Boundary conditions for damming of a large river by fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming II

Journal Pre-proof Boundary conditions for damming of a large river by fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming II

Cornelia Park, Hans-Ulrich Schmincke PII:

S0377-0273(19)30051-4

DOI:

https://doi.org/10.1016/j.jvolgeores.2020.106791

Reference:

VOLGEO 106791

To appear in:

Journal of Volcanology and Geothermal Research

Received date:

26 January 2019

Revised date:

15 January 2020

Accepted date:

20 January 2020

Please cite this article as: C. Park and H.-U. Schmincke, Boundary conditions for damming of a large river by fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming II, Journal of Volcanology and Geothermal Research(2020), https://doi.org/10.1016/j.jvolgeores.2020.106791

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© 2020 Published by Elsevier.

Journal 1 Pre-proof JVGR - Research Article

Boundary conditions for damming of a large river by fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming II a,1

Cornelia Park , Hans-Ulrich Schmincke

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Both authors will be corresponding authors a

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

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

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

Abstract

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The Rhine River (Germany) - the largest river in Western Europe - was dammed by pyroclastic material multiple times during the major Plinian Laacher See Eruption (12,900 BP). Dams formed both upstream and downstream of the broad tectonic Lower Neuwied Basin (LNB) which interrupts the narrow Rhine canyon. Here we document

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upstream damming of the Rhine River at the entrance to the LNB close to the present city of Koblenz due to overloading with tephra fall into the Rhine and its major tributaries, the Moselle and the Lahn. The dam was

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formed repeatedly during rapid pumiceous tephra fall events and breached during breaks in eruptive activity, causing extensive, high-energy flooding throughout the entire basin. The ephemeral Koblenz dams differed significantly from “normal” volcanically-induced dams by consisting principally of washed-together pumice clasts and some driftwood. The porous nature of pumice and its ability to absorb water were crucial factors. Thus, a large volume percentage of the tephra that had fallen into the Rhine floated submerged within the upper part of the water column or swam at the surface. Moreover, the absorption of the river water by the pumice clasts increased the sediment:water ratio of the two-phase flow considerably. We here present a model of dam formation resembling the formation of ice jams. We visualize the Koblenz dams to have been elongate, partly floating and partly grounded, permeable plugs many kilometers long and rising no higher than the flood plain. Damming was most plausibly initiated in the LNB within the area of maximum tephra loading and propagated upstream in a chain reaction comparable to the formation of traffic jams. A major dam was finally accumulated at the bottleneck entrance to the LNB, a site combining several favorable conditions: the upstream multi-channel Rhine was confined to a single channel, change of flow direction by 125°, extremely low gradient (0.19 ‰) starting already 24 km upstream of the bottleneck, constant decrease of flow velocity over many kilometers towards the bottleneck and the Moselle River - largest tributary of the Rhine within the LNB and an important conveyor of additional tephra masses – entered the Rhine only 700 m upstream of the bottleneck. We assume

Journal 2 Pre-proof that the Koblenz dams could only have formed and been stabilized by an extremely long “foot region” that extended many kilometers downstream and that was possibly connected to one or several low-rise secondary jams/dams. The backwater of Lake Brohl that was dammed by pyroclastic flows 7 km downstream of the LNB about halfway through the eruption extended further and further upstream into the LNB during the second Plinian stage of the Laacher See Eruption and was probably a major factor contributing to the formation and large size of Koblenz Dam No.4. The Koblenz dams were probably not completely sealed most of the time. This way the major pre-eruptive Rhine channel received some water. An equilibrium condition was established that enabled the dams to remain stable as long as tephra fell into the Rhine relatively continuously.

Key words River damming; Fallout; Pumice; Flooding; Plinian; Hanging dam

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Abbreviations

Lower Neuwied tectonic Basin

LS

Laacher See

LSE

Laacher See Eruption

LST

Laacher See Tephra

LLST

Lower-LST

MLST

Middle-LST

ULST

Upper-LST

LSV

Laacher See Volcano

NT2

Niederterrasse 2 (Older Lower Gravel Terrace)

NT3

Niederterrasse 3 (Younger Lower Gravel Terrace)

RR

Rhine River

SM

Supplementary Material

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1. Introduction

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LNB

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Damming of rivers or of the outlets of lakes by the accumulation of pyroclastic flow deposits (e. g. Manville et al., 1999; Macías et al., 2004; Kataoka et al., 2008), lava flows (e.g. Crow et al., 2008) or by volcanically induced landslides (e.g. Glicken et al., 1989; Capra, 2011) are common and widely discussed. Scenarios of damming by unconsolidated fallout deposits are rare. The Karymskaya river draining the Karymskoye caldera (Kamchatka) was dammed by fines-poor tuff ring deposits (similar to proximal fall deposits of dry hydroclastic eruptions) in 1996 (Belousov and Belousov, 2001). An eruption sequence in 1995/96 expelled the crater lake of Mt Ruapehu (New Zealand) and dammed a new lake by depositing c. 8 m of unconsolidated tephra over its former overflow channel. Failure of this fragile dam caused a large break-out flood/lahar (Hancox et al., 2001; Manville et al., 2007). Damming of large rivers by high concentrations of suspended particles are, however, rare in volcanic scenarios. A pyroclastic eruption in the Jemez volcanic field is assumed to have choked the ancestral Rio Grande (USA) with pumice that was immediately transported downriver by a catastrophic flood wave or that created a dam that was subsequently breached causing a flood of pumice to move downriver (Mack et al., 1996). Lahars were capable of river damming e.g. following the Pinatubo eruption (Pierson et al., 1992; Umbal and Rodolfo, 1996; Scott et al., 1996). Examples in a non-volcanic context are more common, e.g. damming events of the Yellow River by

Journal 3 Pre-proof overcritical concentrations of suspended silt (Bai and Xu, 2008; Yuchuan and Haijue, 2010). With the exception of the Rio Grande case, the bulk of the suspended particles was significantly smaller than the largely gravel-sized pumice clasts suspended in the Rhine River and, moreover, damming was caused by aggradation at the river base. The mechanisms operating during damming of the Rhine by fallout acted in the opposite way - by accumulation of suspended particles within the upper water body because the pumice clasts of LST were mostly submerged below the water surface on account of their density and vesicularity. This mechanism resembles river damming by ice jams (e.g. Beltaos, 1995; Healy and Hicks, 2006; Shen et al., 2008) which results from high concentrations of suspended ice fragments that - similar to the pumice clasts in the Rhine River – also float at or close to the water surface. The interaction of Laacher See Eruption, characterized by repeated and pronounced changes in eruptive and transport mechanisms, with the high-discharge Rhine River and the special topographic setting of the river area resulted in a highly dynamic damming scenario that was so complex and also in part so unique that we will present the entire evolution in several separate accounts. Park and Schmincke (2020a) and this account are

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closely related companion papers and published in short succession. Papers 3 and 4 will be submitted in the near future.

In Park and Schmincke (2020a), we present a detailed treatment of the multi-phase damming and flood chronology and of the effects of the large-magnitude floods in the Lower Neuwied Basin based on a high-

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resolution analysis of the complex succession of primary and fluvially reworked LST deposits in the LNB unequivocal evidence for upstream dam building and the basis of this paper. Here we evaluate the mechanisms that permitted damming of a large river such as the Rhine solely by tephra fallout. We present a reconstruction of

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several key factors that contributed to damming: the multi-channel morphology of the pre-eruptive Rhine River, its paleo-discharge, and the highly discontinuous mass eruption rates during Laacher See Eruption governed by the

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pronounced compositional zonation of the LSE magma body. We also reconstruct and discuss major hydro- and topographic boundary conditions and eruption-induced factors that operated simultaneously to cause damming of

2. Background

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the Rhine River 15 km upstream of the area of major tephra loading.

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2.1. Topographic and tectonic setting

Laacher See Volcano (LSV) is located on the higher western shoulder of the Neuwied Basin (difference in

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elevation between LSV and basin floor c. 210 m; Fig. 2b). The Neuwied Basin is a c. 20 x 30 km roughly triangular tectonic depression within the still rising Rhenish Shield (Figs. 1, 2) that interrupts the deeply incised Middle Rhine Canyon between Koblenz and Andernach. It has subsided c. 350 m since the Eocene, with faulting continuing to the present (Meyer and Stets, 1996). The Rhine River traverses the lowest part of the Neuwied Basin (here called Lower Neuwied Basin, LNB) from its eastern upstream entrance at Koblenz (23 km east of LSV) to its western outlet at Andernach (10 km east of the volcano) over a distance of c. 22 km. The basin floor is up to 7 km wide in a N-S direction perpendicular to the river. For location of exposures see SM-1.

2.2. Evolution of the Laacher See Eruption The Laacher See Eruption took place during the Allerød interstadial at c. 12,900 BP. It was the first - and so far only - Plinian eruption from a site characterized by magma-focusing, judging from the high density of c. 13 older (250,000-100,000 BP) basanitic/tephritic cones and maars surrounding the vent area (Schmincke, 2009, 2014). The VEI 6 eruption was characterized by repeated and pronounced changes in eruptive and transport mechanisms and by major chemical and mineralogical zonation of the magma reservoir (e.g. Bogaard and

Journal 4 Pre-proof Schmincke, 1984; Wörner and Schmincke, 1984a; Schmincke et al., 1990, 1999; Bourdon et al., 1994; Harms 3

and Schmincke, 2000; Ginibre et al., 2004; Schmincke, 2007, 2008; Fig. 5). The LSE produced c. 20 km of 3

fallout tephra and c. 0.7-0.8 km of pyroclastic flow deposits (new unpubl. data) - equivalent to an erupted magma 3

volume of c. 6.3 km . 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 collapses associated with longer eruptive breaks, phreatomagmatic reopening-phases followed by short Plinian intervals with low eruption columns, and the generation of pyroclastic flows (Bogaard and Schmincke, 1984; Freundt and Schmincke, 1986; Schmincke, 2008, 2014; Schmincke et al., 1990, 1999; Figs. 3, 5). The climactic Plinian fallout stages were synchronous with - and responsible for - river damming at the upstream entrance to the LNB at Koblenz (this paper and Park and Schmincke, 2020a). For the nomenclature of stratigraphic subdivision see SM-2.

2.3. Overview of the overall syn-eruptive damming scenario

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The evolution of the impact of the LSE on the Rhine River is summarized in figures 3 and 5. Lakes were dammed at two pivotal locations and at different times during the LSE (Figs. 1, 2, 3). Tephra fall events during climactic eruptive activity led to massive overloading of the Rhine with particulate material and repeated ephemeral damming events (Koblenz lakes) at its entrance into the Lower Neuwied Basin (LNB). Repeated collapse of the

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unstable Koblenz Dam during breaks in eruptive activity caused one minor (FE1) and four high-magnitude regional flooding events (FE2 to FE5; Park and Schmincke, 2020a). Meanwhile, a major stable dam (Brohl Dam) was formed by pyroclastic flows entering the narrow Rhine valley c. 7 km downstream of the LNB at the end of

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eruptive phase MLST-A, about halfway through the climactic eruption, causing a major lake (Lake Brohl) to accumulate. Flooding Events FE3, FE4 and FE5 triggered by the renewed breach of Koblenz Dam contributed to

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a rapid incremental growth of Lake Brohl (Figs. 3, 5). Subsequent re-establishment of the Koblenz Dam (and flooding in the LNB triggered by its failure) was terminated when the backwater of Lake Brohl reached upstream of the mouth of the Moselle River at Koblenz at the end of the ULST-A phase (after FE5). The Brohl ignimbrite 2

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dam remained stable for more than two weeks, impounding a lake that reached a maximum areal extent of 300 3

km (2.6 km volume), and whose backwater reached possibly as far upstream as the Upper Rhine Graben. Breaching of Brohl Dam - synchronous with the resumption of powerful eruptive activity (ULST-B) after a longer

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break during the terminal stage of the LSE - triggered a major flood wave that propagated along the Lower Middle

3. Methods

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Rhine valley at least as far as Cologne, 52 km downstream of the dam.

3.1. Reconstruction of the pre-eruptive hydrographic setting Detailed reconstruction of the pre-eruptive network of active and abandoned Rhine River channels on the floor of the Lower Neuwied Basin (Fig. 4) was an essential requirement to reconstruct the syn-eruptive flooding events and the damming mechanism. The pre-eruptive morphology of the LNB has been strongly overprinted by both natural processes and human activities since the time of the Laacher See Eruption. The pre-eruptive morphology bordering the lower flanks of the LNB was locally buried below >10 m thick alluvial fan and slope wash deposits both immediately after the LSE (caused by its climate impact; Schmincke, 2014 and unpubl. data) and particularly during the Younger Dryas stadial (starting c. 200 years after the LSE; Merkt and Müller, 1999). These processes affected the northern flank of the basin more strongly (Fig. 4a). Moreover, the morphology of the entire LNB became excessively overprinted by closely-spaced built-up areas, infrastructure, massive mining of Laacher See Tephra and of pre-eruptive gravel deposits, and artificial landfill during the 20th Century (Fig. 4b). Existing rough and incomplete reconstructions of

Journal 5 Pre-proof the Quaternary morphology of the LNB by Thoste (1974) and Ikinger and Weidenfeller (2000) were not detailed enough for our high-resolution analysis of the syn-eruptive flooding events. We reconstructed the course and the hierarchy of the pre-eruptive channels based on field evidence assembled in and correlated between more than 60 often extremely small tephra and gravel pits temporarily opened up during the past 25 years by applying the following methods: 1. In most cases, the type of deposits directly underlying primary or fluvially reworked LST and the type of vegetation buried below, or within the tephra were the only indicators allowing us to evaluate the status of a channel prior to the LSE: e.g. meadow loam at channel base = abandoned channel, trees at channel base = dry channel for an extended period of time, clear gravel = active channel, etc. (cf. Miall, 1996; Bridge, 2003). 2. We determined the incision of the channels into the flood plain by high-resolution elevation measurements with a barometric altimeter, resolution: 1 m (the only practicable method in view of the very short-term existence of the pits) and the aid of 1:5.000 topographic maps. 3. Circa 120-year-old topographic maps (Königlich Preußische Landesaufnahme 1:25.000, issued from 1893 to

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1901 AD) were an essential prerequisite for connecting the isolated exposures and to reconstruct the course of the pre-eruptive channels. These maps show the natural morphology of the LNB prior to massive human overprinting.

