion by the Rhine River and modern construction. The growth mechanisms and structure of such a dam as well as arguments for its most plausible location are discussed in detail in a companion paper (Park and Schmincke 2019b in press).

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In short: The Rhine must have been dammed five times downstream of the confluence of the Moselle River behind a shortlived, unstable dam consisting of an accumulation of mainly pumice and driftwood washed together. Koblenz dams No.1, 2 and 3 were much lower than Koblenz dam No.4 judging from the observation that the FE4 floods reached the highest

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velocities, erosive power and flood levels compared to the other flooding events (Fig. 10). Koblenz Dam No.4 was as high as the flood plain bordering the major Rhine channel including the overlying cover of LST at a maximum (up to c. 10 m, Park and Schmincke 2019b in press). The extremely low gradient of the Rhine River upstream of Koblenz (0.15-0.19 ‰; www.bafg.de, Park and Schmincke 2019b in press) allowed the repeated accumulation of large volumes of water in an

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elongate lake (Lake Koblenz) along the upstream course of the Rhine despite the postulated low height of the dams (Fig. 1). This contrasts with most eruption-induced river damming scenarios. Commonly volcanoes are surrounded by deep valleys and high relief, favoring the formation of deep lakes (Capra 2011) and thus only small lakes could be impounded by low-rise dams. Lake Koblenz No.4 dammed prior to FE4 was potentially the most voluminous of all lake stages. It may have extended

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as much as c. 30 km upstream of the dam for a maximum water level at 70 m asl (Fig. 1). Its backwater must also have extended several kilometers into the valleys of the tributaries Moselle and Lahn. The water volume of the lake could have amounted to approximately 80x106 m³. About 50 Vol.% of the shallow lake comprised

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water filling the Rhine channel along its 30 km long upstream course (at pre-eruptive discharge) already prior to damming (tributaries are not considered in this rough estimate). The actually dammed-up water volume of c. 40x106 m³ could have accumulated within c. 6 to 7 h. Extra time must be accounted for because the dam was potentially not completely sealed most of the time. The time to fill the lake is roughly equivalent to the estimated duration of the MLST-B/C eruptive phases (Park and Schmincke in prep.). The dammed-up water volume would have been more than sufficient to flood all channels and the flood plain in the LNB up to the maximum flood level by a transient breakout flood wave. Addition of tephra components originating from the breached dam and from entrainment by erosion from the ground potentially increased bulk discharge of the flood waves considerably.

17 7.5. The syn-eruptive breakout floods The syn-eruptive breakout floods differ significantly from most breakout floods described in the literature resulting from failures of landslide dams (e.g. Costa and Schuster 1988, Costa 1988), from breakouts of lakes developed in craters and calderas (Manville et al. 1999, Manville and Hodgson 2011) and breakouts from drainages blocked by pyroclastic material (White et al. 1997, Kataoka et al. 2008). 1. The different stages of Koblenz Dam were as high as the floodplain + the overlying tephra cover at a maximum (Park and Schmincke 2019b in press). The gradient of the flood plain and channels in the LNB downstream of the dam was extremely low (0.15- 0.46 ‰; www.bafg.de, Park and Schmincke 2019b in press). Thus, the elevation difference between the crest of the likely highest dam, Koblenz Dam No.4, and the base of the river bed at Andernach c. 21 km downstream was only 16 m at a maximum. This elevation difference is extremely small compared to tens to several hundreds of meters documented along a similar distance for other breakout floods in volcanic context (e.g. Manville et al. 1999, Kataoka et al. 2008, Manville et al. 2010, Manville and Hodgson 2011). 2. The Koblenz Dam was located at a position where the upstream narrow Rhine Canyon opened up into the very broad and slightly inclined floodplain of the tectonic Lower Neuwied Basin (c. 60 km2, up to 7 km wide perpendicular to the river). In

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many other cases, the terrain downstream of a breaching dam is a narrow, deeply incised valley several kilometers in length (e.g. Manville et al.1999, Kataoka et al. 2008) well-suited for confining an outburst flood. The syn-eruptive floods debouching into the Lower Neuwied Basin spread sideways into the multiple active and abandoned channels. This, and the low gradient may have been mainly responsible for the limited downward erosion into the pre-eruptive deposits at the base of the active side channels (Figs. 3, 4).

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We speculate that the Koblenz dams were breached instantaneously owing to their composition of a cohesionless mass of low-density grains and driftwood. This contrasts with many other cases of outburst floods that were initiated by a more gradually enlarging breach due to overflowing (e.g. Manville and Hodgson 2011). The low elevation difference between the

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dammed-up Koblenz lakes and the area downstream and the potentially sudden breach of the dam more closely resemble the conditions and processes associated with ice jams (cf Beltoas 1995). In Park and Schmincke 2019b, we discuss in detail mechanisms of dam formation that may have closely resembled ice jam formation. The floating and partly grounded Koblenz

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dams were probably not completely sealed most of the time. This way the major pre-eruptive Rhine channel would have received some water and an equilibrium condition was established that enabled the dam to remain stable as long as tephra fell into the Rhine relatively continuously (Park and Schmincke 2019b). The breaches of Koblenz Dam obviously caused voluminous high-energy breakout floods despite the low height of the dam.

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The large dimension of erosion and reworking of freshly fallen tephra in general and the shifting and overthrusting of large floated bodies of primary tephra with the floor area of a garage or a house (even in abandoned channels and on the flood plain) indicates that the particle-laden flood waves - bulking due to entrainment of additional tephra by erosion - were able to exert enormous forces (more than plain water; cf Alexander and Cooker 2016) on their way through the tephra-filled channels.

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Heavy fragments - many square meters in size - of the up to 25 cm thick MLST-A sequence (consisting predominantly of cohesive ash layers) were lifted-off the underlying LLST by FE3, overturned, folded or rolled up like wall paper and dropped at the channel base. Trees were ripped off (potentially together with large entire MLSTB/C tephra bodies in which they had

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been buried) by the FE4 floods above the MLST-A ash layers on the flood plain. Several superimposed cycles of upper flow regime deposition followed by sedimentation of fine-grained pumiceous sand and silt or mud draping within and on top of the deposits of Flooding Events FE3, FE4, and FE5 either indicate significant fluctuations in discharge and flow velocity within a single flood wave or two or more successive flood waves. This suggests rapid renewed blockage at Koblenz or further downstream (backwater effect; see Park and Schmincke 2019b) owing to the large volumes of buoyant pumice clasts and drift wood mobilized by lake drainage and flooding. Moreover, the Moselle River and the Rhine upstream of Koblenz constantly supplied additional tephra masses and driftwood. Incomplete drainage and repeated renewed damming of a lake by lahars was observed after the Pinatubo eruption (Umbal and Rodolfo 1986).

18 7.6. Scenario following Flooding Event FE4 We have briefly mentioned in Ch. 4 that - about halfway through the eruption at the end of MLST-A - a second lake started to accumulate behind a pyroclastic flow-generated dam blocking the narrow Rhine valley at Brohl, c. 28 km downstream of Koblenz Dam and c. 7 downstream of the Lower Neuwied Basin. Prior to FE4, the backwater of Lake Brohl potentially only reached up to Weissenthurm (6 km upstream of the outlet of the LNB at Andernach) within the major axial channel (Figs. 1, 2; Figs. 2, 9 in Park and Schmincke 2019b in press). The release of upstream Lake Koblenz No.4 during the FE4 flood waves led to a rapid and substantial expansion of Lake Brohl. Following FE4 and prior to deposition of the MLST-D tephra, the water of Lake Brohl had filled the deepest sections of the active channels above the level of the FE4 flood deposits and in part also the deepest sections of abandoned channels in the LNB (depending on their elevation; Fig. 10). The flood waves of flooding Event FE5 (discussed in detail in SM-5) occurred after MLST-D. They were less powerful than the previous events because of the low elevation difference between the already relatively high level of the backwater of Lake Brohl within the channels in the LNB and the level of Lake Koblenz No.5 (Figs. 2, 10; see SM-5). Plane-parallel bedding, however, still indicates upper flow regime conditions (Figs. 12, 13A, 14B; Figs. 2Ab1 and 3A in SM-5). The damming of the Rhine by Plinian fall into the river ended, and the resulting flood waves then terminated when the

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backwater of Lake Brohl reached upstream behind the mouth of the Moselle river at Koblenz after FE5. 8. Summary and conclusions

The systematic interbedding of fluvially reworked tephra with primary LST units related to successive eruptive phases allows to infer an unequivocal causal relationship between eruptive activity and repeated distinct flooding events. There is clear

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evidence for one minor (FE1) and four major (FE2-FE5) highly energetic flooding events affecting the entire LNB during the climactic stages of Laacher See Eruption.