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3.2. Estimate of the pre-eruptive discharge and flow velocity of the Rhine River There is no information on the discharge and flow velocity of the Rhine River during the Allerød. We estimated the pre-eruptive discharge of the Rhine by considering its discharge from 1900 AD - prior to recent climate warming -

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during the likely season of the LSE and adjusting for major controlling factors that had changed since the Allerød such as the human-manipulated river-architecture, glaciation of the Alpine catchment, precipitation, etc. (for

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details see section 4.2).

3.3. Revised event-based high-resolution correlation of LST

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During the last few years, we have initiated a more process-oriented approach in subdividing the Laacher See Eruption in order to understand the temporal and spatial evolution of this highly discontinuous eruption, even on the scale of subphases. We have studied the LST at c. 150 outcrops around the LSV up to c. 25 km from the

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vent. This large database allowed high-resolution step-by-step computer-based correlations between true-toscale photographs of closely spaced outcrops supplemented by XRF- and lithological analyses (unpubl. data).

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The occurrence of breaks in eruptive activity and their duration was reconstructed based on classical break indicators such as the degree of weather-induced erosion, indicators of vent collapse, vent migration, change in the direction of fallout distribution, etc. and analogy with recent eruptions. A detailed knowledge of phases of massive fallout tephra input into the Rhine River and of the duration of intervening pauses was imperative for the reconstruction of the complex damming and flooding scenario.

4. Results and discussion 4.1. Reconstruction of the network of active and abandoned river channels prior to the LSE Three gravel terraces (Niederterrasse NT1, NT2, NT3) are distinguished from each other in the LNB (Schirmer, 1990; Fig. 4 in Park and Schmincke, 2020a). The Late-Weichselian NT2 gravel terrace was deposited by the Rhine River prior to the Bølling interstadial and thus prior to the Allerød interstadial during which the LSE took place (NT2 = Older Lower Terrace; Weichsel = youngest glacial period in Northern Europe and northern Central Europe). It formed the floor of the Lower Neuwied Basin and represented the flood plain at the time of the Laacher See Eruption.

Journal 6 Pre-proof Our fieldwork shows that, at this time, the NT2 surface was dissected by a network of active channels differing in depth of incision (Figs. 2, 4), strongly contrasting with the present single-thread course of the Rhine River. This agrees with the findings of Erkens et al. (2009) for the northern Upper Rhine Graben farther upstream of the LNB, and of Erkens (2009) for the Lower Rhine Embayment farther downstream. There the Rhine was a multi-channel river in many stretches during the entire Late Glacial and part of the Holocene until c. 9000 BP. The pre-eruptive Rhine River in the LNB was degradational, cutting into the NT2-terrace with well-developed sandy point bar complexes. It would be best classified as transitional between its glacial braided-river-heritage 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 was located within the central, up to 3 km wide part of the Lower Neuwied Basin (Figs. 2, 4). Subsequent to the LSE, this axial zone was scoured during the Allerød and later refilled by NT3 gravel deposits during the Younger Dryas (NT3 = Younger Lower Terrace containing reworked LST components; Younger Dryas = final short stadial at the end of the Weichsel glacial,

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starting c. 200 years after the LSE). It was again scoured during the Holocene. These events destroyed any evidence of the major channel.

A broad, shallow, meandering active side channel, incised c. 6 m into the NT2, was located south of the main channel. It represented an originally major Rhine channel that had been almost abandoned prior to LSE (Fig. 4).

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Another much narrower active side channel was located to the north of the major channel. These side channels were connected to the major branch of the pre-eruptive Rhine by several subordinate channels - some most probably cut by small tributary rivers from the steep flanks of the LNB - as well as by chute channels transecting

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the large point bar complexes of the side channels.

The fact that both side channels were completely abandoned following drainage of Lake Brohl (dammed at the

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downstream ignimbrite dam; see section 2.3.; Figs. 1, 2, 3) for weeks or even months is convincing evidence both for the existence of the major channel in the center of the LNB and for its deeper incision into the NT2 compared to the side channels. The elevation difference cannot have been large considering that both side channels had

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been permanently active prior to the eruption. An incision of the major branch of the Rhine River by c. 8 m into the NT2 seems reasonable. This is 2 m deeper than the incision suggested by Thoste (1974), but agrees with the

of the side channels.

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depth of incision postulated by Bibus (1980) for the Lower Middle Rhine valley, and 2 m deeper than the incision

The NT2 flood plain - south and north of the major channel - was also intersected by a multitude of abandoned

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channels differing in vertical elevation, hierarchy and age (Fig. 4). A shallow channel extending along the boundary between the NT2 terrace and the lower northern flank of the Lower Neuwied Basin represented the oldest and highest abandoned channel - a relic of an originally broad and deep major marginal channel that had been largely filled by the time of the LSE. It was covered by locally >10 m thick alluvial fan and slope wash deposits after the eruption, especially during the Younger Dryas. This is why its course could only be deduced from rare outcrops and drilling data (Ikinger and Weidenfeller, 2000). The difference in vertical elevation of the abandoned channels is due to progressive downcutting of the Rhine River into the NT2 but was also modified by variable degrees of tectonic displacement of the various fault blocks (forming the floor of the LNB) since these channels had been active (unpubl. data).

4.2. Discharge, water depth and flow velocity of the Rhine River at the time of the LSE 4.2.1. Discharge The present discharge of the Rhine River is controlled by many factors linked in a complex way (Belz et al., 2007). A detailed reconstruction of the pre-eruptive discharge is not the main focus of this study, but important factors need to be addressed:

Journal 7 Pre-proof a. Season of the year: The imprints of blooming lilies of the valley (Schweitzer, 1958) and dandelion within Laacher See tuff (Baales, pers. com.), the hoof prints of horses with a neonate foal on a tuff layer about halfway through the eruption (Baales, 2002; Baales et al., 2002), dendrochronological (Kaiser, 1993; Friedrich et al., 1999) and varve studies (Zolitschka, 1988; Hajdas et al., 1995; Brauer et al., 1999a; Brauer et al., 1999b; Litt and Stebich, 1999; Merkt and Müller, 1999) all agree that Laacher See Volcano erupted in late spring or early summer because the seasons were delayed during the Allerød compared to the present (Atkinson et al., 1987; Coope et al., 1998; Boogart et al., 2003b). b. Influence of the Alpine catchment: Large parts of the Alpine catchment of the Rhine River were still glaciated at the time of the LSE (Grimm, 2006, 2011). Glacial melting that currently takes place between June and September (Belz et al., 2007) probably had only a minor impact on the discharge of the Rhine River at the time of the eruption because of the delayed seasons. Erkens (2009) assumes that the timing of seasonal snowmelt in the Alps was basically similar to that during the Holocene (April, May, June; Belz et al., 2007). We assume that the present snowmelt peak in June due to snowmelt in the highest regions of the Alps was delayed due to the

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glaciation at the time of LSE. Moreover, a larger percentage of precipitation was possibly retained as snow within the large glaciated areas during May and June compared to the present.

c. Flow regime: The discharge of the Rhine during the Allerød was probably precipitation-dominated within the Middle-Rhine valley (i.e. it was not so strongly influenced by run-off events in its Alpine catchment) similar to the

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conditions reconstructed for the Lower Rhine Embayment during that time (Erkens, 2009) and to the present conditions (Belz et al., 2007).

d. Climate and precipitation: The climate at the time of the LSE was temperate and humid with slightly lower

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annual temperatures and cooler winters compared to the present. The LSE postdates the Inter-Allerød Cold Phase (IACP = GI-1b) by about 70 years and predates the onset of the Younger Dryas by c. 200 years (Baales et

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al., 2002). Today, the snowmelt within the catchment downstream of the Alps occurs in January and February (Belz et al., 2007). We assume that, at the time of the LSE, higher discharges than at present occurred in early spring caused by the thaw of thicker snow covers formed in this area during the cooler winters. However, soil

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formation on the floodplain in the LNB and even in low-lying abandoned channels as well as the growth of trees and bushes may indicate the absence of large, long-lasting floods (cf. Turner et al., 2013). Decreased precipitation and water levels are assumed at the Jeetzel river (Elbe valley, northern Germany) during the second

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half of the Allerød. These relatively low water levels during the late Allerød were detected at several other localities in northeastern Germany and are probably bound to strong summer insolation, lower precipitation, fluvial

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incision and reduced groundwater discharge (Turner et al., 2013). e. Damping of flood events: The Rhine River and its tributaries were braided river systems in the Alps at the time of the LSE. The Rhine was a multi-channel transitional system between a braided and a meandering river in the northern Upper Rhine Graben (Erkens et al., 2009), parts of the Upper Middle Rhine Canyon (see section 4.6.1) and the LNB (see section 4.1). This implies that the syn-eruptive Rhine River and its tributaries were bordered by vast flood plains. These offered large retention space damping the discharge during flood events, but are cut off today to a large extent by river correction and training installations (e.g. dams). The catchment area of the rivers was significantly affected by land-use changes due to deforestation, intensification and industrialization of agriculture and sealing of the land with impermeable surfaces, etc. which has an intensifying effect on floods compared to the time of LSE. Reconstruction of Holocene paleo-peak flow regimes for the Rhine River (de Molenaar, 2012) showed that smaller flood peaks were damped by 30% under pre-human conditions compared to the present and extreme floods by about 10%. f. The rivers were significantly narrowed and the channels shortened since the 19th century (Herget et al., 2005) causing an increase in flow strength. The present man-made straight single-channel course of the Rhine River

Journal 8 Pre-proof and of its tributaries reduces friction at the riverbanks and thus increases the discharge compared to the time of the LSE.

The mean discharge of the Maas river during the Allerød, a tributary to the Rhine in the Netherlands, was basically similar to that during the Holocene (prior to climate warming) according to Bogaart (2003). The annual mean discharge in the Middle Rhine Region has steadily increased over the past 120 years due to man-made 3

climate warming. It has increased by 13.3 % (248 m /s) within the LNB (at gauging station Andernach; Belz et al., 3

2007; www.bafg.de) based on a trend analysis and amounted to c. 1800 m /s c. 120 years ago. The decisive factor for this increase was an increase in precipitation in winter due to a growing number of “west weather conditions” resulting from climate change. The averaged mean discharge in May and June was c. 2000 m³/s c. 120 years ago based on a trend analysis (www.bafg.de). It is difficult to estimate the pre-eruptive discharge precisely because it was influenced by many factors (cf Vandenberghe, 2003). A major factor is the unknown mean precipitation in May/June. We suspect that the pre-

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eruptive discharge was between 1800 and 2000 m³/s taking the mean discharge 120 years ago and the modifying factors mentioned above into account.

4.2.2. Water depth

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The mean water depth within the dredged navigable channel of the present single-thread Rhine channel is c. 5.1 m within the LNB between Rhine-km 591 and 614 at mean discharge (www.bafg.de). The water depth in the

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multiple and locally extremely wide pre-eruptive channels (esp. the southern side channel) was most probably significantly shallower as shown by the small elevation difference between the degradational gravel-sheeted channel base and sandy lower point bar deposits of the southern side channel (Loc. 655) and because the likely

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smaller water volume (due to the lower discharge) was partitioned among the various active channels. The maximum water depth was possibly c. 1-2 m within the pre-eruptive side channels and c. 3-4 m within the main channel at mean discharge. This and the depth of incision of the channels into the NT2 suggest that the pre-

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eruptive mean water level was around 4-5 m above that of the present Rhine River.

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4.2.3. Flow velocity

The mean (depth- and width-averaged) flow velocity within the navigable channel of the present Rhine River in the LNB fluctuates between 0.78 - 1.52 m/s (data based on German reference water level GIW; www.bafg.de). It

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was certainly much lower at the time of the Laacher See Eruption due to the lower discharge and because the multiple shallow channels increased frictional processes at the channel margins and bases.