The floods, especially Flooding Event FE4, deposited striking large-scale, largely gravel-grade bedding structures including

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in-phase wave draping, antidunes and chute-and-pool configurations formed under upper flow regime conditions. The prominent black-and-white striped stratification of the flood deposits is due to the strongly contrasting colors and densities (pumice and lithoclasts) of the fluvially reworked tephra components.

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Bulk densities of the LST deposits were <1 g/cm3 (owing to the large percentage of incorporated pumice) - strongly contrasting with the density of deposits normally eroded by rivers - and allowed unique en masse erosion of LST both from the channels and the flood plain - to our knowledge not previously described from other fluvial environments. Tephra sheets (up to several m thick) were undercut along impermeable boundaries. Extensive tephra sections - with the floor area of a house - were

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shifted and thrust one above the other. Large primary tephra bodies were lifted off and transported downstream for tens or even hundreds of meters. They finally disintegrated completely or – less commonly – were grounded and sheared to hydrodynamically shaped hummocks.

We present strong evidence that large water masses must have been released from upstream of the LNB during five times:

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(a) Extensive and extremely widespread erosion and reworking of freshly deposited tephra sequences and (b) upper flow regime conditions within all active side channels, abandoned channels and even vast flood plain areas, (c) extremely high peak flood levels and the extremely broad flooded area and (d) the rapidity of water level increase. We can exclude a

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meteorological trigger for the flooding. The repeated formation and breach of a dam at the upstream entrance of the Lower Neuwied Basin near Koblenz (Koblenz Dam) consisting of principally fallout components and some driftwood washed together is the most convincing explanation for the phenomena observed. Flooding generally occurred during breaks and not during eruptive activity. Tephra sequences representing complete eruptive cycles repeatedly fell on drained ground (mud drapes) in between the documented flooding events - even at the base of side channels that had been active prior to the Laacher See Eruption and that had been flooded by a preceding flood event. This is strong evidence that Koblenz Dam formed and remained stable only as long as tephra fell into the river during Plinian phases and became breached during longer breaks in eruptive activity. Thus, the strongly pulsating nature of the LSE associated with multiple interruptions of eruptive activity was the fundamental factor controlling the damming and flooding

19 dynamics. The extremely low gradient of the Rhine upstream of Koblenz allowed the accumulation of large volumes of water, forming elongate lakes (Koblenz lakes) along the course of the river for up to c. 30 km, although the dams were less than 10 m high. The syn-eruptive breakout floods differ from other breakout floods in volcanic areas. Koblenz Dam was located at a position where the upstream narrow Rhine Canyon opened up into the very broad and only slightly inclined (0.15- 0.46 ‰) floodplain of the tectonic Lower Neuwied Basin. These boundary conditions may have been mainly responsible for the limited downward erosion into pre-eruptive deposits. In the LSE/Rhine-case, Plinian fall was capable to dam a major river and cause massive flooding while the climactic eruptive activity was in full progress. A comparable scenario would be a major challenge for mitigation efforts - aside from the destructive effects caused by primary volcanic transport processes. Damming of a large river would rapidly increase the threat to life and property because huge volumes of water could quickly be dammed. Today, the LNB is a highly industrialized area with a population close to 300.000. The Rhine itself is a major transport artery between Switzerland and the Netherlands with dozens of barges and tourist boats crossing the LNB daily and important European railroad lines along both banks. High-risk industries (e.g. chemical industry, terminals for oil, gas and other chemical products, etc.) are preferentially located close to

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the river and would severely pollute the entire LNB as well as vast areas downstream if a similar flooding scenario happened today. Deep low frequency earthquakes interpreted to be generated by fluids (e.g. magma) rising through the mantle/crust below Laacher See Volcano have been recently detected (Hensch et al. 2019) possibly signaling a reawakening of LSV.

Funding Information

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Our studies were initially supported by the Deutsche Forschungsgemeinschaft, grants Schm 250/58 and 250/84 and partly by the Stiftung Zukunft/Sparkasse Koblenz.

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Acknowledgments

We are especially grateful to an anonymous reviewer for critical comments and insightful suggestions that led to major improvement of the manuscript. We thank V. Manville, K. Kataoka and J. Taddeucci for helpful suggestions on an earlier draft

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of this manuscript and T. Aigner, J. Alexander, H.-R. Bork and R. Cheel for helpful comments. We also thank M. Sumita for technical support and valuable suggestions.

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–kartierung, Ruhr-Universität Bochum, unpubl.

Telford WM, Geldart LP, Sheriff RE (1990) Applied Geophysics. Cambridge Univ Press: 770 pp Thorne CR (1982) Processes and mechanisms of bank erosion. In Hey RD, Bathurst JC, Thorne CR (Eds): Gravel bed rives.

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Wiley: 227-271 pp

Thoste V (1974) Die Niederterrassen des Rheins vom Neuwieder Becken bis in die Niederrheinische Bucht. InauguralDissertation, Universität Köln: 131 pp

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Todd SP (1989) Stream-driven, high-density gravelly traction carpets: possible deposits in the Trabeg Conglomenrate Formation, SW Ireland and some theoretical considerations of their origin. Sedimentology 36: 513-530 Umazano AM, Melchor RN, Bedatou E, Bellosi ES, Kraus JM (2014) Fluvial response to sudden input of pyroclastic sediments during the 2008–2009 eruption of the Chaitén Volcano (Chile): The role of logjams. J South Am Erath Sci 54: 140-157

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Umbal JV, Rodolfo KS (1996) The 1991 lahars of southwestern Mount Pinatubo and evolution of the lahar-dammed Mapanuepe lake. In Newhall CG and Punongbayan RS eds: Fire and Mud; Eruptions and Lahars of Mount Pinatubo. Philippine Institute of Volcanology and Seismology. University of Washington Press: 951-970 pp

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Wagner B, Weidenfeller M (2000) Hydrogeologische Querschnitte. In: Geol Landesamt Rheinland-Pfalz (ed): Hydrogeologische Kartierung Neuwieder Becken, Mainz, CD Waitt Jr RB (1980) About forty Last-Glacial Lake Missoula jökulhlaups through southern Washington. J Geol 88: 653-679

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White JDL, Houghton BF, Hodgson KA, Wilson CJN (1997) Delayed sedimentary response to the A.D. 1886 eruption of Tarawera, New Zealand. Geology 25: 459-462

White JDL, Manville V, Wilson CJN, Houghton BF, Riggs NR, Ort M (2001) Settling and deposition of AD 181 Taupo pumice in lacustrine and associated environments. In White JDL and Riggs NR (Eds): Volcanogenic Sedimentation in Lacustrine Settings. Spec Publs int Ass Sediment 30: 141-150

Whitham AG, Sparks RSJ (1986) Pumice. Bull Volcanol 48: 209-228 Wörner G, Schmincke H-U (1984a) Mineralogical and chemical zonation of the Laacher See Tephra sequence. J Petrol 25: 805-835

23 Figures Fig. 1 a. Map showing Laacher See Volcano, areal distribution of total Laacher See Tephra deposits (isopachs in m) after Bogaard and Schmincke (1984), isopach axes of individual eruptive phases relevant for damming at Koblenz after Staps (1976 unpubl: LLST), Freundt und Bednarz (1982 unpubl: MLST-A2), Bogaard and Schmincke (1984: MLST-B/C/D2), areal distribution of ignimbrites (in red) after Freundt and Schmincke (1986) and new data (unpubl.). Damming scenario in tectonic LNB: Reconstruction of pre-eruptive channels in the LNB based on new data (Park and Schmincke 2019b in press) and Ikinger and Weidenfeller (2000). Reconstruction of pre-eruptive flood plain at Koblenz based on new data. Max. extent of Lake Koblenz No.4 dammed at Koblenz Pumice Dam No.4 during the deposition of the MLSTB/C tephra after Flooding Event FE3 and prior to Flooding Event FE4 (based on potential maximum water level at 70 m asl) had the largest depth and length of all lakes. Early stage of Lake Brohl prior to FE4 (assumed water level <59 m) dammed at downstream ignimbrite dam cannot be reconstructed and is only schematically drawn because the broad central zone in LNB bordering the major Rhine channel was scoured after LSE.