4.3. Favorable implications of the eruptive scenario for river damming Our revised stratigraphic subdivision of the Laacher See Tephra (LST), presented and applied for the first time in this paper, reflects the pronounced fluctuation of eruption dynamics in a more realistic way. We maintain the basic subdivision into a Lower (LLST), Middle (MLST) and Upper (ULST) Laacher See Tephra for practical purposes. It is simple and easily-recognized at most localities and conforms to the older publications on the LSE. We have partly changed and supplemented the previous subdivision into subunits to better agree with the actual course of eruptive activities. We have: (i) reassigned the basal part of the former MLST-B subunit terminating with major pyroclastic flow generation to subunit MLST-A; (ii) newly subdivided MLST-B into subunits MLST-B1 and MLSTB2; (iii) modified the stratigraphic assignment of the upper part of subunit MLST-C by newly introducing subunit MLST-D; and (iv) subdivided the ULST into subunits -A, -B and –C. For the nomenclature of the stratigraphic subdivision see SM-2.

Journal 9 Pre-proof We were able to correlate LST deposited east of the LSV with deposits to the south even on the scale of individual subphases - a previously unsolved problem because of the in part strongly diverging transport directions during the various eruptive events. We were also able to correlate LST deposited at the outer rim of the fallout fans with deposits in central positions as well as LST deposits at medial distances with deposits closer to vent. This enabled a complete overview of the succession of eruptive events and pauses. Systematic evaluation of c. 400 XRF bulk rock analyses taken at c. 75 localities around LSV since 1998 and the new possibility to unequivocally assign an analysis to a specific subphase allows a detailed reconstruction of the extreme zonation of the LS magma reservoir. We recognize a pattern that was basically repeated three times, with some modifications: a large chemically homogenous magma batch was underlain by increasingly smaller volumes of increasingly less evolved magma (Fig. 5). Magma tapped from voluminous, chemically distinct sections of the magma reservoir was ejected in high eruption columns (LLST-A, MLST-B1, MLST-C) associated with high mass eruption rates (Fig. 5). The underlying layers of increasingly less evolved magma and decreasing volume were ejected in progressively lower eruption columns (LLST-B to –F, MLST-B2) as documented by the

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systematically shifted areal distribution of the fallout fans during these eruptive phases due to wind shearing (Fig. 6 in Park and Schmincke, 2020a).

The extremely complex eruption dynamics of the LSE comprising repeated and pronounced changes in eruptive style associated with a range of transport mechanisms (fallout, pyroclastic flows, surges, etc.), widely diverging

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transport directions and many interruptions of eruptive activity associated with repeated collapse and migration of the vent was governed by the interplay of internal and external forcing factors (Schmincke et al., 1990). A major result of our new reconstruction of the evolution of the LSE on the scale of subphases is the evidence that the

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fine-scale pattern of chemical and mineralogical zonation within the magma reservoir was the dominant governing factor (Fig. 5).

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The discontinuous eruptive activity of the LSE was the crucial forcing factor controlling the repetitive upstream damming scenario and the subsequent flooding events (Figs. 5, 7). (a) Multiple phases of fast and excessive fallout deposition into the Rhine River - peaking especially at the

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beginning of both Plinian cycles (during LLST-A1 and MLST-B1) when large chemically distinct magma batches were tapped – resulted in a particularly rapid initiation of the damming processes as shown by field evidence (Park and Schmincke, 2020a). LLST-A was the subphase with the highest eruption columns and mass eruption

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rates of the entire eruption as indicated by the large tephra thickness and large diameters of pumice clasts and lithoclasts even as far as 25 km from the vent (Locs. 1021, 1022, 1041) compared to the overlying tephra units.

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(b) We estimate that the main climactic episode of high discharge fallout and pyroclastic flow generation (LLST and MLST-A/B/C/D), that was synchronous to upstream Rhine damming, lasted 4-6 days but was interrupted by multiple, in part extended breaks in eruptive activity (Fig. 5). These multiple interruptions were pivotal in triggering the breach of the tephra dams that were only stable as long as fallout deposition persisted (see section 4.5.1 and Park and Schmincke, 2020a). The major breaks in eruptive activity followed severe collapses of the vent and conduit, and in part also vent migration, largely influenced by the compositional zonation in the magma reservoir (Fig. 5). The multiple collapses are indicated in the LST deposits by prominent layers extremely rich in Devonian lithoclasts (“Big Bang” events; Schmincke, 2004, 2009; Park and Schmincke, 2020a). The unusually strong and variable stratification of LST, accentuated by multiple repetitions of these lithic-rich layers, is one major criterion distinguishing the LST from the more “simple” deposits of most other Plinian eruptions. The large volumes of dense lithoclasts contributed to the damming of the Rhine River (see section 4.6.2).

4.4. Tephra loading of the Rhine River

Journal 10 Pre-proof The aim of the following calculations was to merely provide a rough estimate of the tephra loading of the Rhine River. A more realistic dynamic analysis would have required elaborate computer modeling or lab simulations and was not the aim of our work. For analytical methods and calculation processes see SM-3 4.4.1. Areal distribution of tephra loading The total thickness of primary LLST trough MLST-D varies from 1 to 6 m (LLST up to 1.7 m, MLST-A up to 40 cm, MLST-B/C up to 2.6 m, MLST-D up to 1.1 m) along the 22 km long section of the present Rhine within the Lower Neuwied Basin (Figs. 6, 9). Tephra loading was at its maximum in the river section around Weissenthurm where the cumulative fallout fan axes of the main Plinian phases (except for MLST-A) crossed the Rhine (Figs. 2, 6, 9). 4.4.2. Grain size In the Lower Neuwied Basin, the LLST and MLST-B/C tephra consists 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; Fig. 6). Pumice clasts are generally one or two size classes larger than the lithoclasts due to aerodynamic sorting during transport within the eruption clouds. Silt and clay-sized components >4 phi are largely lacking - except for ash

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coatings 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 - is only between 3 and 25 cm thick in the central LNB. The grain size of LST generally decreases upstream (Fig. 6).

4.4.3. Volume percentage of buoyant and non-buoyant components within primary LST

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The percentage of buoyant components – exclusively juvenile pumice - decreases from c. 78 Vol.% in LLST to 37 Vol.% in MLST-C and <13 Vol.% in MLST-D (average of 3 tephra sections close to the Rhine in the Lower Neuwied Basin; Fig. 6). This is a result of the strong compositional, mineralogical and volatile zonation of the

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Laacher See magma reservoir, an increase in phenocryst abundance, and an increasing phreatomagmatic impact starting with MLST-C (Fig. 5). The non-buoyant fraction consists mostly of Devonian slates and sandstones plus

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lesser tephritic to basanitic volcanics within the LLST to MLST-B tephra units/subunits and an almost negligible amount of non-buoyant pumice. In the MLST-C to MLST-D tephra subunits, the amount of non-buoyant components (especially that of non-buoyant pumice) increases enormously from 63 Vol.% in MLST-C (average of

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3 tephra sections) to 92 Vol.% in the MLST-D (average of 2 tephra sections). The percentage of lithoclasts in the LST is generally high compared to other Plinian eruptions (see section 4.3) and increases upriver (Fig. 6). 3

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The density of buoyant LST pumice clasts increases from 0.47 g/cm in LLST, 0.48 g/cm in MLST-B1 and 0.47 3

3

3

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g/cm in MLST-B2 to 0.59 g/cm in MLST-C and 0.74 g/cm in MLST-D (average of size fractions -5 phi to -2 phi at four localities in the LNB close to the Rhine; see Fig. 7 in Park and Schmincke, 2020a). Non-buoyant, less 3

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vesicular MLST-C/D pumice has a density of c. 1.2-1.5 g/cm . For methods of density measurements see SM-3. 4.4.4. Total volume of the LLST tephra that fell into the Rhine We calculated the total volume of tephra that fell into the Rhine River in the Lower Neuwied Basin only for the LLST unit (using procedures described in SM-3). The LLST eruptive stage is the only one that reflects the true response of the Rhine River to fallout deposition because the Brohl ignimbrite dam built-up farther downstream at the end of MLST-A did not exist during this stage (Figs. 2, 3). The water surface affected by fallout during the LLST phase was probably 3.5 to 4 times larger than that of the present single–thread Rhine because: (a) the Rhine was a multi-channel system within the LNB (Fig. 4) at the time of the LSE, (b) the Rhine had overtopped its banks in a major way due to overloading with tephra, which led to an additional increase in the water surface exposed to fallout. Hence, the volume of total LLST (buoyant and non-buoyant) that fell into the pre-eruptive Rhine (with a 4 times larger surface area) would have been c. 27.9 x 6

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10 m³ (for a potential average inter-particle pore space of 15 Vol.% within primary LLST) and 29.5 x 10 m³ (for 10 Vol.% pore space). 4.4.5. Ratio of the volume of buoyant LLST components that fell into the Rhine versus the water volume

Journal 11 Pre-proof The volume of buoyant LLST components (for 10 Vol.% inter-particle pore space) that fell into the Rhine would 6

have been 23.6 x 10 m³ for a water surface 4 times larger than today (for alternative calculations see Table 1). The water volume filling the multiple pre-eruptive channels within the LNB, when LLST started, was possibly largely similar to that of the deep present single-thread Rhine channel because these pre-eruptive channels were much shallower (see section 4.2). The water volume filling the 24 km long section of the pre-eruptive Rhine within 6

the LNB would have amounted to 31.5 x 10 m³ and the water volume supplied from upstream during the LLST 6

stage (assumed to have lasted 6 hours) would have been 38.9 x 10 m³ for a pre-eruptive discharge of c. 1800 m³/s. The water volume filling the channels plus the water volume supplied from upstream during the LLST stage 6

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would have added up to a total volume of 70.4 x10 m³ for a discharge of 1800 m /s. For alternative calculations see Table 1. The ratios of buoyant LLST tephra volume to water volume (two calculated alternatives) for a discharge of 1800 3

m /s (Tab. 1) show an extreme concentration of floating particles. The ratios range between 1:3.4 and 1:3 – equivalent to 23-25% buoyant components by volume.

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Pumice is defined as finely vesicular pyroclastic material that, when cool and dry, will float on water (i.e. density 3

<1.0 g/cm ; Whitham and Sparks, 1986; Manville et al., 1998; Manville et al., 2002). The majority of LLST pumice clasts floated below the water surface in our lab experiments (unpubl.) - despite the extremely low densities measured in a dry state. This implies that their density rapidly increased to around 1 g/cm

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due to an

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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 experiments,

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independent of their specific properties (Whitham and Sparks, 1986; Manville et al., 1998, 2002). There are several reasons for the high absorption rate of the LST pumice lapilli. First, many of the LST lapilli have tubular

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vesicles permeating an entire clast. Secondly, the vesicles are wide open at the clast surface because the majority of the lapilli are fragments of originally larger ones. Moreover, many lapilli have a thin absorptive ash cover. This implies that the LLST pumice clasts were not distributed equally within the Rhine River in reality, but

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concentrated within its upper part. Moreover, rapid absorption of water by the pumice clasts would have reduced the volume of water in the river, increasing considerably the volumetric concentration of particles in the two-phase flow (V. Manville, pers. com.). This must have resulted in extreme particle concentrations of >70% buoyant

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components by volume (see section 4.7). For comparison: hyperconcentrated flows contain >5-60% and debris flows >60% sediment by volume (Pierson, 2005a, 2005b).

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The above calculations were carried out for the total LLST unit and not for single subunits because several factors such as the interpolation of total unit thicknesses were better controlled. However, the quantification of tephra loading over the entire duration of the LLST phase does not entirely simulate the conditions when only part of the tephra volume had fallen. Rough calculations for shorter time intervals during which only part of the total tephra thickness was deposited (Tab. 2) show that the tephra loading of the Rhine River increased with increasing duration of a single tephra fall event because the diluting effect of the large water volume filling the channels when the tephra fall started was naturally more efficient than that of the water supplied from upstream during short tephra falls. However, the volume flux of tephra components entering the Rhine River in the LNB due to fast and excessive fallout deposition may have exceeded the volume flux of water entering the system from upstream, especially at the beginning of both Plinian cycles (during LLST-A1 and MLST-B1) when large chemically distinct magma batches were tapped (see sections 4.3 and 4.7.1). The aim of this simple model was to roughly estimate the potential concentration of floating particles within the Rhine River by a quick procedure. Several dynamic aspects are completely neglected, such as: (1) part of the tephra would be transported out of the studied area by the river during the studied time interval, (2) an initial incremental deceleration of the river would have enhanced further deceleration and (3) etc.

Journal 12 Pre-proof 4.5. Evidence arguing for major dam building at the upstream entrance to the LNB at Koblenz There are no exposures left at the upstream entrance to the LNB that would allow exact location and study of the various manifestations of the repeatedly reestablished Koblenz Dam, because: (a) the dams were destroyed by breaching and flooding during the ongoing LSE; (b) the area was widely scoured during the post-eruption Allerød, the Younger Dryas and the Holocene (section 4.1); and because (c) the surviving remnants of the pre-eruptive flood plain are today completely covered by buildings in this area. However, several lines of evidence both up and downstream of the entrance to the LNB (see section 4.6.1) strongly argue for the reality of the Koblenz dams.