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b. The Koblenz lakes temporarily dammed at the repeatedly reestablished Koblenz Dam were very elongate reflecting the narrow upstream Rhine canyon. Lake Koblenz No.4 may have extended as much as c. 30 km upstream due to the low gradient of the Rhine - despite the low height of Koblenz Dam No.4.

c. Morphological cross-section from Laacher See Volcano through the Lower Neuwied Basin up to its northern rim Fig. 2

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Overview of the succession of fluviodynamic processes resulting from the impact of different types of eruptive activity on the Rhine River during the LSE.

a. Revised event stratigraphy of LST at Loc. 758 at the northern rim of the LNB, c. 14 m above the mean pre-eruptive water

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level of the Rhine, where a stranded pumice raft intercalated between ULST-A and –B indicates that Loc. 758 was submerged below water during the eruptive break between ULST-A and -B. This is unequivocal proof of the damming and drainage of a huge lake (Lake Brohl) during the LSE.

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b. Correlation of lake damming and flooding events with stratigraphy. For explanation see Ch. 4 of main text. c. Correlation of breaks in eruptive activity deduced from proximal LST deposits based on classical break indicators (red arrows) with breaks deduced from the overall flooding scenario (blue arrows). Estimates of the duration of specific eruptive

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phases and breaks based on analogy with recent eruptions and many, in part newly applied criteria (degree of weatherinduced erosion, indicators of vent collapse, change in direction of fallout distribution, impact of LSE on the Rhine River, water saturation experiments with LST pumice, calculations of the time it took to accumulate a specific water volume of growing Lake Brohl up to a specific level until a particular eruptive phase started Lake Brohl, etc.; new unpubl. data).

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Fig. 3A

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d. Events during the entire damming scenario discussed in this account and companion paper Park and Schmincke 2019b.

a. Area in central LNB flooded by Flooding Event FE4 (following deposition of MLST-B/C) triggered by breach of Koblenz Dam No.4. FE4 flooded all active and abandoned channels as well as most of the flood plain north and south of the major pre-eruptive Rhine channel (dark blue) except for the highest parts >67.5 m asl (light and dark orange). This massive flooding reflects a huge water volume released from upstream during FE4. b. Central part of LNB prior to LSE. Reconstruction after Park and Schmincke (2019b in press)

Fig. 3B Legend for figure 3A and overview map

24 Fig. 4 Cross sections through Lower Neuwied Basin a. Pre-eruptive: The NT2 gravel terrace forming the base of LNB was intersected by a multitude of channels of different age and elevation. The Rhine was a multi-channel river at the time of LSE. The water depth within the active channels were much shallower than within the present single Rhine channel. The central area of LNB was scoured after LSE, see section c. Thus, the active major Rhine channel and its bordering flood plain is reconstructed only schematically. The pre-eruptive topography - buried below a blanket of primary and reworked LST deposits of strongly varying thickness today (see c) - was reconstructed based on present topography prior to exploitation of gravel and tephra (according to old maps), our own measurements of elevations and tephra thicknesses in the field and drilling data by Wagner and Weidenfeller (2000). The geologic framework is based on a hydrogeologic cross section by Wagner and Weidenfeller (2000) and Giebel et al. (1990). b. Syn-eruptive: Flooding Event FE4 (post-MLST-B/C tephra fall) inundated the entire base of LNB (6.5 km wide within this cross-section). Only the highest flood plain areas were spared. Areas were flooded that had been dry land for a long time prior to LSE (as indicated by soil formation and trees). Along all channels - irrespective of their elevation - primary LLST+MLST-A/B/C tephra (up to 4.7 m thick) was removed and replaced by largely gravel-sized, upper-flow-regime deposits

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largely consisting of non-buoyant LST components. The exclusively high flow velocities within the entire area, the large volume of reworked, coarse-grained tephra and the high flood level argue for damming and release of a very large water volume farther upstream (compare to “normal” floods in c).

c. Present: The center of LNB was repeatedly scoured and refilled following LSE. The water volume of an average modern flood and even of the once-in-a-century flood in 1926 was small compared to that of FE4. Terrace flight modified after data

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by Bibus (1980) and Schirmer (1990).

Fig. 5

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Legend for all photographs Fig. 6

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Identification of primary tephra relics intercalated within flood deposits based on the LST stratigraphy: Photos a. and b. illustrate the striking facies variation of primary LST in the LNB that does not allow straightforward correlations, but require an excellent familiarity with the stratigraphy of LST also closer to vent even on the scale of subphases. The facies variation of LLST is most significant because of the pronounced stepwise rotation of the fallout fans of successive

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LLST-phases from northeast to southeast. Subunits LLST-B to –F terminate with a layer or a succession of layers extremely rich in Devonian lithoclasts indicating a repeated collapse of the deeper conduit and attempts to clear it (“Big Bang” events). The bipartite Big Bang layer at the top of subunit LLST-B at Loc. 610 (located close to the western margin of the LNB and close to the axis of the LLST-B depositional fan) is much less prominent and much finer-grained only c. 2 km to the southeast

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at Loc. 642 (located farther away from the fan axis). LLST-C is much thicker at Loc. 642 compared to Loc. 610 due to the shift of the depositional fan of LLST-C further to the south compared to that of LLST-B. The succession of the 3 Big-Bang events in subunit-C is much less pronounced than the 2 Big Bang events of LLST-B due to a constant decrease of eruptive

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power and of the height of the eruption column during the course of LLST. LLST-D was not deposited at Loc. 610 in contrast to Loc. 642 indicating that the depositional fan of LLST-D was rotated even more strongly to the southeast. Photo c. shows an isolated relic of primary LST intercalated within syn-eruptive flood deposits that was transported as a raft by the floods and then grounded. A familiarity with the variable LST stratigraphy allows to identify it as LLST (compare to photo a.). The prominent Big Bang layer below MLST-A indicates that the tephra relic was eroded in an area represented by the LLST-B facies shown in Fig. 6a.

Fig. 7

25 Variation of diagnostic properties of the juvenile components in LST from base to top that are fundamental to stratigraphically assign tephra relics intercalated within the flood deposits. a. Stratigraphy of LST at Loc. 758 (see Fig. 2). b. and c. Pumice clasts become more mafic and crystal-rich and thus darker due to the pronounced chemical and mineralogical zonation of the magma reservoir. Phenocryst abundance after Bogaard and Schmincke (1985) d. and e. The density of buoyant pumice clasts increases and the volume percentage of buoyant LST components (mean of bulk samples from 4 localities close to the Rhine) decreases due to the zonation of the magma reservoir enhanced by an increasing phreatomagmatic impact towards the end of LSE (starting with MLST-C). f. Decrease in Zr concentration within the stratigraphy (based on XRF-analyses of pumice clasts from Loc. 758). The high Zr concentration of the pumice raft layer does not fit to the more mafic composition of the primary tephra layers below and above – one of the strong proofs for its origin. g. Typical example of the complex intercalation of primary and fluvially reworked LST sequences (section abstracted from a photo). Correct stratigraphic assignment of the layers was achieved by several characteristic criteria: color, phenocryst abundance, grain-size, density and content of the stratigraphically diagnostic trace element Zr of incorporated pumice clasts

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as well as the abundance and lithology of non-buoyant components. The fluvially reworked layer characterized by very high Zr concentrations (stratigraphically corresponding to LLST) was washed onto MLST-A by Flooding Event FE3. The layer with the very low Zr concentrations (corresponding to MLST-C) contrasting very strongly with the in-situ units above and below represents a non-buoyant lag. It was left behind by Flooding Event FE4 that occurred after MLST-B/C had been deposited. The layer is the result of undercutting of the originally c. 2.5 m thick MLST-B/-C section (and the underlying pumice layer left

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behind by Flooding Event FE3) during the early stage of FE4. The stratigraphic identification of the grounded raft as pseudoprimary MLST-B (based on color and vesicularity of pumice) is confirmed by its chemical composition.