4.5.1. Evidence within the LNB downstream of the Koblenz bottleneck We have found strong evidence for one minor (FE1) and four major flooding events (FE2, FE3, FE4, FE5) that inundated both active and abandoned channels, as well as large parts of the adjacent floodplain, downstream of

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the Koblenz bottleneck during the climactic phase of the LSE, as detailed in Park and Schmincke (2020a). Criteria that allow to infer an upstream trigger of these flooding events include: a. Flooding events occurring during breaks in eruptive activity

The systematic interbedding of fluvially reworked tephra with incremental primary LST relics related to successive

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eruptive phases allows us to infer an unequivocal causal relationship between repeated distinct flooding events in the LNB and eruptive activity. At first sight, repeated flooding synchronous with active Plinian fallout pulses would be the most plausible scenario. However, the arguments listed below exclude major flooding during Plinian fallout

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deposition and are strong evidence that the flooding events occurred during breaks in eruptive activity: (i) 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 LSE and that had been flooded by a preceding flooding event (Figs. 7, 8a, 8b). Mud drapes deposited during the waning stage

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of each flooding event indicate that the channels had been temporarily abandoned; (ii) 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 LNB. These lithoclasts (especially large ones)

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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

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of the fluvial sequences in the LNB was “disturbed” by simultaneous Plinian fall (Fig. 8a); (iii) The dense, gray, easily identifiable MLST-C pumice lapilli were incorporated already into the very base of the up to 2 m thick aggrading deposits of FE4 (consisting of reworked MLST-B/-C tephra) even within the active side channels. The lowermost unit of the FE4 channel sequence locally can be followed towards the flood plain up to tens of meters away from the channel margin, where it overlies the complete MLST-B/C tephra sequence and underlies MLST-D reworked by FE5 (Figs. 8a/b). This indicates that flooding must have occurred after the deposition of the complete MLST-B/C unit and prior to deposition of MLST-D. 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 (Fig. 7). b. Massive and extremely widespread erosion and reworking of freshly deposited tephra sequences The syn-eruptive floods eroded and reworked large volumes of fresh tephra deposits - up to 1.7 m thick LLST was eroded and reworked by FE2 and FE3 and up to 2.5 m thick MLST-B/C by FE4 - from all active side and subsidiary channels and even from abandoned channels and vast flood plain areas (Fig. 8b). Approximately 7580 % of the tephra-covered area below c. 70.5 m asl within the central Lower Neuwied Basin shown in figure 4 –

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an area >40 km in dimension - was affected by syn-eruptive erosion and reworking (see Fig. 3 in Park and Schmincke, 2020a). c. Upper flow regime conditions The sedimentary structures deposited by the flooding events indicate high flow velocities (Park and Schmincke, 2020a). FE4 was characterized by even higher flow velocities than the previous flooding events. Erosion and reworking of the entire MLST-B/C sequence and the large-scale, gravel-sized antidunes and chute and pool structures indicate that upper flow regime conditions (cf. Cheel, 1990; Cheel and Udri, 1996; Manville et al., 1999; Alexander et al., 2001; Manville et al., 2002; Fielding, 2006) had been reached throughout the entire flooded area even on the flood plain and in the highest abandoned channel (Fig. 8c). d. Extremely high peak flood levels and extremely broad flooded area The maximum water levels (Fig. 7; Park and Schmincke, 2020a) were high despite the extremely broad flooded area (Fig. 4). 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 the LBN; Figs. 4, 7) significantly overtopped flood plain areas whose elevation

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had been raised by up to c. 4.5 m of fallout tephra (LLST + MLST-A/B/C), even within the central LNB. The flood level of each of the syn-eruptive flooding events was possibly at least 2 m higher within the major channel of the Rhine River (field data are lacking due to post-eruptive scouring) because it was more deeply incised. An extremely large cross-section area (up to 7 km perpendicular to flow direction) was flooded during peak discharge

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of FE4.

e. 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 above the flood plain

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(elevated by the primary tephra deposits) - were reached during the initial stage of the syn-eruptive flooding events, indicating a sudden inflow of large volumes of water. After that, flood levels fell and flow was diverted

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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 discharge hydrograph and duration (days to weeks) of

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“conventional” meteorologically-induced floods in the Rhine River area (Park and Schmincke, 2020a). We can exclude the initiation of lahars from the flanks of the LNB by heavy regional rainfalls as a cause of the syn-eruptive flooding events in the LNB, because we did not find indicators of effective surface or sub-surface

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run-off, contemporaneous with the LLST and MLST-B/C/D phases, even on very steep terrain within the area 2

surrounding the LNB and the LSV (1200 km in size; for more details see Park and Schmincke, 2020a). We can

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also exclude heavy rainfalls in the headwaters of the Rhine River as a cause because the flood waves would have taken too long to reach the LNB. Moreover, even an extreme weather-induced “normal” flood and thus a peak inflow into the LNB could not have provided the huge water volumes needed to reach the erosive power, transporting capacity and flow velocities simultaneously in all active and abandoned channels on the very wide flood plain (for more details see Park and Schmincke, 2020a). Further, the multiple and short-term repetition of flooding during LSE could not be explained by this type of trigger. The repeated formation and breach of a major dam at the upstream entrance of the Lower Neuwied Basin near Koblenz (Koblenz Dam), consisting of fallout components and driftwood washed together, is the most convincing explanation for the repeated drainage and successive powerful flooding of the channels in the LNB during the climactic Plinian stages of LSE. 4.5.2 Evidence upstream of the Koblenz bottleneck We found a small temporary exposure of LST (Loc. 697) at Koblenz main train station which is (a) upstream of the mouth of the present Moselle River, (b) c. 2.7 km upstream of the postulated ephemeral Koblenz Dam, (c) located within an abandoned channel connecting the Moselle to the Rhine River, and (d) at an elevation of c. 68

Journal 14 Pre-proof m asl (base of LST) which is c. 4 m above the mean water level of the pre-eruptive Rhine (Figs. 2, 9). MLST-C consists of 41 Vol.% buoyant pumice within the LNB (versus 34 Vol.% non-buoyant pumice and 25 Vol.% nonbuoyant lithoclasts; mean of two exposures close to and south of the Rhine River; see SM-6 in Park and Schmincke, 2020a). None of the MLST-C components of Loc. 697 could float when placed into water. This suggests that the horizontally bedded MLST-C tephra had been deposited by non-buoyant components settling down through a standing water column while the buoyant components had been floated off. This provides sedimentological evidence that Lake Koblenz No.4 which accumulated during the MLST-B/C subphases (the second major Plinian eruptive stage of the LSE, following Flooding Event FE3) must have risen up to c. 68 m asl during the eruptive break between phase MLST-B and –C (Fig. 5), because MLST-B had been deposited on still dry ground. There is no field evidence for MLST-D because the top of the LST section was eroded after the LSE.

4.5.3. When did the dams form? One of the most significant aspects of the LSE is the strongly pulsating succession of distinct eruptive phases

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separated from each other by major eruptive breaks (see section 4.3). In section 4.5.1 and in more detail in Park and Schmincke (2020a), we have presented convincing evidence that flooding in the LNB generally occurred during pauses in eruptive activity and not during peaks. This indicates that the pumice dams formed during eruptive phases and were stable only as long as tephra fell into the Rhine River and its tributaries. Several

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independent and unequivocal indicators document that each of these eruptive phases was followed by a major break in eruptive activity (Fig. 5). The order of events is shown in Figures 3, 5 and 7.

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4.6. Topo- and hydrographic preconditions and eruption-induced factors responsible for major dam formation upstream of the main tephra loading area

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LST deposits are thickest around Weissenthurm close to the downstream end of the Lower Neuwied Basin (Figs. 2, 6, 9). However, the main damming site must have been located c. 15 km further upstream at the eastern entrance of the narrow Rhine Canyon into the LNB close to Koblenz (Figs. 2, 9) as indicated by several lines of

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evidence discussed in section 4.5 and in detail in Park and Schmincke (2020a). However, it seems very likely that damming was initiated in the LNB and propagated upstream in a chain reaction. Below, we discuss topo- and

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hydrographic conditions within both crucial areas.

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4.6.1. Conditions at and upstream of the Koblenz bottleneck

4.6.1.1. Conditions at the Koblenz bottleneck (a) The section of the present Rhine River between Rhine-km 593 and 594 forms a narrow constriction less than 720 m wide at 70 m asl, the cross section of the present Rhine being only c. 270 m in this area (Figs. 2, 9). This river section was already a bottleneck at the time of LSE, judging from the fact that it is - and was - bordered by steep canyon walls directly to the east. To the west, it is - and was - bordered by a broad flood plain formed by the NT2 gravel terrace (Gad et al., 2007). The pre-eruptive Rhine channel was deeply incised - by c. 8 m - into this terrace based on our findings further downstream in the LNB (see section 4.2). (b) The pre-eruptive Rhine was a single-thread channel within the area of the bottleneck in contrast to the conditions further up- and downstream (Fig. 9). The NT2 terrace was not transected by an active side channel at the time of the LSE judging from its natural morphology prior to massive human overprinting, as shown on an old topographic map (Königlich Preußische Landesaufnahme, Blatt Koblenz 1901 AD). The narrow marginal channel between the NT2 terrace and the adjacent western margin of the elevated area at Koblenz-Lützel was not active at the time of the LSE, as inferred from its minor incision into the NT2 compared to that of the active side channels downstream in the LNB (Figs. 2, 4).

Journal 15 Pre-proof (c) The Koblenz bottleneck is located only 700 m downstream of the mouth of the Moselle River. The Moselle is 3

the largest tributary of the Rhine (311 m /s present mean (1931-2017 AD) discharge; www.dgj.de, DGJ 2017) 3

within the LNB area that contributes 15.5 % of its present mean (1931-2017 AD) discharge (2030 m /s at Andernach; www.dgj.de, DGJ 2017) and was an important conveyor of additional tephra masses during the LSE (see below). (d) The present Rhine River turns its flow direction abruptly by 125° directly upstream of the Koblenz bottleneck and also did so at the time of LSE forced by similar topographic conditions (Figs. 2, 9). (e) The mean gradient of the present riverbed is zero within the river sections (up to 0.5 km in length) both directly up- and downstream of the bottleneck (Fig. 9). The mean gradient of the low water level of the present Rhine River is extremely low – only 0.19 ‰ within the bottleneck (www.bafg. de, www.dgj.de). The gradient abruptly increases, but is still low (ranging between 0.24 and 0.46 ‰) within the 15 km long river section downstream of the bottleneck (Fig. 9). The gradient of the pre-eruptive Rhine was most probably similar to the present conditions because the variation in gradient reflects the differing degree of displacement of the various tectonic elements

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within and outside of the LNB that was most probably similar 12,900 years ago (cf. Giebel et al., 1990; Gölz, 1990).

(f) The water depth of the present Rhine (based on the mean water level) is low shortly upstream of the Koblenz bottleneck compared to the river section further upstream within the Rhine Canyon. The water depth is lower up

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to 7-8 km downstream of the bottleneck than further downstream (www.bafg.de; Fig. 9). This is most plausibly also a result of tectonics judging from our tectonic reconstruction (unpubl.). Conditions were likely similar at the time of the LSE.

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(g) The mean (depth- and width-averaged) flow velocity of the present Rhine River (based on German reference water level GIW; www.bafg.de) within the navigable channel is at a minimum (0.85 m/s) at the downstream end of

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the Koblenz bottleneck where the Rhine River changes its flow direction (Fig. 9). This extremely low flow velocity (the lowest within the 27 km long river section between the mouth of the Lahn River into the upstream Rhine Canyon and the bottleneck outlet of the LNB at Andernach) is most probably due to a backwater effect caused

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both by the constriction and the change of flow direction. Conditions were likely similar at the time of LSE. In general, however, the flow velocity of the pre-eruptive Rhine was much lower than within the present navigable

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channel due to the lower discharge as discussed in section 4.2.

4.6.1.2. Conditions upstream of the Koblenz bottleneck

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(a) A detailed reconstruction of the river architecture of the Rhine upstream of the LNB would have been very complicated due to post-eruptive scouring and the lack of exposures. The pre-eruptive Rhine was most plausibly a multichannel river directly upstream of the Koblenz bottleneck and within large and wider sections of the Rhine Canyon as far upstream as the mouth of the Lahn River (Fig. 9; areas further upstream not studied), judging from channels incised into remnants of the NT2 (shown on old topographic maps) and in analogy to the conditions in the LNB and to those in the Upper Rhine Graben (Erkens et al., 2009) and the Northern Rhine Embayment (Erkens, 2009). (b) The mean gradient of the present Rhine River is, and was, extremely low - only 0.19 ‰ - within the Rhine Canyon between Boppard, c. 24 km upstream, and the Koblenz bottleneck (gradient of mean low water level; www.bafg.de, www.dgj.de). (c) The Present Rhine slows down along the 5.5 km long river section between the mouth of the Lahn River and the Koblenz bottleneck. The mean (depth- and width-averaged) flow velocity within the navigable channel decreases from 1.13 to 0.85 m/s (Fig. 9; www.bafg.de). In general, the flow velocity is significantly lower within the Rhine Canyon than within the downstream LNB which is most probably due mainly to the lower gradient within the Rhine Canyon compared to that within the LNB controlled by the tectonic activity within the LNB. The

Journal 16 Pre-proof flow velocity of the pre-eruptive Rhine was lower in general (see above), but the differences between the Rhine Canyon and the LNB were most probably similar.