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Fig. 8 Criteria that distinguish syn- from post-eruptive flood deposits

a. Syn-eruptive flood deposits: upper flow regime structures, poorly defined bedding planes due to sedimentation

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simultaneous with erosion, exclusively LST components, standard Rhine components lacking, rich in lithoclasts (Devonian slate and sandstone, less abundant but diagnostic tephritic and basanitic lava and scoria), minor non-buoyant (i.dr.st.) pumice, buoyant (i.dr.st.) pumice largely washed away, pumice clasts surrounded by original ash covers (not abraded) indicating rapid syn-eruptive reworking and short-distance transport

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b. Post-eruptive flood deposits: structures reflecting lower flow velocities, sharp bedding planes, admixture of minor percentage of standard Rhine sand and gravel, high percentage of buoyant pumice, pumice clasts abraded (ash covers removed = white/tan appearance), in part subrounded to rounded abp = abraded, subrounded buoyant pumice (off-white), Ds = Devonian slate (dark gray), l+Rs = lithoclasts (dark gray, black,

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brown; Devonian and basaltic/tephritic rock fragments originating from LST) with minor standard Rhine sand (sand-colored), n-bp = non-buoyant pumice (light to dark gray), ts = tephritic scoria (reddish brown)

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Fig. 9

Example of the complex interdigitating successions of primary and syn-eruptive fluvially reworked LST filling the pre-eruptive Rhine channels in LNB. The section illustrates the difficulty in distinguishing syn- and post-eruptive fluvial deposits because both types of deposits consist of coarse reworked LST see Fig. 8. Fig. 10 Schematic overview of the characteristic erosional and depositional effects of the five syn-eruptive flooding events and the complex intercalation of primary fallout and flood deposits within the southern side channel of the Rhine River. This evolution is representative for the processes within all channels that were active prior to LSE. Stages are omitted during which fallout

26 fell on drained ground even at the channel base after each flooding event due to renewed damming upstream (but shown in Fig. 7A in Park and Schmincke 2019b). The five flooding events took place during major breaks in eruptive activity. The morphology of the southern side channel represents that of an originally major Rhine channel that had almost been abandoned at the time of LSE due to progressive downcutting by the Rhine during the Allerød. Trees (not to scale) and paleosol formation on the lower channel margins indicate dry ground for an extended time prior to LSE. Maximum flood levels were measured within the southern active side channel in central LNB (see Fig. 3). The maximum flood levels were measured versus the mean bed level. The values are not equivalent to maximum flow depth because of the tephra fill in the channels. For further definition of maximum flood level see Ch. 5.5. View perpendicular to flow direction

Fig. 11 Example of efficient reaming of LST filling a channel by undercutting and flotation illustrated by LLST undercut by FE2 in an abandoned channel. Tephra fill was undercut along the entire length of the channel (up to 10 km) and spanning its complete width (400 m). Secondary channels were carved by en mass erosion of primary tephra bodies. Tephra bodies with the size of a house were overthrust one above the other parallel to the channel in flow direction and perpendicular to flow direction

Fig. 12

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towards the channel margins. Undercut tephra bodies toppled into the newly carved secondary channels.

Overview of large-scale, coarse-grained upper flow regime structures deposited by Flooding Event FE4 (a) at the base of the northern active side channel and (b) on the bordering flood plain. Details of these overview sections are shown in Figs. 13Aa,

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13Ac, 13B, 14Bb. For explanations see these figures and Ch. 6.2, 7.1 and 7.2.

Fig. 13A

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Effects of flooding event FE4 (post-MLST-B/C) in three different morphological settings (from the flood plain to the channel base) at the cut bank of two different side channels active prior to LSE - the broad southern (Loc. 636) and the narrow northern side channel (Locs. 702, 703). Exposures are roughly parallel to flow direction. The thickness of primary LST (most

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prominently that of MLST-A) varies depending on the location of exposures with respect to the orientation of the fallout fans. a. Flood plain adjacent to channel (Loc. 702)

The area had been dry land prior to LSE as shown by branch molds. MLST-B/C tephra (up to 1.8 m thick) was undercut by FE4 along the base of MLST-B above impermeable MLST-A layer (overlain by FE3 mud drapes) over wide areas bordering

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the channel. Large primary tephra sections were removed by flotation except for relics anchored by trees and bushes buried within MLST. These tephra relics were strongly hydrodynamically eroded at the top. Undercutting caused additional erosion at the base. Antidune accretion (consisting of non-buoyant lag) at stoss-side of relic indicates upper flow regime conditions. Non-buoyant lag: dark gray = lithoclasts and dense MLST-C pumice, light-gray = vesicular MLST-C pumice >1g/cm3.

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Subsequent deposition of chute and pool structures (larger view in Fig. 13B). Top of exposure removed by quarrying. b. Upper inner channel margin (Loc. 636)

Tree molds indicate dry land prior to LSE. MLST-B/C (2.2 m thick) originally deposited on drained ground after transgression

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of FE3 was completely removed by FE4. LLST was protected from erosion by impermeable MLST-A + FE3 mud drapes. Trees were snapped off above MLST-A or damaged. Chute and pool structures consisting of non-buoyant lag indicate upper flow regime conditions. Buoyant pumice clasts were largely washed away. Poor separation of lithoclasts and pumice indicates rapid simultaneous erosion and sedimentation, most probably from a highly concentrated traction carpet. Three individual bedsets overlain by mud drapes that can be correlated across the entire LNB indicate transgression of three individual FE4 flood waves. c. Channel base (Loc. 703) Coarse gravel at the base (buried below thin FE1, FE2 and FE3 flood deposits not quarried = brownish area in foreground) indicates that the channel had been active prior to LSE. LLST (c. 1.6 m thick) and MLST-A (4 cm thick) were eroded by FE1,

27 FE 2 and FE3. MLSTB/C (c. 1.8 m thick) was deposited on drained ground (FE3 mud drape) even at the base of this active channel after transgression of FE3 due to renewed upstream damming. It was removed by undercutting and flotation by FE4 after MLST-C. The tan hydrodynamically sheared bodies represented grounded tephra rafts that enhanced the formation of stationary water-surface waves. In-phase wave draping and antidune accretion at the stoss-side of grounded tephra rafts (resulting from up-stream breaking of the waves) indicate upper flow regime conditions. The tan, coarse-grained, longstretched lensoid and pumice-rich layers most plausibly resulted from excessive shearing of these rafts.

Fig. 13B Reconstruction of the effects of the three successive flood waves of FE4 (only major stages shown) based on the working hypothesis that each flood wave may have consisted of body, head and tail; detail of figure 13Aa (Loc. 702). 1. The powerful head of first FE4 flood wave (FE4a) undercuts MLST-B/C along the base of MLST-B above impermeable MLST-A (overlain by FE3 mud drapes) and erodes large bodies of MLST-B/C by flotation. 2. Relic of MLST-B/C remains anchored by a tree (buried within the tephra) from which only branch molds are visible in this cross section. Hydrodynamic erosion of the tephra relic by the body of the first flood wave (FE4a). A current separation eddy

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behind the tephra relic potentially caused the coil-shaped structure consisting of unconsolidated primary MLST-B1 tephra seemingly hanging in “midair”. Its preservation implies simultaneous erosion of primary MLST-B/C and resedimentation of non-buoyant tephra components (lithoclasts+smaller volumes of non-buoyant pumice, see Fig. 8a) from a highly condensed suspension at the base of the flood wave.

3. The head of the second flood wave of FE4 (FE4b) - forming an upstream breaking wave - accreted an antidune consisting

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of a non-buoyant lag at the stoss-side of the primary tephra relic. The body of flood wave 2 subsequently deposited chute and pool structures consisting of non-buoyant lag (not shown, but visible in Figs. 12b, 13Aa)

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Fig. 14A

Examples of en masse mobilization of thick and large tephra sections by undercutting and flotation by FE3 and FE4 a. Abandoned northern marginal channel (Loc. 743)

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Dry ground prior to LSE as indicated by soil and branch molds. Complete LLST+MLST-A tephra fill was undercut by FE3 along loosely packed FE1 deposits above impermeable paleosol and then became floated and vertically lifted over large areas within and along entire broad channel prior to deposition of MLST-B/C. Huge segments of floating LLST with the size of a house were shifted and thrust one above the other. This indicates that extreme shoving power was generated by FE3

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between paleosol and cover of impermeable MLST-A. Overthrusting was achieved by washout and loss of mainly buoyant tephra components along the thrust plane escaping through tension cracks in the MLST-A cover. Non-buoyant lithoclasts were partly left behind. The dimension of shortening is indicated by folding of MLST-A. MLST-B/C was deposited on completely drained ground following transgression of FE3.