4.6.1.3. Interpretation Strong erosion and upper flow regime reworking of LST in the broad northern abandoned side channel (Fig. 4) and on the adjacent flood plain (Locs. 742, 760), and similar conditions in and along abandoned channels south of the major axial Rhine channel within the eastern section of the LNB (Locs. 678, 688) imply that the major damming site and the breaching water reservoir must have been located farther upstream than the upstream turnoff of these abandoned side channels (Figs. 4, 9). The Koblenz bottleneck represented an ideal damming location because of several conditions operating together: Focusing of the upstream massively tephra-laden multichannel Rhine into a narrowed single-thread channel, the drastic change of flow direction, the low gradient, the low flow velocity, considerable additional tephra input by the Moselle River, and favorable conditions for many kilometers upstream of the bottleneck within the Rhine canyon

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(the low gradient, low water level, flow velocities decreasing towards the bottleneck, the locally multi-channel river structure increasing the surface area for tephra capture, additional sources of tephra input, see below). These topographic and hydrographic preconditions strongly suggest that the crest of Koblenz dams No. 2, No.3 and No.4 (for dam No.1 and No. 5 see section 4.6.3) was located within this bottleneck.

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Areas of low velocity and marked slope reductions are prime jamming sites in the formation of (analogous) ice jams because of increased surface ice concentration and possible congestion, while the formation of grounded ice jams may be promoted by shallows (Beltaos, 1995). Logjams played an important role in the fluvial response

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to the sudden input of pyroclastic sediments during the 2008-2009 eruption of the Chaitén Volcano (Chile; Umazano et al., 2014). Several factors controlling their formation were roughly similar to those operating during

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the LSE. The logjam zone formed within a sharp curve of the Blanco River where its gradient decreased markedly.

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4.6.1.4. Sources of the tephra forming the Koblenz Dams (a) Tephra that fell directly into the Rhine River upstream of the Koblenz bottleneck. 3

(b) The Moselle River (the largest tributary of the Rhine River, see above; c. 270 m /s mean discharge 120 years

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ago prior to major climate warming based on a trend analysis; www.bafg.de) traversed the 1 m-LST-isopach for more than 34 km. It has - and had - a much steeper gradient than the Rhine River (0.4 ‰; Fig. 9) and was

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probably not blocked or much decelerated by tephra fall during the LSE. The Moselle was thus an important and constantly working conveyor of additional tephra masses (preferentially LLST and MLST-A tephra) towards the Koblenz bottleneck also during the subsequent eruptive stages MLST-B/C/D. 3

(c) The even steeper Lahn River (0.78 ‰, 47 m /s present mean discharge, mouth c. 6 km upstream of the LNB; Fig. 9) contributed large additional volumes of tephra in a similar way. (d) A multitude of small creeks deeply incised into the flanks of the Rhine, Moselle and Lahn Canyons and their hinterland were most plausibly not blocked due their very steep gradient and the reduced tephra thickness in these areas. They were possibly important suppliers of additional tephra masses into the larger rivers, considerably enlarging the surface area from which tephra was transported into the Rhine. (e) The Rhine River upstream of the Koblenz bottleneck, and the Moselle and the Lahn Rivers are flanked by steep canyon walls, in part rocky cliffs, at the stoss-side of meanders. The cliffs west of the Rhine are inclined up to 80° toward the river (Kraft and Dietz, 2007) within the Koblenz bottleneck and further upstream (Fig. 9). LST potentially slid off as shallow syn-eruptive landslides which would have also increased the surface area from which material could enter the Rhine.

Journal 17 Pre-proof The Moselle and Lahn rivers and the Rhine River upstream of the LNB received a higher percentage of small gravel- to sand-sized pumice clasts and ash - except during the MLST-A phase (see below). Smaller pumice clasts quickly saturate with water and sink much faster than larger ones according to our own laboratory experiments and similar to studies by Manville et al. (1998). They would have contributed to dam building because they could be transported easily and quickly by the Rhine, had a better chance to be deposited than still buoyant larger pumice clasts and because frictional interlocking becomes more efficient as grain size decreases (cf. Manville et al., 2002). During the MLST-A phase, associated with eruption clouds at lower altitudes, the focus of fallout deposition was sheared more to the south by a lower wind system and mainly affected the Moselle River and the critical area at the eastern entrance to the LNB where the Moselle River enters the Rhine. The grain size spectrum of the MLST-A lapilli layers was comparable to that of LLST and MLST-B/C in the LNB, comprising dominantly gravel- to coarse sand-sized tephra components.

4.6.2 Conditions contributing to efficient slowing and damming the Rhine River within the LNB

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The effective impact of fallout deposition onto the Rhine River is fundamentally due to the fact that the dispersal axes of the major Plinian stages of the LSE (LLST and MLST-B/C/D) crossed the Rhine River within the extremely broad tectonic Lower Neuwied Basin where the pre-eruptive Rhine was subdivided into three major parallel channels and many active subordinate channels (Figs. 2, 4). Favorable conditions for additional

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downstream damming of the Rhine include:

(a) The water surface affected by major fallout was three to four times larger than that of the present single-thread

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Rhine would be.

(b) The water depth within the channels was much shallower compared to the present Rhine supporting entrapment of floating particles especially at the channel margins (see section 4.7.1).

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(c) The total tephra accumulation (LLST and MLST-A/B/C) significantly overtopped the mean pre-eruptive water level - within the major axial Rhine River along a c.14 km long section between 3 km downstream of the Koblenz bottleneck and 2 km upstream of the Andernach bottleneck (Figs. 6, 9).

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(d) The non-buoyant tephra components sank to the riverbed in the major channel and possibly stayed largely in situ. The increasing diameter of these particles in downstream direction between Koblenz and the culminating

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fallout fan axes at Weissenthurm led to a sequential reduction in the originally very low gradient of the riverbed by aggradation.

(e) After the formation of Brohl Dam by pyroclastic flows at the end of the MLST-A phase, the backwater of Lake

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Brohl would have progressively reached farther upstream along the major Rhine channel, supplied by each breach of the Koblenz Dam (Figs. 2, 3, 9). (f) We found primary ignimbrite deposits c. 1.5 km upstream of the Brohl Dam (Locs. 902, 903). This proves that the pyroclastic flows entering the Rhine through Brohl valley (7 km downstream of the LNB) moved not only downstream but also several km upstream within the Rhine valley (unpubl. data). They may even have reached the Andernach bottleneck. (g) Pyroclastic flows may have reached the LNB and even the Andernach bottleneck through the Nette river valley at the end of the MLST-A phase (Figs. 2, 3, 9). There is, however, no evidence to prove this. Ignimbrites have been documented as far as c. 4 km upstream of the mouth of the Nette river into the Rhine (Freundt and Schmincke, 1986). (h) Tributaries of the Rhine downstream of the Koblenz bottleneck potentially did not much contribute to damming. Only 3 tributary rivers into the LNB (Nette, Saynbach, Wied) managed to re-incise and to reach the Rhine River following the LSE. Smaller confluents drowned in their own alluvial fans of reworked LST (conditions prior to exploitation of LST; see old topographic maps (Königlich Preußische Landesaufnahme, 1893 to 1901

Journal 18 Pre-proof AD). This suggests that they were buried relatively quickly by tephra during the first Plinian phase (LLST) of LSE. The Nette River was definitely blocked by pyroclastic flows at the end of the MLST-A phase (Fig. 2). We can exclude that the strong and widespread erosion and reworking of LST within the LNB was caused by the drainage of a temporary lake dammed downstream in the LNB because: (a) Convincing facts argue for voluminous transient flood waves entering the LNB from upstream (section 4.5.1 and discussed in more detail in Park and Schmincke, 2020a); (b) A temporary tephra dam located close to the outlet of the LNB and consisting of tephra and driftwood washed together would have had to be 14-17 m high to dam a lake that could have inundated the flood plain in the eastern LNB (Fig. 9). This seems implausible. (c) The main axial channel was incised slightly deeper into the NT2 terrace (by c. 2 m) than the side channels (see section 4.1). LLST, MLST-A and MLST-B/C fell on drained ground, in between the flooding events, even at the base of the side channels and even at the base of a chute channel (Loc. 642; connecting the southern side channel with the main axial channel;) corresponding to Rhine-km 606, only 2 km upstream of the area of maximum tephra loading. This implies that the

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maximum water level of any lake dammed-up downstream was up to 58-59 m asl at a maximum and the lake thus confined to the major axial channel (Fig. 9); (d) A potential ignimbrite dam at the Andernach bottleneck would have been formed at the end of MLST-A but would have breached repeatedly (after MLST-A, MLST-C and MLSTD) as indicated by the drainage of the channels. This seems implausible.

The entire 2.5 km wide axial zone of the LNB was scoured after the LSE. We can thus only speculate where a

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jam/dam could have been located: (i) The bottleneck outlet of the LNB at Andernach would have been a highly plausible damming site (Fig. 9) as discussed in our first publication on Rhine damming (Park and Schmincke,

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1997). Arguments include: the multi-channel pre-eruptive Rhine River was confined to a narrow single-thread river within this bottleneck and the mean gradient of the river bed is - and was - extremely low (0.15 ‰) starting 3

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km upstream of the Andernach bottleneck. The present Rhine slows down significantly within this zone. Its mean (depth- and width-averaged) flow velocity within the navigable channel is only 0.78 m/s (based on German reference water level GIW; www.bafg.de).

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(ii) Field evidence indicates that a large water volume (dammed upstream during the MLST-A phase that lasted 12 days) was released during Flooding Event FE3 (Park and Schmincke, 2020a). This must have caused an abrupt increase in the volume, surface elevation and area of early Lake Brohl. It seems plausible that the water

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level of Lake Brohl could have reached 58-59 m asl and extended upstream as far as the area of maximum tephra loading at Weissenthurm (Fig. 9). The flow velocity of the Rhine would have been abruptly reduced

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upstream of the area where it entered the standing water body of Lake Brohl, possibly creating a secondary dam trapping pumice incoming from upstream. Any kind of downstream damming caused backwater and flow resistance effects (cf. Chow, 1959) that possibly extended farther upstream than usually because of the extremely low gradient of the Rhine that was further reduced by the non-buoyant tephra components accumulating at the channel base (see above). Moreover, the constant tephra fallout caused over-banking through volumetric expansion of the river water and reduction in channel capacity, increased flow resistance through bed drag and particle interlocking and thus additional decrease in water surface gradient and flow velocity within the river section upstream. This may have triggered a chain reaction. In general, basic mechanisms responsible for the Rhine damming possibly resembled those also active in other multiple-particle systems such as in the formation of traffic jams (Sugiyama et al., 2008) or ice jams. Once initiated, ice jams propagate upstream (Beltaos, 1995). The major “accident” had happened in the area around Weissenthurm where tephra loading was highest but the significant deceleration of the river was propagated farther upstream. Major dam formation finally occurred where several favorable conditions were operating jointly. In other words, the dynamic interaction of the river with the tephra load was critical - rather than the presence of the tephra alone.

Journal 19 Pre-proof The chain reaction must have started quickly, because the Rhine was already partly dammed shortly after the onset of major fallout phases when the mass eruption rates were especially high. Almost the entire tephra section of such an eruptive phase fell on drained ground on top of the mud drape of the preceding flooding event, even at the base of active side channels. The major Rhine channel possibly received only a reduced volume of water from upstream (passing below the enlarging floating dam, see below) that constantly decreased.

4.6.3. Potential dimensions of the Koblenz dams The Koblenz dams must have been lodged against the embankment of the major Rhine channel carved deeply into the NT2 terrace (Fig. 10b). The flood levels and the erosive power of the flood waves inundating the downstream LNB increased from Flooding Event FE1 through Flooding Event FE4 (Fig. 7) indicating that the height of the various stages of Koblenz Dam and thus the water volume of the lakes confined behind them increased. Koblenz Dam No.1 – the lowest dam - was probably only slightly higher than mean water level. Dam No.4 was possibly the highest dam. Lake Koblenz No.4 developed simultaneously with the formation of this dam

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during deposition of the MLST-B/C tephra (after Flooding Event FE3 and prior to Flooding Event FE4) and had 6

probably the largest maximum depth, length and volume (c. 80x10 m³; for more details see Park and Schmincke, 2020a) of all lakes (Figs. 1, 2). It may have extended as much as c. 30 km upstream due to the low gradient of the Rhine - despite the low height of the dam.

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None of the Koblenz dams was probably higher than the NT2 terrace plus the overlying LST cover, at a maximum ≤ 10 m (Fig. 10b). A dam reaching higher than the flood plain could only efficiently dam a lake if it expanded over

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the entire width of the flood plain perpendicular to the Rhine (c. 1.5 km at the Koblenz bottleneck). This would have been clearly unrealistic mechanically for a dam consisting of loose pumice clasts. Moreover, there would not have been enough tephra in this area to build such a large dam. A log jam damming the Blanco River after the

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Chaitén eruption (2008-2009, Chile) consisting of logs and trapped volcanic components was at least 6 m high (Umazano et al., 2014).

It is difficult to define the length of the Koblenz dams parallel to the Rhine. They could only be formed and

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stabilized by an extremely long and low-rise “foot region” that may have even extended up to the Andernach bottleneck - 15-20 km downstream - and was connected to one or several secondary downstream jams/dams of

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unknown location and dimension. The foot of Koblenz Dam No.4 probably extended c. 8 km downstream of the Koblenz bottleneck up to the place where the Rhine entered Lake Brohl and a low-rise downstream dam (Weissenthurm Dam) had probably been formed due to backwater effects (Figs. 2, 9).