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b. Abandoned northern marginal channel (Loc. 742)

Dry ground pre-LSE as indicated by molds of bushes and trees (outside of photo). Phase 1: Undercutting and flotation of large sections of MLST-B/C (1.8 m thick) along the base above impermeable MLST-A + FE3 mud drapes (that prevented

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LLST from being eroded) by the first flood wave of FE4. Separation of non-buoyant lag (base) and pumice (layer of pure pumice underlying MLST-B1) derived by erosion at the base of MLST-B1. Phase 2: MLST-B/C was removed en masse as large tephra bodies via flotation except for relics anchored by trees/bushes buried within MLST. Transport of floated MLSTB/C relic to the left and overthrusting at stoss-side of sheared pseudo-primary tephra relic was caused by the second flood wave of FE4. Upward flexure of unconsolidated primary MLST-B relic seemingly hanging in “midair” implies simultaneous erosion, transport and resedimentation of non-buoyant tephra components. Buoyant components were removed. Top of exposure removed by quarrying. c. Floodplain bordering southern side channel (Loc. 631)

28 Dry ground pre-LSE as indicated by molds of bushes and trees. Same scenario as in photo b. Floating relic of MLST anchored by tree buried within LST behind sectional plane (as indicated by branch molds) was hydrodynamically rounded by erosion by FE4. MLST-C and top of MLST-B were removed. MLST-B1 was eroded at base by undercutting. Condition of flotation was “frozen-in” by simultaneous deposition of non-buoyant lag. Top of exposure removed by quarrying Fig. 14B Different depositional modes of floated tephra bodies by FE4 a. Mouth of more deeply incised channel of small tributary Gladbach (active prior to LSE but buried below LST already in its headwaters during syn-eruptive flooding) into broad older northern abandoned marginal Rhine channel running at right angle to tributary (Loc. 715). Main flow direction during syn-eruptive flooding caused by breaches of Koblenz Dam from right to left. Flow direction during drainage of Lake Brohl towards observer. MLST-B/C was deposited on drained ground on top FE2 and FE3 mud drapes overlying fluvially reworked LLST indicating renewed upstream damming following transgression of FE3. MLST-B/C was undercut by FE4 along base of MLST-B above FE3 mud drape. MLST-B/C was removed en masse from abandoned channel and center of tributary channel. Undercutting of remaining tephra fill along margins of abandoned channel

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caused large bodies of primary MLST-B/C to topple and collapse into the channels newly scoured into the LST fill (see overview sketch at base). This indicates that fresh tephra deposits were sufficiently coherent to be moved and transported without disaggregating.

b. Channel base of northern active side channel (Loc. 703): Close-up of figures 12a and 13A. Photo shows different degrees of hydrodynamic shearing of grounded rafts consisting of MLST-B/C ranging from deformed hummocks with preserved

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Core region of Koblenz Pumice Dam No.4 prior to FE4, max. height ≤10 m, up to c. 70 m asl

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Lake Koblenz No.4 dammed at Koblenz Dam No.4 prior to FE4, max. depth ≤ 10 m, max. level c. 70 m asl,

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Damming and flooding effects

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Terrain 100 - 450 m asl

Active side channel pre-LSE

Towns

Abandoned channel pre-LSE

Present rivers

Neuwied Tectonic Basin Fault active during the Quaternary

Crest of Brohl ignimbrite dam at >85 m asl

Elevation of top of Devonian basement < 100 m asl

Early stage of Lake Brohl dammed at Brohl Dam prior to FE4, max. level at <59 m asl (suspected)

Elevation of top of Devonian basement 100 - 150 m asl

Small lakes dammed by ignimbrites in side valleys (susp.)

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Adobe Illustrator CS6

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This account, Park and Schmincke 2019b

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Breaks deduced from drainage of Lake Koblenz and flooding events in the LNB

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Breaks deduced from rise and drainage of Lake Brohl

2 d Duration of eruptive phases

and breaks roughly calculated based on rise of Lake Brohl Breaks deduced from the primary tephra record

1-2d Duration (h=hours, d=days)

of eruptive phases/breaks estimated based on classical break indicators and analogy with recent eruptions

Fine ash and accretionary lapilli Laminated fine and coarse ash

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b Correlation of lake damming and flooding events with stratigraphy

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Loc 758 Koblenz Dam

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Revised event stratigraphy of LST in Lower Neuwied Basin

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Multiple formation of pumice dam at Koblenz due to overloading of Rhine River with fallout

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Koblenz Dam

ULST

Lake Brohl

oo

Brohl Dam

ine n Rhanyo C

W

Lower Neuwied Basin Rhine Canyon

od Floave w

Rhine Canyon

Adobe Illustrator CS6

c

d

Duration of Documentation eruptive events and breaks

Break in eruptive activity (8-10 d)

Syn-Flooding Event FE4

b

Gladbach Heimbach Weis

Heddesdorf

Bendorf Neuwied

Engers Kaltenengers Urmitz

Sankt Sebastian

ro of

Rhine

Weissenthurm

Kettig

0.5

1

1.5

Kärlich 2 km

Mühlheim

Pre-eruptive

-p

0

Bubenheim

re

Gladbach

a

Heimbach

lP

Heddesdorf

ur

na

Neuwied

Weissenthurm

Weis

Rhine

Bendorf

Engers Kaltenengers Urmitz

Sankt Sebastian

Jo

Kettig

0

0.5

Figure 3A

1

1.5

Kärlich 2 km

Mühlheim

2 column fitting image

Bubenheim

Adobe Illustrator CS6

b

Terrain flooded during Flooding Event FE 4

(post-MLST-C tephra fall)

Active and abandoned channels flooded by FE 4

Relic flood plain areas that remained dry

Major pre-eruptive Rhine channel (not retracable due to post-eruptive scouring) symbolized by present single-thread Rhine channel

High flood plain c. 66,25-65 m asl overlain by up to 4.1 m primary LLST + MLST-A/B/C tephra Highest and oldest flood plain areas c. 67,5-66,25 m asl overlain by up to 4.1 m primary LST

Active and abandoned channels of different age, hierarchy and vertical spacing

Highest and oldest flood plain areas c. 68,75 -67,5 m asl overlian by up to 4.1 m primary LST

Flood plain areas on NT2 flooded by FE 4

Brohl

Channel margins and variable lower terrain 63,75 - 62,5 m asl overlain by up to 4.1 m primary LLST + MLST-A/B/C tephra

Laacher See Volcano

Low flood plain c. 65 - 63,75 m asl overlain by up to 4.1 m primary LST

Brohl Dam Neuwied

Koblenz Dam

Andernach

Lake Brohl Koblenz

Lake Koblenz

ro of

4 km

ine n Rhanyo C

Reference area where flood levels of syn-eruptive food waves were documented

Morphology in central Lower Neuwied Basin pre-LSE

Pre-eruptive flood plain on NT2

Kettig Towns in LNB, outline not shown Exposures of syn-eruptive flood wave deposits originating from breaches of Koblenz Dam No pre-eruptive topo data available due to building sites and other infrastructure

Channel margins and variable lower terrain 63,75 - 62,5 m asl

Present single-thread Rhine channel

High flood plain c. 66,25-65 m asl

-p

Low flood plain c. 65 - 63,75 m asl

Highest and oldest flood plain areas c. 67,5-66,25 m asl

Active Rhine channels pre-LSE

Highest and oldest flood plain areas c. 68,75 -67,5 m asl

Area where major pre-eruptive Rhine channel was located; the pre-eruptive morphology cannot be reconstructed because the NT2 terrace was scoured during the Allerød post-LSE and refilled by the NT3 terrace during the Younger Dryas

Chute channels in point bars Abandoned Rhine channels pre-LSE

ur

na

Channels of different hierarchy and vertical spacing, abandoned pre-LSE for some time (paleosol, bushes, trees) Narrow relic gully along southern boundary of filled up northern marginal channel; oldest and highest situated abandoned channel; buried below pre- and especially post-eruptive slope wash and alluvial fan deposits

Figure 3B

Pre-eruptive course of small tributary rivers suspected Eastern boundary of Gladbach valley scoured into northern abandoned channel

lP

Active tributary channel between side channels and main channel, in part likely associated with small tributary rivers from the shoulders of the LNB

Land surface on NT2 considerably modified by flooding and scouring post-LSE

re

Active side-channels pre- LSE; southern side- channel represents originally major Rhine channel that was almost abandoned and shallow at the time of LSE