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The present Rhine River is split into 3 arms by mid-channel bars in the area directly downstream of the Koblenz bottleneck where it changes flow direction. There is no evidence of the pre-eruptive topography in this area due to post-eruptive scouring. However, a similar river architecture most probably also already existed downstream of the bottleneck prior to the LSE resulting from the topographic, hydrographic and tectonic conditions discussed above. Gravel islands and mid-channel bars would have increased the probability of freeze-up processes moderated by friction along the significantly enlarged embankment line (cf. Beltaos, 1995). Within this region, the “foot” of the Koblenz dams was probably thicker than further downstream. We illustrated only this “core region” of the Koblenz dams (5-6 km) on our maps. The core region of Koblenz Dam No. 1 was probably located farther downstream than that of the successive dams and connected to a larger downstream dam. Koblenz Dam No. 5 was possibly located upstream of the Koblenz bottleneck and was not as high as No.4 because the backwater of Lake Brohl potentially reached upstream of the bottleneck by that time. A secondary, considerably smaller and shorter dam must have plugged the abandoned channel west of the Andernach bottleneck (see section 4.6.1; Figs. 2, 10).

4.7. How can a large river become dammed up by fallout tephra?

Journal 20 Pre-proof We can only speculate on the mechanisms of dam formation and the architecture of the dams because no remnants are left (see sections 4.1 and 4.5). The mean flow velocity of the pre-eruptive Rhine River was likely much lower than that of the present Rhine (0.85 m/s at the Koblenz bottleneck, see section 4.6.1) but to dam a river with a discharge of c. 1800 - 2000 m³/s is still a challenge. The pumice dams blocking the Rhine River certainly differed significantly from dams accumulated by landslides (Costa and Schuster, 1988) or other volcanic natural dams (Capra, 2011). The porous nature of pumice and thus its ability to absorb water were crucial factors contributing to damming aside from the topo- and hydrographic preconditions and eruption-induced factors discussed in section 4.6. Major aspects were: (1) A large volume percentage of the tephra that had fallen into the Rhine could float and be easily and quickly water-transported. (2) The masses of buoyant pumice were submerged within the upper part of the water column or swam at the surface according to our lab experiments (unpubl.) – owing to their specific physical properties (see sections 4.4.3 and 4.4.5) and similar to the experiments by Bagnold (1955) using wax spheres in water flows. This way, the Rhine may have separated into a lower and an upper water body with different hydrodynamic properties. (3) The absorption of the river water by the porous

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and permeable pumice clasts not only increased their density but also reduced the effective volume of the water available to dilute the sediment influx, increasing the sediment:water ratio of the two-phase flow considerably (V. Manville pers. com.).

The upper water body may have become transformed into a highly concentrated water-particle mixture with >70%

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buoyant components by volume (see section 4.4.5) probably displaying Non-Newtonian behavior. Bagnold (1955, 1956) demonstrated that neutrally buoyant spheres in water maintained Newtonian flow behavior up to 50% sediment by volume. The lower part of the water column may still have behaved as a Newtonian fluid within its

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upper section but may have also been hyperconcentrated within its basal section due to the high percentage of non-buoyant pumice, lithoclasts and large rafts of primary LST (Park and Schmincke, 2020a) transported close to

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the base. Considerable boundary forces were probably effective between these water layers. The Rhine damming scenario may have resembled the formation of ice jams (e.g. Beltaos, 1995; Healy and Hicks, 2006; Shen et al., 2008) resulting from high concentrations of suspended ice fragments that, similar to the

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pumice clasts in the Rhine River, also float at or close to the water surface. Normally, damming mechanisms attributed to high concentrations of sediment in rivers operate in the opposite way (see section 1).

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4.7.1. Mechanisms of jam initiation

One of the most significant aspects of the LSE is its very discontinuous eruptive activity characterized by the

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repeated succession of distinct eruptive phases separated from each other by major eruptive breaks (see section 4.3). Multiple phases of fast and excessive fallout deposition into the Rhine River - peaking especially at the beginning of both Plinian cycles (during LLST-A1 and MLST-B1) when large chemically distinct magma batches were tapped – most probably resulted in a particularly rapid initiation of the damming processes. Field evidence shows that the Rhine River must have become already slowed down and partly dammed during the LLST-A1 subphase when tephra representing an only up to 35 cm thick deposit on land rained rapidly into the river (see section 4.3, Figs. 3, 5, 7; SM-5 in Park and Schmincke, 2020a). Favorable conditions include: (a) The Rhine River was loaded with tephra also up- and downstream of the Lower Neuwied Basin area. An about 630 km long section of the Rhine (up to the foot of the Alps) and large parts of its tributaries were affected by fallout deposition (Bogaard and Schmincke, 1984, 1985). (b) All of the buoyant tephra components that fell into the studied section of the Rhine during LSE had no horizontal velocity and thus had to be accelerated by the river. This may have had a significant decelerating 6

effect. For example, a volume of c. 23.6x10 m³ buoyant components (Tab. 1) had to be accelerated during LLST. (c) The primary mode of jam initiation was probably that the large volumes of pumice clasts falling into the Rhine caused it to volumetrically bulk. As it spread out of its channel it increased its surface area capable of directly

Journal 21 Pre-proof intercepting further tephra fall, producing a feedback loop. Shallow inundation of fast flood plain areas and abandoned channels covered by LST caused increased bed shear and flow resistance and thus led to a considerable reduction of flow velocity. Moreover, absorption of the river water by the porous pumice clasts increased the effective sediment:water concentration. Thus, fallout into the Rhine may have resulted in a sediment-choked and floating “moving-dam” that likely behaved as a cohesionless granular mass plugging the Rhine within the entire LNB as well as several kilometers upstream and downstream (Figs. 1, 2, 9). (d) The second step in jam initiation was probably that further congestion caused an increase in particle concentration relative to the available width and the surface water velocity. The frictional freeze-up process possibly started along the channel margins and also around potential mid-channel bars - similar to present conditions downstream of the Koblenz bottleneck (see section 4.6.2; Figs. 2, 10). This further reduced the width of the channel surface and further impeded the passage of incoming pumice clasts. (e) Hanging dam stage: A stoppage of the viscous, highly concentrated water-pumice mixture within the upper water body of the Rhine - e.g. due to an irregularity of the channel margin, especially where drift wood had

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become entangled within still standing trees and bushes - may have triggered a chain-reaction also affecting the underflow and promoting the entrapment of masses of bedload components. This may have finally led to a complete halt of the river. Frictional freezing was probably reached by interlocking of the pumice clasts especially when the hanging dam grounded at least incrementally. This process transformed a section of the very

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elongate “moving-dam” into a hanging dam.

4.7.2. Factors and mechanisms that contributed to stabilize and thicken the Koblenz dams (Fig. 10)

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The duration of a volcanic dam depends primarily on the volume of the obstructing mass, its textural characteristics, on its geometry and also how long it is before the dam is overtopped (Capra, 2011; Costa and

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Schuster, 1988). The Koblenz dams were obviously strong enough to withstand the forces applied by the current and by gravity although they were primarily maintained only by internal friction due to the lack of cohesion. Stability of the Koblenz Dams was probably enhanced by a range of factors:

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(a) The considerable length of the dams (5-6 km within the core region and up to 20 km for the entire dam, see section 4.6.2) and the extremely low gradient of the riverbed (0.19 – 0.34 ‰). (b) Once water saturated, the depositional behavior of pumice much more closely resembles that of non-volcanic

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components (White et al., 2001; Manville et al., 2002). The large volumes of sand-sized pumice clasts contributed by the Moselle and Lahn rivers and the Rhine River upstream of the LNB contributed to stabilize especially the

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crest of the Koblenz dams close to the Koblenz bottleneck because they saturated with water faster than larger pumice clasts and thus had a better chance to be deposited than the larger, still buoyant pumice clasts (see section 4.6.1). The foot region of the Koblenz dams (most probably initiated by damming further downstream in the LNB) was formed by a larger grain size spectrum. (c) The pronounced surface roughness of the angular and vesicular pumice clasts in general increased intergranular friction within the dam. Manville et al. (2002) 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 (2 mm). Thus, the sand-sized particles filling the interstices between larger pumice clasts may have much contributed both to the damming process and in holding the dam together. (d) The incoming pumice clasts began to accumulate upstream of the hanging dam and also to submerge at its upstream side. After diving, the buoyant clasts rose and were added at the base of the hanging dam contributing to its thickening. (e) Incorporated driftwood – trunks and branches ripped from riparian trees and shrubs upstream of the dam when the slowing-down Rhine River increasingly overtopped its banks – may have contributed considerably to form and stabilize the dam.

Journal 22 Pre-proof (f) Large and heavy rafts of primary tephra with a base area of several meters (that had been mobilized from the channel banks by undercutting and flotation and that were possibly transported preferentially at the base of the floods as documented in the LNB downstream of the dam; Figs. 8, 10; Park and Schmincke, 2020a) possibly became integrated preferentially within the basal section of the dam contributing to its stability. (g) Progressive shoving and shortening due to the compression exerted by the water dammed upstream may have been another way by which the dam thickened. (h) The accumulation of fallout (pumice and lithoclasts) that fell onto the growing hanging dam formed a heavy cover that both stabilized the underlying dam, contributed to its thickening and further forced it to ground. (i) The free flow area below the dam was probably progressively reduced and thus the passage of water extremely inhibited. This caused a significant increase in water level at the upstream side of the dam. Eventually, the floating dam became intermittently grounded near its toe at one or possibly several places. Grounding is plausible because flow velocity and discharge were extremely reduced already upstream of the dam by additional tephra masses plugging the Rhine upstream of the dam and contributed by the Moselle River (Fig. 9).

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The longevity of a dam decreases as mud content increases and permeability decreases (Capra, 2011). The mud content of the Koblenz dams was negligible. In the case of extensive grounding, only a limited water volume would have passed as seepage through the voids of the pumice accumulation comparable to water migration through a sand-gravel deposit. However, it is more likely that the Koblenz dams were not completely sealed. This

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way the major pre-eruptive Rhine channel within the Lower Neuwied Basin downstream of the dam would have received some water fed by the reduced water flow below the dam but the higher located side channels (active prior to LSE) fell dry as shown by field evidence (Park and Schmincke, 2020a). The permeability of the dam

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possibly contributed to its constant growth allowing newly arriving pumice to be strained out of the water. An approximate steady state was obviously established as long as the Laacher See Eruption was in progress and

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tephra fell into the Rhine relatively continuously (see section 4.5 and discussed in more detail in Park and

5. Summary and conclusions

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Schmincke, 2020a).

1. The Rhine River - the largest river in Western Europe - was dammed five times by tephra fall alone during the LSE. The very discontinuous eruptive activity of LSE is one major aspect contributing to dam formation. The

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dams were formed during rapid and voluminous tephra fall events and breached during major eruptive breaks. 2. The effective impact of fallout deposition onto the Rhine River is fundamentally due to several favorable

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conditions: (a) Laacher See Volcano is located very close to the Rhine River (10-23 km); (b) the broad tectonic Lower Neuwied Basin (LNB), interrupting the narrow Rhine canyon for some 22 km, was the major area affected by massive fallout deposition; (c) the Rhine was a multi-channel meandering system (a crucial new conclusion) throughout the LNB. This meant that the water surface exposed to fallout was three to four times larger than that of the present single-thread Rhine channel greatly enhancing rapid overloading of the river. 3. The discharge of the Rhine River, its flow velocity and the depth of the channels were in part significantly lower at the time of the eruption compared to the present. These factors strongly facilitated damming, as did the extremely low river gradient (0.15-0.46 ‰). 4. Basic mechanisms responsible for the Rhine damming possibly resembled those active in other multi-particle systems such as in the formation of traffic jams or ice jams. It seems very likely that the major “accident” = initial damming had occurred in the LNB where tephra loading was highest but the significant deceleration of the river was propagated farther upstream in a chain reaction. 5. There is strong evidence that major dam formation occurred upstream at the eastern entrance of the narrow Rhine Canyon into the LNB where several favorable conditions were operating together. Evidence for upstream damming includes: Extensive and extremely widespread erosion and reworking of freshly deposited tephra

Journal 23 Pre-proof sequences and upper flow regime conditions within all active and abandoned channels and even vast flood plain areas in the LNB, extremely high peak flood levels and the fact that tephra sequences representing complete eruptive cycles repeatedly fell on drained ground in between the documented flooding events even at the base of active channels within the LNB. 6. Favorable conditions for upstream damming include: a. bottleneck entrance to the LNB where the upstream multi-channel Rhine was constricted to a single channel, change of flow direction by 125° directly downstream of the bottleneck, extremely low gradient (only 0.19 ‰) starting already 24 km upstream of the bottleneck and within the bottleneck, constant decrease of flow velocity starting at least 5.5 km upstream of the bottleneck, lowest flow velocity at the downstream end of the Koblenz bottleneck where the Rhine River changes its flow direction, the bottleneck being located only 700 m downstream of the mouth of the Moselle River - the largest tributary of the Rhine within the LNB area and an important conveyor of additional tephra masses during LSE. 7. We can only speculate on the mechanisms of dam formation and the architecture of the dams because no remnants are left. The pumice dams blocking the Rhine differed significantly from “normal” volcanically induced

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dams. The porous nature of the pumice clasts and their ability to absorb water were crucial factors. Thus, a large volume percentage of the tephra that had fallen into the Rhine could float and be easily and quickly watertransported. The masses of buoyant LST pumice floated submerged within the upper part of the water column or swam at the surface due to their specific density and vesicularity. This way, the Rhine may have separated into a

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lower and an upper water body with different hydrodynamic properties. Moreover, the absorption of the river water by the pumice clasts not only increased their density but also reduced the effective volume of the water available to dilute the sediment influx, increasing the sediment:water ratio of the two-phase flow considerably.

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The basic concept of dam formation resembles the formation of ice jams. Jam initiation possibly started by stoppage of the highly concentrated water-pumice suspension within the upper water body along the channel

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margins - especially where drift wood had become entangled within still standing trees and bushes. Thickening of the dam was potentially controlled by the accumulation and diving of incoming pumice clasts below the upstream base of the dam, by progressive shoving and shortening and by fallout that fell onto the dam. The dam was

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possibly stabilized by its extreme length, by interlocking of the angular pumice clasts, by incorporated drift wood, by the heavy fallout cover on top and by heavy primary tephra packages incorporated preferentially within the basal part.

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8. The height of the various ephemeral Koblenz dams and thus the water volume of the lakes confined behind them increased during the LSE. None of the Koblenz dams was higher than the NT2 terrace plus the overlying

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LST cover, at a maximum ≤ 10 m. Lake Koblenz No.4 potentially had the largest depth and length of all lakes. It may have extended as much as c. 30 km upstream due to the low gradient of the Rhine - in spite of the low height of Koblenz Dam No.4.

9. The Koblenz dams could only be formed and stabilized by an extremely long downstream “foot region” that may have even extended up to 15-20 km downstream to the Andernach bottleneck, and that was possibly connected to one or several low-rise secondary jams/dams within the LNB. The foot of Koblenz Dam No.4 probably extended only 8 km downstream of the Koblenz bottleneck up to the place where the Rhine entered early Lake Brohl, which had been dammed by pyroclastic flows 7 km downstream of the LNB about half way through the climactic eruption, and which had extended into the LNB at that time. This was probably a major factor contributing to the formation and large dimensions of Koblenz Dam No.4. 10. The Koblenz dams were probably not completely sealed most of the time. This way the major pre-eruptive Rhine channel received some water and an equilibrium condition was established temporarily that enabled the dam to remain stable as long as tephra fell into the Rhine relatively continuously during individual eruptive phases.

Journal 24 Pre-proof Funding Information Our work was initially supported by the Deutsche Forschungsgemeinschaft, grants Schm 250/58 and 250/84 and partly by the Stiftung Zukunft/Sparkasse Koblenz and by the Vulkanpark GmbH.

Acknowledgments We thank an anonymous reviewer for helpful suggestions. V. Manville provided an exceptionally detailed review that greatly helped to improve the understanding of specific aspects and to strengthen our arguments. We also appreciate comments by J. Taddeucci and K. Kataoka on an earlier draft of this manuscript. Many thanks to M. Sumita for technical support and valuable suggestions. S. Beltaos (Environment and Climate Change, Canada) and T. Mauer (Bundesanstalt für Gewässerkunde) kindly reviewed our model of dam formation. Many thanks to M. Voloschina for carrying out the pumice density measurements and volume calculations co-supervised by A. Freundt (GEOMAR). We thank J. Belz, N. Busch, E. Gölz, M. Reiß and S. Vollmer (Bundesanstalt für Gewässerkunde at Koblenz) who kindly supplied fluviodynamic data for the present Rhine River and G. Erkens

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Figures

Journal 29 Pre-proof Fig. 1 Overview of the Rhine damming scenario. Distribution of LST (total thickness of fallout tephra, ignimbrite deposits excluded; after Bogaard and Schmincke, 1985) relative to the geometry of the Rhine River and the Lower Neuwied Basin. The Koblenz lakes temporarily dammed at the repeatedly re-established Koblenz Dam due to fallout into the Rhine River and its tributary rivers, the Moselle and the Lahn, were very elongate, reflecting the narrow upstream Rhine canyon. Lake Koblenz No.4 was dammed during the deposition of the MLST-B/C tephra following Flooding Event FE3 and prior to Flooding Event FE4 and had the greatest depth and length of all lakes. It 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.

Fig. 2 a. Detailed Rhine damming scenario in the Lower Neuwied Basin (LNB) following deposition of MLST-B/C tephra and prior to Flooding Event FE4. Map shows Laacher See Volcano, areal distribution of total Laacher See Tephra

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deposits (isopachs in m, ignimbrites excluded) 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 and –D2), areal distribution of ignimbrites after Freundt and Schmincke (1986) and new data (unpubl.).

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Overview of the damming scenario is described in section 2.3 and Fig. 5. Speculative low-rise Weissenthurm Dam formed (during MLST-B/C eruptive phase) where the highly pumice-laden Rhine entered the early backwater stage of Lake Brohl impounded by an ignimbrite dam farther downstream at Brohl (at the end of the

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MLST-A eruptive phase). Areal extent of Lake Brohl based on a suspected maximum water level of <59 m asl. and speculative Weissenthurm Dam are shown schematically because the broad axial zone in LNB bordering the

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major Rhine channel was scoured following the LSE. Deceleration and damming of the Rhine propagated upstream where major damming occurred at the Koblenz bottleneck. Maximum areal extent of Lake Koblenz No.4 dammed behind Koblenz Pumice Dam No.4 is based on a suspected maximum water level at 70 m asl.

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Reconstruction of pre-eruptive flood plain at Koblenz based on new data. Reconstruction of pre-eruptive channels in the LNB based on new data and Ikinger and Weidenfeller (2000), for details see figure 4.

Fig. 3

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b. Cross-section from Laacher See Volcano through the Lower Neuwied Basin to its northern rim

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Summary and timing of the various syn-eruptive lake filling and spilling episodes resulting from the impact of different types of eruptive activity on the Rhine River with reference to the eruptive phases of the LSE. a. Chronology of the LSE shown by the revised event stratigraphy of LST at Loc. 758 at the northern rim of the LNB.

A stranded pumice raft intercalated between ULST-A and –B - c. 14 m above the mean pre-eruptive water level of the Rhine River - 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 large lake (Lake Brohl) during the LSE. b. Correlation of the various lake damming and spilling events with stratigraphy. For explanation see section 2.3. c. Events during the entire damming scenario discussed in this account and companion paper Park and Schmincke (2020a). Fig. 4 a. Reconstruction of the pre-eruptive morphology (12,900 BP) in central Lower Neuwied Basin based on new data and Ikinger and Weidenfeller (2000). For discussion see section. 4.1. The reconstruction of the area south of the major Rhine channel is verified by a large number of outcrops. To the north, exposures are largely lacking due to

Journal 30 Pre-proof extensive extraction of pumice and gravel. Essential points of our reconstruction are the observations that the Rhine was a multi-channel system at the time of LSE and that all types of channels on the flood plain, irrespective of their status quo prior to LSE (active or abandoned since various time spans and located at different elevations), were reoccupied during flooding following the multiple breaches of the ephemeral tephra dam at Koblenz (Park and Schmincke, 2020a). Locations of outcrops are also shown as listed in SM-1. b. Today, the Lower Neuwied Basin is densely populated (c. 280,000 people). Large areas are covered by buildings or have been modified by the extraction of gravel and pumice. The satellite image gives an idea of the difficulties in reconstructing the pre-eruptive morphology. c. Overview

Fig. 5 Eruption-induced forcing of damming and flooding events in the Lower Neuwied Basin. Figure 5 shows that the discontinuous eruptive activity of LSE, governed by the pronounced compositional

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zonation within the LS magma reservoir, was the crucial forcing factor controlling the Rhine damming scenario. a. Event-based, volume-normalized stratigraphy of LST (revision of existing subdivision by reinterpreted and newly defined phases/subphases; volumes are estimated based on tephra distribution) b. Correlation of the repetitive upstream damming of the Rhine River at Koblenz (Koblenz dams, KD 1-5) and

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flooding events (FE 1-5) due to breaches of these dams with the course of eruptive events. Repeated phases of fast and excessive fallout deposition into the Rhine River - peaking especially at the beginning of both major Plinian cycles (during LLST-A1 and MLST-B1) when large chemically homogeneous magma batches were

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tapped – resulted in a particularly rapid initiation of the damming processes (Koblenz dams No.1, 2 and 4) as shown by field evidence. The dams became breached during major breaks in eruptive activity (see i.).

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c. Correlation of damming at Brohl with the eruptive events. A major stable dam (Brohl Dam, BD) was formed by pyroclastic flows entering the narrow Rhine valley c. 7 km downstream of the LNB at the end of eruptive phase MLST-A. A large lake (Lake Brohl, LB) accumulated. The breach of the ignimbrite dam synchronous with the

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powerful resumption of eruptive activity (ULST-B) after a long break (8-10 d) during the terminal stage of LSE triggered a major flood wave downstream.

d. Total phenocryst abundance in juvenile clasts based on thin section analyses (Bogaard and Schmincke, 1985)

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e. Zr concentration based on whole rock XRF-analyses of pumice samples (mean values of analyses from

data)

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indicated number of exposures up to 15 km from vent and referring to unequivocally isochronous layers, new

f. Eruptive mechanisms after Bogaard and Schmincke (1984, 1985) and new data g. Vent/conduit collapse events indicated by layers extremely enriched in xenoliths and/or by increased ballistic activity, pyroclastic flows, significant changes in the direction of tephra distribution, vent migration (new and archive data) h. Mass eruption rate: new semi-quantitative assessment based on new detailed reconstruction of fluctuations of eruption column height based on tephra distribution following the criteria of Carey and Sparks (1986) i. High resolution temporal assessment of LSE. Correlation of breaks in eruptive activity deduced from proximal LST deposits (grey triangles to the left) based on classical break indicators and in part newly applied criteria (degree of weather-induced erosion, indicators of vent collapse, change in direction of fallout distribution, etc.) with breaks deduced from the overall flooding scenario (blue triangles to the right; this account, Park and Schmincke (2020a) and unpubl. data). Breaches of Koblenz Dam are synchronous with major eruptive breaks. Estimates of the duration of specific eruptive phases and breaks based on analogy with recent eruptions and newly applied criteria (see above; red bars to the left) and based on the impact of LSE on the Rhine River (water saturation experiments with fluvially reworked LST pumice, calculations of the time it took to accumulate a

Journal 31 Pre-proof specific water volume of growing Lake Brohl up to a specific level until a particular eruptive phase started, etc.; blue bars to the right); new unpubl. data. d = days, h = hours; LB 14-17d = estimated persistence of Lake Brohl.

Fig. 6 a. Topography in LNB: Multiple active and abandoned Rhine channels at the time of LSE. Major pre-eruptive Rhine channel (course not traceable due to later scouring) simulated by present Rhine. Isopachs of different Plinian fallout phases of LSE indicate maximum tephra loading. For details see figures 2 and 4. b. Modeling of the tephra loading of the major pre-eruptive axial Rhine channel exclusively by fallout that fell directly into the river illustrated by a longitudinal profile along the Rhine River from Koblenz to Andernach. Tephra accumulation is shown for those individual eruptive phases relevant for damming at Koblenz. Tephra thickness was interpolated from tephra sections located as close as possible to the Rhine. Supply of tephra by tributaries and from upstream section of the Rhine as well as syn-eruptive reworking and removal by the Rhine are neglected in this model. Total thickness of tephra aggradation is to scale; superimposed on mean bed level

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morphology of present Rhine after Gölz (1990) due to lack of reliable data for the syn-eruptive Rhine. Tephra thickness is shown in relation to mean pre-eruptive water level that is assumed to have been c. 2 m lower than that of the present Rhine due to a lower discharge and because the water volume was partitioned across multiple active channels. Tephra accumulation would overtop the mean water level for a distance of c. 14 km and

c. and d.

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effectively reduce the gradient of the riverbed.

channel. Components <1 mm were not analyzed.

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Hydrologic properties of the tephra that fell into the Rhine at 3 localities along and close to the southern side

c. The volume and weight percentages of buoyant tephra components significantly decreased within successive

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stratigraphic units/subunits of LST (MLST-D removed by quarrying at Loc. 617). The relative proportion (Vol.%) of non-buoyant components is larger upstream.

d. The weight percentage of buoyant clasts clearly decreases and that of non-buoyant clasts increases with

Fig. 7A

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significant for MLST-D.