Jo

a

Lower Neuwied Basin

ine n Rhanyo C

Lowest flood areas bordering the major pre-eruptive Rhine channel

NT2

2 column fitting image

Alluvial fans of tributary rivers and slope wash deposits accumulated pre-LSE; significantly enlarged post-LSE by reworked LST due to climate impact of LSE and during Younger Dryas especially at northern rim of LNB; post-eruptive slope wash deposits locally >10 m thick Boundary of older lower terrace NT2 defining the outline of the floor of LNB at the time of LSE reduced in size by pre-eruptive alluvial fans and slope wash deposits Older lower gravel terrace 2 (Niederterrasse 2, NT2), Weichselian, pre-Bølling interstadial, forming the basement of Lower Neuwied Basin (LNB) prior to Laacher See Eruption (LSE)

Adobe Illustrator CS6

c

Gladbach

S Present

Present single-thread N Rhine channel

m asl 90

Urmitz

Rhine 80

Present floods

70 60 m

60

NT3

MHWL Mean high water level MWL

b

Mühlheim

HHWL Highest high water level since 1880 in 1926

HT

NT2 + OHF

50

2 km

Mean water level

Syn-Flooding Event FE4 (post-MLST-C tephra fall) N

S Widespread erosion and reworking of primary LST (up to 4.7 m) and locally FE1-3 flood deposits by FE4 within all types of channels and deposition of mainly gravel-sized upper flow regime deposits

m asl

ro of

90

Flooding of all active and abandoned channels on the NT2 irrespective of their hierarchy and vertical spacing

80

6.5 km

70

FE4

>11 m

60

60 m

Pre-eruptive MW

50

a

-p

x 33

Pre-eruptive 12,900 BP

Lower Neuwied Tectonic Basin

S Active southern side channel

90

Major active channel

Abandoned channels

re

m asl

lP

80

Active northern side channel

70 60

60 m

NT2 + OHF

NT2

NT3

na

NT1

Rhine

uMT

ur

Pre-eruptive

HT

Jo

Terrace flight at base of LNB

Pre-eruptive mean water level (assumed)

a

Pre-eruptive multi-channel Rhine River Paleosol, trees and shrubs indicating dry ground for extended time pre-LSE NT2 + OHF Devonian/Tertiary basement/other pre-NT2 deposits, not spec.

Figure 4

Pre-eruptive MW

HT

Holocene terrace

NT3 Younger lower gravel terrace (Weichselian, Younger Dryas)

0

500

1000

1500

N

Fault

Reconstructed area

Posteruptive

50

Relic channel at margin of filled-up abandoned Abandoned northern channels marginal channel

2000

2500 m

OHF Older high flood deposits overlying NT2

NT2 Older lower gravel terrace (Weichselian, pre-Bølling) NT1

Olderst lower gravel terrace (Weichselian)

uMT Lower Middle gravel terrace

b Max. flood level and areal extent of FE4 c. 9 km downstream of upstream end of Koblenz Dam Water volume and area flooded by FE4 LST reworked by floods FE1 to FE4, mainly gravel-sized, upper flow regime deposits Total primary LST (LLST+MLST-A/B/C) deposited prior to FE4, up to 4.6 m thick

2 column fitting image

Present flood levels Water volume and area flooded by present floods Present single-thread Rhine River

c

Complete package of primary LST and LST reworked during syn-eruptive flooding, climate impact of LSE and Younger Dryas Holocene flood plain deposits NT3

Adobe Illustrator CS6

LST reworked by post-eruptive lahars due to climate impact of LSE (Allerød) and during Younger Dryas Reworked ULST-C tephra supplied by lahars into inactive channels; locally overlain by primary tuff layers of final ULST-C eruptive pulses deposited on drained ground

MLST Middle-LST LLST Lower-LST -f LST reworked by flood waves due to breach of Koblenz dams -ij Reworked autochthonous and allochtonous LST injected by flood waves between previously deposited tephra layers along an impermeable bedding plane -u Autochthonous LST reworked by flood waves via undercutting of tephra sections above impermeable bedding planes -l LST reworked by lahars

Lower units of ULST-B deposited into draining Lake Brohl, upper units into standing water Lake Brohl deposits (small water-soaked pumice lapilli) reworked or eroded during lake drainage

-ps Pseudo-primary LST = floated and/or fluvially transported LST relic still showing some primary characteristics such as lithoclasts mixed with pumice clasts or faint stratification -gLB LST deposited into growing Lake Brohl -d LST reworked by drainage of Lake Brohl -dLB LST deposited into draining Lake Brohl

ro of

LST reworked by Flooding Events FE 1-5 due to breach of Koblenz dams and prior to major rise of Lake Brohl Primary LST deposited on dry/drained ground

Meadow loam (post-Weichselian) + paleosol (Allerød) Sands (Allerød) Gravel (Weichselian)

-p

Pre eruptive

FE1-5 Flooding Events 1-5 LST Laacher See Tephra ULST Upper-LST

This Account

Post eruptive

S yn - er u p t ive

Artificial infill (present)

Figure 5

Jo

ur

na

lP

re

2 column fitting image

Adobe Illustrator CS6

Stepwise rotation of

fallout fan axes of successive subphases MLST-A

MLST-A C

„Big Bang“ Events

D

„Big 3 Bang“ Events

2

C

2

Rotation of fallout fan axes of successive LLST-subphases

1 1 „Big 2 Bang“ Events 1

LLST

LLST

B

LSV

610

B

742

A

642

a

Loc. 642

-p

Loc. 610

Standing water

re

Lake Brohl

A

b

Drainage of Lake Brohl

lP

FE 5

MLS

na

Abandoned channel north of present Rhine River

Jo

Loc. 742

Figure 6

T-A

ur

c

LLS

2 column fitting image

ULST-B-d ULST-A-f + MLST-D-f

-B/C-f

T-B

FE 3 FE 1

A

LLS

FE 4

ULST-C ULST-C-l

Non-buoyant LST components

T-B 1 MLS T-

Rafted primary LLST relic

LST reworked by flooding events Primary LST

MLST

10 cm

A

10 cm

ro of

4 km

1m

MLST-A-f LLST-A1-f

Color code and abbreviations see Figure 5

Adobe Illustrator CS6

B C

D E

f g/cm3

Vol-% Black Dark gray

b

Sample positions for whole rock composition

20

40

c

N.D. No data

.3

.4

.5

.6

Gladbach

.7

610 In-situ primary relic of MLST-B1

1997

Pseudo-primary MLST-B2 tephra raft

508 501

75%

Buoyant

MLST -B/C-f

861 835 Tan

1810 Off-white

LLST

Non-buoyant, fine lapilli to ash-sized lag of LST lithoclasts + medium gray-green pumice + juvenile crystals

FE 4

Non-buoyant, lapilli-sized lag of LST lithoclasts + light gray-green pumice + minor buoyant pumice

FE 3 FE 4

Mud drape deposited during vaning stage of floods Pseudo-primary MLST-B2 = mixture of pumice and lithoclasts Deposit consisting entirely of buoyant LLST pumice lacking xenoliths

1m Loc. 637

d - 4 phi Loc. 610 - 5 phi to - 2 phi Lower Neuwied Basin Mean of Locs. 610, 617, 678, 689 Bulk Samples

0 20 40 60 80 100

e Lithoclasts + non-buoyant pumice Lithoclasts Lower Neuwied Basin Mean of Locs. 610, 617, 678, 689 Bulk Samples

0

800

1600

Primary LST

2400

g

f Fluvially reworked LST Primary LST

Mühlheim

FE 5

MLST-B1 LLST-ij

678 689

617 2 km

ULST-A-f + MLST-D-f

84%

Urmitz Rhine

Flow direction

MLST-A

.8

758

Identification of primary and fluvially reworked LST in complex intercalations by a combination of diagnostic criteria

82%

0.39 0.47

Proximal Locs. 1, 9, 15

Figure 7

e-

0.42 0.48

0

a

Pumice raft at Loc. 758

41%

0.46 0.47

Jo ur

Loc. 758

N.D.

13%

na l

Off-white

1m

?

LLST

MLST-A

0.69 0.74 0.61 0.59

Tan

-B1

N.D.

Pr

-B2

Zr (ppm)

100%

Tan

MLST

-C

Vol-%

N.D.

Tan Light graygreen

-D

Whole rock composition

N.D.

Dark gray

Medium gray-green

ULST-C ULST-B Pumice raft ULST-A

Buoyant and non-buoyant components

MLST-B/C-u

Density of buoyant pumice

oo

Total phenocryst abundance in juvenile clasts

pr

Color of pumice

Non-buoyant

Event stratigraphy

Outer point-bar of southern side channel

FE 4 Flooding Event LST reworked by flood waves -f -u

Loc. 758

flood wave deposit reworked by undercuttig Primary LST

2 column fitting image

Adobe Illustrator CS6

abp

ts

10 cm

10 cm

n-bp

l+Rs

Ds

a

b Syn-eruptive

Loc. 660

Post-eruptive

ro of

Loc. 703

Figure 8

Jo

ur

na

lP

re

-p

2 column fitting image

Adobe Illustrator CS6

Flooding events Posteruptionflooding

ULST-A-f + MLST-D-f

MLST -B/C-f MLST-B Rafted primary MLST-B relic

MLST-A

Drainage of Lake Brohl Lake Brohl

FE 5 FE 4 FE 3 FE 2

ro of

LLST LLST-A1-f

Rafted primary LLST relic

FE 1

Sand

Gravel

1m

-p

Pre-eruptive base of channel

Gladbach

MLST-B

ML

ST -B

FE3

FE5 LLST LLST

FE3

LL

FE1

Urmitz

Rhine 650

2 km

Mühlheim

Cross section through tributary between active southern side channel and main active channel

na

Loc. 651

FE4

ST

lP

LLST

re

FE2

Post-eruptive

Jo

ur

Figure 9

2 column fitting image

Adobe Illustrator CS6

This account

LLST

ULST-C-l ULST-B-d

Post-MLST-D

m 4

MLST-D

12 m

2

- up to 1.05 m thick, > 90 vol-% heavy, non-buoyant components

-2

- erosive power of FE 5 reduced due to backwater

-4 -6

e2

-8

4

Overthrusting of huge tephra blocks

Post-MLST-C MLST-B/C

11 m

2

Upper flow regime structures Non-buoyant lag

0 -2

Mobilization of huge tephra sections by undercutting and flotation

-4

Grounded, hydrodynamically sheared pseudo-primary rafts of MLST-B/C tephra

-6 -8 6 4

d2

Post-MLST-A

Sealing, impermeable MLST-A

MLST-A

0

-p

-4 Overturned, rolled-up piece of cohesive MLST-A several m 2 in size

-6

Post-LLST Dunes of exclusively fine gravel-sized, water-soaked pumice clasts (lithoclasts lacking) deposited during the waning stage of FE2 subsequently removed by FE3

-4 -6

Coarse, non-buoyant lag of lithoclasts and pre-eruptive cobbles

7.5 m

Scouring of pre-eruptive channel base

6 4

Southern active side channel

2

-2

Jo 200

400

LLST-A1

MW

800

MLST-B/-C MLST-A

- thick mud drape at base of channel indicates renewed damming upstream - LLST-A1 fell on dry ground prior to FE1 except at the channel base - up to 35 cm thick

FE1

- removal without trace is indicative of bulk erosion in a dry state - flood deposits lack unequivocal fallout components indicating that FE1 took place during an eruptive break - thick mud drape overlain by a stranded pumice raft at the channel base indicates renewed damming upstream

Flooding Event 1 to 5 Maximum flood level of FE1-5 measured in southern side channel c. 12 km downstream of the upstream end of the Koblenz dams Flooded area Mud drape deposited during waning stage of Flooding Events indicating renewed damming upstream

LLST

Figure 10

- removal without trace is indicative of bulk erosion in a dry state

Backwater of growing Lake Brohl in channels of LNB FE1-5

Primary LST deposited on dry/drained ground

Soil horizon overgrown with trees and shrubs indicating dry land for extended time pre-LSE

- up to 1.35 m thick

1000 m

MLST-D deposited into backwater of Lake Brohl

MLST-D

b2

a2 600

FE2

- flood deposits lack unequivocal fallout components indicating that FE2 took place during an eruptive break

Flow direction

Point bar

4m

-4

-8

Post-LLST-A1

ur

0

-6

Plane-parallel bedded, exclusively fine gravel-sized, water-soaked pumice clasts (lithoclasts lacking) + mud drape deposited during waning stage of FE2

na

-8

- LLST-B to-F fell on drained ground prior to FE2 even at the channel base

LLST

lP

0 -2

c2

re

-8

2

FE3

- MLST-A fell on drained ground prior to FE3 even at base of channel

- huge overturned, folded or rolled-up pieces of cohesive MLST-A were deposited at channel base

8.5 m

-2

4

- MLST-B/C fell on drained ground prior FE4 to FE4 even at base of channel - up to 2.6 m thick - MLST-B/C was undercut above impermeable MLST-A, large tephra bodies were transported downstream as floating rafts, grounded at channel base and hydrodynamically sheared - rafts imbedded within non-buoyant lag consist of buoyant pumice and minor lithoclasts - in-phase wave draping and antidunes indicate upper flow regime conditions

- up to 40 cm thick, cohesive ash layers alternating with lapilli layers

2

6

- plane-parallel bedding indicates upper flow regime conditions

ro of

6

FE5

- MLST-D fell into backwater of growing Lake Brohl in lower part of channels

Non-buoyant lag

0

Major characteristics of FE

LST reworked by flood waves MW

Pre-eruptive mean water level (assumed)

2 column fitting image

Adobe Illustrator CS6

m

Adandoned channel

4 2

Post-LLST Newly reamed secondary channels

Flotation, fragmentation and overthrusting of LLST cover

Flow direction LLST

4.5 m

0 -2

100

200

300

400

500 m

- up to 1.35 m thick - flotation of LLST cover within entire channel by undercutting along paleosol + overlying FE1 mud drape

- erosion of secondary channels within the tephra cover filling the channel

ro of

b2

Relics of LLST anchored by trees/bushes

2 column fitting image

Jo

ur

na

lP

re

-p

Figure 11

FE2

- fragmentation of LLST cover into large rafts and overthrusting of rafts on top of each other

MW

-4

- LLST-B to-F fell on drained ground prior to FE2 even at the channel base

Adobe Illustrator CS6

f Loc. 703 Base of northern active side channel

FE 2/3-u *

b

Figure 12

Relic of MLST-B anchored by bush undercut by FE4

Non-buoyant lag

MLST-B2 -C -D

MLST-A-f *

-B1

Length of tephra section c. 19 m

Wave length up to 8 m

* Not exposed/below base of outcrop

Upstream dipping laminae/ Relic of MLST-B antidune accretion anchored by tree buried within tephra undercut by FE4 Fig. 13Aa, 13B

Thickness of primary LST

Relic of MLST-B1 undercut by FE 4

Flow direction

-B1

FE 4-u FE 3

FE 4

Jo ur

FE 5

Upstream dipping laminae/ antidune accretion

na l

Relic of MLST-B

Gravel

MLSTB/C-f MLST-B1

-A

1m

Loc. 702 Flood plain bordering the northern active side channel

LLST

Paleosol Branch mold

2 column fitting image

Length of tephra section c. 24 m Top of section removed by quarrying

Adobe Illustrator CS6

LLST-u *

* Not exposed/below base of outcrop

MLSTB/C-f

LLST-f *

FE 3* FE 1/2*

-A

Pr

1m

ULST-A-f + MLST-D-f

LLST

FE 4

ULST-C-l ULST-B-dLB

MLST-A-f + MLST-B/C-u

FE 5

Flow direction

LLST

Lake Brohl

Drainage of Lake Brohl

Fig. 14Bb

Thickness of primary LST

MLST-B2 -C -D

Standing water

a

Upstream dipping laminae/ antidune accretion

e-

Drainage of Lake Brohl

Mud drape

pr

Hydrodynamically sheared pseudo-primary tephra rafts grounded at stoss-side of underlying obstacles Fig. 13Ac

oo

Upper flow regime structures deposited by FE 4

LST reworked by flooding events Primary LST Hydrodynamically truncated Antidune relic of primary MLST-B stoss-side anchored by tree accretion