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decreasing particle size for all eruptive phases. The grain size generally decreases upstream. This is most

Overview showing the systematic fivefold succession of massive fallout into the Rhine River causing repeated

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lake damming at Koblenz upstream of the LNB followed by breaches of Koblenz Dam during breaks in eruptive activity triggering large flood waves in the LNB (order of events from bottom to top). This resulted in complex intercalations of primary fallout and flood deposits. Evolution - here shown for the southern side channel of the River Rhine - is representative for the processes within all channels that were active prior to LSE. 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) indicate dry ground for extended time prior to LSE. Vertical scale shows relative elevations above/below the lower flood plain in m. Flood levels 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 figure 4a). These values refer to the mean bed level and are not equivalent to maximum flow depth because the riverbed was overlain by fallout tephra of the preceding eruptive phase and possibly partly by the deposits of the preceding floods when each flood arrived. 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. Flood levels were most probably higher towards the breaching dam where the LNB is narrower. The major axial Rhine channel (not depicted here) was already partly filled with the backwater of Lake Brohl when FE4 took place in contrast to FE1, FE2 and FE3 (see

Journal 32 Pre-proof section 4.6.2). Prior to FE5, even the higher elevated active side channels had been partly filled by Lake Brohl. See figure 7B for legend and figure 10 in Park and Schmincke (2020a) for more detailed information. Fig. 7B Legend for figure 7A

Fig. 8 Horizontal bright red lines indicate base and top of the syn-eruptive primary and/or fluvially reworked LST section in the LNB. Vertical magenta lines mark part of the section discussed in this account. Bars to the right and left of the photos indicate primary LST deposits (yellow, brown) and LST deposits reworked during the syn-eruptive flooding events as well as the type of reworking. Labelling of the right bars indicates the stratigraphic assignment of primary and fluvially reworked LST. Labelling to the left indicates the flooding event, by which the tephra unit was reworked.

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The systematic interbedding of fluvially reworked tephra in the LNB with incremental primary LST relics related to successive eruptive phases (as shown in the photos) allows to infer that large volumes of water were repeatedly dammed upstream of the LNB at Koblenz during fallout phases and released during breaks in eruptive activity causing powerful and widespread flooding in the LNB. This indicates repeated formation and breach of an

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upstream dam.

a. Marginal base of highest located abandoned channel (Loc. 715) where flow velocities of the syn-eruptive floods were lower and thus the deposits of the waning stage of the flooding events thicker and not eroded by successive

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floods.

Primary LLST deposits (mixture of coarse-grained, off-white pumice and lithoclasts) had been eroded by Flooding

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Event FE2. Deposition of the dunes consisting entirely of coarse ash- to small lapilli-sized LLST pumice lapilli during the waning stage of FE2 was clearly not disturbed by synchronous fallout (lithoclasts lacking). This indicates that FE2 took place during an eruptive break between LLST and MLST-A. For description of intervening

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stages see figure 2A in SM-5 of Park and Schmincke (2020a). Primary MLST-B/C (c. 1.8 m thick) was deposited on drained ground post-FE3 indicating renewed damming upstream. MLST-B/C was removed by FE 4 after deposition of MLST-C (sheared pseudo-primary relic of MLST-B preserved at left). The FE4 flood deposits were

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not disturbed by Plinian fall indicating that FE4 took place during the eruptive break between MLST-C and MLST-

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b. Margin of abandoned channel south of the major axial channel (Loc. 678). Early stages not described (for information see Park and Schmincke, 2020a). The channel fell dry after FE3 (mud drape) indicating renewed damming upstream and was then covered with primary MLST-B/C (c. 2 m thick) deposited on drained ground. MLST-B/C was removed by FE 4 after deposition of MLST-C as shown by large relic at right indicating that FE4 took place during the eruptive break between MLST-C and -D. Large tephra bodies were removed by undercutting. FE4 left a non-buoyant lag consisting of lithoclasts and dark- and light-gray non-buoyant MLST-C pumice clasts behind. Red-iron-staining of LLST was caused by groundwater. c. Channel base of northern active side channel (Loc. 703) Striking example of large-scale, coarse-grained upper flow regime structures deposited by Flooding Event FE4 post MLST-C indicating that a voluminous flood wave was triggered by the release of a large water body upstream. In-phase wave draping above grounded and sheared tephra rafts (consisting of primary MLST-B/C) and antidune accretion at their stoss-side result from stationary and up-stream breaking water-surface waves and point to upper flow regime conditions. For detailed information see figures. 13Ac and 14Bb in Park and Schmincke (2020a).

Fig. 9A

Journal 33 Pre-proof Synoptic view of major dam formation at the Koblenz bottleneck (upstream of the main tephra loading area at Weissenthurm) governed by salient topo- and hydrographic preconditions and eruption-induced factors – shown parallel to a 30 km-longitudinal profile of the Rhine River (major axial channel) from the mouth of the Lahn River in the upstream Rhine canyon (right) to the bottleneck outlet of the LNB at Andernach (left). State prior to Flooding Event FE4 after deposition of MLST-B/C tephra. For discussion see section 4.6. Database explained from bottom to top of figure: (a) Mean gradient of present Rhine based on mean low water level (database from 2006-2010, www.bafg.de). (b) Location of present mid-channel bars versus Rhine-km, elevation not considered; bars could have been located at similar locations at the time of LSE due to similar topographic and tectonic conditions (see section 4.6). (c) Mean bed level (MBL) of navigable channel of present Rhine (no data for the pre-eruptive Rhine due to post-eruptive scouring) measured spanning the entire width of the shipping channel and in 100 m-intervals parallel to the river (measuring campaigns 1985, 2004; www.bafg.de). The bed level curve was shifted upwards by 5 m, because the pre-eruptive major axial Rhine channel was less deeply incised by c. 5 m into the flood plain compared to the present Rhine according to our

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field data. (d) Thickness of tephra that fell directly into the axial channel – shown separately for buoyant (red) and non-buoyant tephra (gray) - was interpolated from tephra sections located as close as possible to the channel (Fig. 6). Tephra thickness is only shown for eruptive phases relevant for damming at Koblenz (LLST+MLST). The volume percentage of buoyant and non-buoyant components was determined in lab experiments, see SM-3. (e)

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Mean discharge of Rhine upstream of Lahn and Moselle c. 120 years ago based on a trend analysis (at gauging station Kaub; Belz et al., 2007). (f) Mean pre-eruptive water level (MWL) based on present mean water level (data from 2006-2010, www.bafg.de) assumed to have been c. 2 m lower than that of the present Rhine due to a

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potentially lower discharge and because the water volume was divided in multiple active channels at the time of LSE. Areas of low water depth of present Rhine shown by blue shading (based on mean water level and mean

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bed level DGM-2004, www.bafg.de). (g) Elevation of backwater level of Lake Brohl in major Rhine channel after transgression of Flooding Event FE3 and prior to rupture of Koblenz Dam No.4, estimated. (h) Modelling of Koblenz Dam, potential Weissenthurm Dam and Andernach Jam is hypothetical because dams were destroyed

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already during LSE and because the damming sites were scoured after LSE. The dams were most probably superimposed on a non-buoyant lag at the base of the major axial channel consisting of components that had directly fallen into the Rhine as well as of reworked non-buoyant LST. (i) Pre-eruptive flood plain and overlying

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LLST/MLST tephra cover on flood plain was interpolated based on field data and data shown in Fig. 6. (j) Junction of pre-eruptive active side channels from axial channel; for reconstruction see section 4.1. (k) Field evidence in

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LNB supporting the existence of Koblenz Dam No.4 (only some typical localities shown): Massive and widespread erosion and upper flow regime reworking of freshly deposited MLST-B/C within all active and abandoned channels and on the flood plain up to 2 km away from major axial Rhine channel and up to 70 m asl^nn (up to 14 m above base of axial Rhine channel at Loc. 631) indicates flood wave from upstream. Evidence upstream of the dam: MLST-C fell into standing water at Loc. 697 (68 m asl) indicating that Lake Koblenz No.4 had risen up to that level during an eruptive break between MLST-B and MLST-C. (l) Mean flow velocity (2006-2010 AD, depthand width-averaged) within the navigable channel of the present Rhine based on German reference water level GIW (www.bafg.de). The flow velocity of the pre-eruptive Rhine was likely much lower; see section 4.2. (m) Red arrows symbolize input of major additional tephra by tributary rivers. (n) The dotted areas upstream of Koblenz Dam represent river sections flanked by steep canyon slopes (rocky cliffs up to 80° inclined) at the stoss-side of meanders from where additional tephra may have been fed into the Rhine by slumping. The red vertical lines indicate tephra input by small creeks from the steep canyon flanks. (o) Reconstruction of pre-eruptive river architecture of the Rhine: for the central LNB verified by field evidence (this study; Fig. 4), for the area up – and downstream of the central LNB suspected from analysis of 120-year-old topographic maps (see section 3.1, 4.1) and analogy to the central LNB. (p) Mean gradient of the Moselle and Lahn was calculated based on high water

Journal 34 Pre-proof levels and spanning 40 km-long sections upstream of their mouth into the Rhine because of modern barrages: Moselle based on 50-year flood (HQ50) in 1925, Lahn based on 100-year flood (www. bafg.de). (q) Present long term mean discharge of tributaries (1931-2017 AD; www.dgj.de)

Fig. 9B Legend for figure 9A

Fig. 10 Virtual model of the crest of hanging pumice dam at Koblenz prior to complete grounding. Different factors and mechanisms contributing to stabilize and thicken the dam – especially once the hanging dam started to ground incrementally - are shown.

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a. longitudinal profile, b. cross section

Supplementary material SM-1

List and location of exposures of primary and fluvially reworked Laacher See Tephra within the Lower Neuwied

Supplementary material SM-2

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Stratigraphic subdivision and nomenclature of LST

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Basin, within the Lower Middle Rhine Valley as far downstream as Brohl and within the Westerwald region

Supplementary material SM-3

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Tephra loading of the Rhine River – calculation procedure

Journal 35 Pre-proof Author statement The work described here has not been published previously. It is not under consideration for publication elsewhere. Its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out. If accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copyright-holder.

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Cornelia park Hans-Ulrich Schmincke

Journal 36 Pre-proof Declaration of interest

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We have no competing interest to declare.

Journal 37 Pre-proof Discharge of RR [m3/s]

Water volume filling the RR channels in LNB at start of LLST fall [m3]

Water volume supplied from upstream during LLST fall (6 h) [m3]

Total water volume affected by LLST fall [m3]

Volume of buoyant LLST calculated for 15 vol% interparticle pore space in primary LLST [m3]

Volume of buoyant LLST calculated for 10 vol% interparticle pore space in primary LLST [m3]

Ratio of volume of buoyant LLST versus total water volume for 10 vol% inter-particle pore space

Buoyant components by volume [%]

Volume of LLST that fell into the multiple pre-eruptive RR channels assumed to have been 3.5 times larger than that calculated for the present single-thread RR due to enlarged water surface exposed to fallout 1600

29,144,000

34,560,000

63,704,000

1800

31,485,000

38,880,000

70,365,000

2040

32,812,000

44,064,000

76,876,000

19,533,850

20,682,900

1 : 3.1 to 3.3

24.4 – 23. 3

1 : 3.4

22.7

1 : 3.7

21.3

1600

29,144,000

34,560,000

63,704,000

1800

31,485,000

38,880,000

70,365,000

2040

32,812,000

44,064,000

76,876,000

22,324,400

23,637,600

1 : 2.7 to 2.9

27- 25.7

1 : 3.0

25

1 : 3.2

23.8

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Table 1

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Volume of LLST that fell into the multiple pre-eruptive RR channels assumed to have been 4 times larger than that calculated for the present single-thread RR due to enlarged water surface exposed to fallout

Ratios of the buoyant LLST volume (that fell into the Rhine River/RR between Koblenz and Andernach) versus

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the water volume (filling the channels in LNB when LLST fall started and supplied from upstream during the 6 h duration of LLST). Volumes of buoyant tephra (calculated for the present single-thread Rhine channel) were

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corrected for a water surface that was 3.5 to 4 times larger than that of the present single-thread Rhine due to the many active channels at the time of LSE. The water volume filling these channels is assumed to have been equivalent to that of the present Rhine. Options for an inter-particle pore space in primary LST of 10 Vol% and of 3

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15 Vol% and varying discharge rates are shown (present mean discharge = 2040 m /s; assumed syn-eruptive 3

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discharge = 1800 m /s; discharge significantly lower than the potential syn-eruptive discharge = 1600 m /s).

Journal 38 Pre-proof Tephra thickness average [cm]

Duration of tephra fall assumed [min]

Total water volume [m³]

Volume of buoyant LLST calculated for 10 vol% inter-particle pore space in primary LLST and water surface of pre-eruptive channels exposed to fallout 4 times larger than that calculated for present RR [m³]

Buoyant tephra/water ratio

Buoyant components by volume [%]

1

2

31,485,000

207,360

1 : 152

0,7

10

20

33,645,000

2,073,600

1 : 16

5.9

20

30

34,725,000

4,147,200

1 : 8.4

10.6

Table 2

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Buoyant tephra/water ratios for short tephra fall events and thus smaller average tephra thicknesses compared to total LLST and a discharge of 1800 m³/s. Buoyant tephra volumes were calculated for an inter-particle pore space of 10 vol% in primary LLST and a water surface of the many active pre-eruptive channels exposed to fallout 4

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times larger than that calculated for the present Rhine River (RR).

Journal 39 Pre-proof Highlights Rhine River, largest river in Western Europe dammed repeatedly by Plinian fallout Damming mechanism resembled formation of ice jams Dam was a partly floating, partly grounded plug of buoyant pumice throughout many km

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Major damming site located 12 kilometers upstream of maximum tephra loading

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9r1

Figure 9r2

Figure 10