Top removed by quarrying

MLST-B/C-f

FE 4 Undercutting by FE 4a

MLST-A

2 km

Current separation eddy

1m

FE 5

Chute and pool structures in non-buoyant lag

lP

FE 2/3-u

re

Trees snapped off by FE 4

Upper inner channel margin (Loc. 636)

na

Lake Drainage

ur Lake Brohl

Eroded by FE 4

Lithic lags capped by mud drapes*

Eroded by FE 1,2 + 3

LLST

MLST-D-f

6m

703

300 m

Position of exposures in Fig. 13A at cut banks of active side channels

FE 1-3

c

Impermeable boundary layer

MLST-A

LLST

Paleosol*

LLST-u

Flow direction Terminal ULST-C Post-erupt. reactivation of channel

Grounded and hydrodynamically sheared pseudo-primary tephra rafts

Slack water deposit of reworked ULST-C

Non-buoyant lag Dark-gray

Lithoclasts and non-vesicular pumice

Light-gray

Vesicular pumice >1g/cm3

ULST-B-dLB ULST-A-f + MLST-D-f

FE 5

FE 4

Branch mold

1m

Channel inactive after drainage of Lake Brohl

Jo

c. 1.8 m

b

MLST-B1 MLST-B2/C

636

MLST-B/C-f

FW1

Dry ground prior to LSE

c. 1.6 m

LLST-u

-p

FE 4

Mühlheim

ro of

Flow direction

FW3

FE 3 mud drapes

Eroded by FE 4

MLST-B2/C MLST-B1

c. 2.2 m

Flood plain adjacent to channel (Loc. 702)

Mud drapes of 3 successive FE 4 flood waves

FW2

636

LLST

702

Dry ground prior to LSE

a

Urmitz

Rhine

Paleosol*

Figure 13A

Gladbach 703 702

FE 5

FE 2/3-u

FE 3 mud drapes

Chute and pool structures in non-buoyant lag

Eroded by FE 4

MLST-B1 MLST-B2/C

c. 1.8 m

Primary tephra thickness Flooding eroded by FE 4 (to scale Events with photos to the right)

MLST-B/C-f

In-phase wave draping in non-buoyant lag

1m MLST-A-f* LLST-A1-f*

Gravel*

Running water prior to LSE

Channel base (Loc. 703)

Flow direction * not exposed/ below base

2 column fitting image

Adobe Illustrator CS6

Erosional structure in completely unconsolidated MLST-B tephra supported by extremely particle-laden water and resedimentation of non-buoyant components almost synchronous to erosion

FE 5

Branch molds

MLST-B2

Non-buoyant lag

FE 4c

Primary MLST-B relic anchored by tree

10 cm

MLST-B1

FE 4b

Non-buoyant lag

FE 4a

FE4a undercuts MLST-B1

Impermeable boundary layer

MLST-A

FE 3 mud drapes

LLST

Flood plain adjacent to northern active side channel Loc. 702

ro of

FE 3 mud drapes

Flow direction

Storyboard: Effects of successive flood waves of FE 4 on flood plain adjacent to active side channel Flotation and detachment of large tephra blocks Head of flood wave FE 4a

Body of flood wave FE 4a

Nonbuoyant lag

B1

FE 3 mud drapes

MLST-A LLST

B1

Buoyant pumice Lithoclasts, non-buoyant pumice

Figure 13B

2

Flow direction

lP

Flow direction

Tree buried inside tephra behind quarry wall, only branch molds visible

ur

na

2 column fitting image

Jo

Antidune accretion

Tree buried by fallout

1m

Paleosol

1

Upstream breaking wave

-p

MLST

re

B2

Tephra-laden water

Buoyant pumice removed

C

Head of successive flood wave FE 4b

3

Flow direction Non-buoyant lag

Adobe Illustrator CS6

LST reworked by flooding events Primary LST

Flow direction

a

MLST-B2

Folding of cohesive impermeable ash layers of MLST-A

1m

Drainage through tension cracks

Non-buoyant lag

LLST

Gravel

LLST

3

ro of b

Branch mold

Overthrusting of large sections of LLST by FE 3

Flow direction

Abandoned northern marginal channel (Loc. 742) FE4 MLST-C

Sheared pseudo-primary tephra relic phase 2

Undercutting phase 2 Undercutting phase 1

MLST-B1

MLST-B/C-u

Non-buoyant lag phase 1

MLST-A

FE4

Nonbuoyant lag phase 1 FE3 mud drapes

LLST

MLST-A-f

Flood plain bordering southern side channel (Loc. 631)

MLST-C MLST-B2 MLST-B2

MLST-A

MLST-B1

MLST-B1

MLST-A

LLST

1m

LLST

Tree buried within tephra

Eroded by FE4

Paleosol

Hydrodynamic rounding/erosion of floating, but still anchored (by tree buried within tephra) relic of MLST-B by FE 4

A

Figure 14A

Pure pumice

Grounded and hydrodynamically sheared tephra rafts

Undercutting

Branch mold

FE3 mud drapes

LLST

Phase 2

MLST-B/C-f

MLST-B1

Nonbuoyant lag

MLST-B1

c

MLST-B2

MLST-B1

MLST-B2

Flow direction

ur

FE 4

Jo

FE 3 mud drape

Non-buoyant lag of FE 4

Pure pumice

LLST

Top of section removed by quarrying

c

Nonbuoyant lag

Seemingly self-sorting relic of fresh unconsolidated tephra supported by sedimentation of non-buoyant lag by FE 4 synchronous to erosion

na

b

MLST-B2

Phase 1

MLST-A

lP

Pure pumice phase 1

1m

MLST-A-f

FE 4

-p

MLST-B2

re

FE 3 mud drapes

1.8 m

Top of section removed by quarrying

Non-buoyant lag phase 2

Nonbuoyant lag Paleosol

Top of impermeable boundary layer

Lag of non-buoyant lithoclasts, buoyant pumice removed

a

FE3

MLST-A

FE

Impermeable MLST-A

Non-buoyant lag

1.48 m

MLST-B1

Abandoned northern marginal channel (Loc. 743)

2.45 m

FE 4

LLST-f + MLST-A-f

FE 3 non-buoyant lag + mud drape

Flooding Events

2 column fitting image

Adobe Illustrator CS6

FE3 mud drapes

Flow direction

Standing water Drainage of Lake Brohl

742 715 703

ULST-B-d

Toppled primary tephra blocks

MLS

Lake Brohl

FE 5

2 km Mud drape of FE 4

MLST -B/C-f

a

-p

For primary tephra thickness see Fig. 13Ac

re

Increasing degree of shearing of grounded tephra rafts

1

ULST-A-f + MLST-D-f

FE 5

Non-buoyant lag of FE 4

1m

Channel base

MLST -B/C-f Primary stratification deformed, but visible = pseudo-primary Gravel

na

FE1-3

lP

Lake Brohl Lithic lags capped Drainage of Lake Brohl by mud drapes

Slack water deposits of reworked ULST-C

ULST-B-dLB

MLST-A-f LLST-f

Not-exposed, below base

2

Grounded and extremely hydrodynamically sheared tephra rafts

ur

b

Northern active side channel (Loc. 703)

Non-buoyant lag of FE 4

Flow direction

Sheared pseudo-primary MLST-B rafts

FE 4

Northern abandoned marginal channel (Loc. 715)

b

Intact tephra blocks toppled from margin into channel (due to undercutting by FE4) indicating that fresh tephra deposits were coherent and could be transported without disaggregating

Standing water

Mühlheim

ro of

Mixture of buoyant pumice and lithoclasts (disintegrated MLST-B/C) injected by undercutting by FE4

a

Urmitz

631

40 cm

MLST-C

MLST-B2

2 T-B

FE 4

Rhine

ULST-A-f + MLST-D-f

743

Jo

Fig. 12Ba

FE 4 mud drape

Non-buoyant lag of phase 2 of FE 4

MLST -B/C-f FE 4

Fig. 8Ab1 MLST-B/C

FE 2 + FE 3 mud drapes Large, lithoclast-free dunes of FE 2 consisting entirely of small lapilli-sized LLST pumice

deposited on drained ground after FE 3

3

MLST-B/C-f

LLST-f FE 2

Hydrodynamically sheared raft, ± structureless mixture of buoyant pumice and lithoclasts

Flotation of cover of MLST-B/C (2.1 m thick) by undercutting

Primary tephra blocks topppled by undercutting

1m

Gravel Undercutting during phase 1 of FE 4 between FE 3 mud drape and base of MLST-B and injection of mixture of pumice and xenoliths originating from disintegrated MLST-B/C

Extremely sheared raft, structureless mixture of buoyant pumice and lithoclasts

B

Figure 14B

2 column fitting image

Adobe Illustrator CS6