Channel adjustments and vegetation cover dynamics in a large gravel bed river over the last 200 years

Geomorphology 125 (2011) 147–159

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Channel adjustments and vegetation cover dynamics in a large gravel bed river over the last 200 years F. Comiti a,⁎, M. Da Canal b, N. Surian c, L. Mao b, L. Picco b, M.A. Lenzi b a b c

Faculty of Science and Technology, Free University of Bozen-Bolzano, piazza Università 5, 39100 Bolzano, Italy Dept. of Land and Agroforest Environments, University of Padova, viale Università 16, 35020 Legnaro, Italy Dept. of Geography, via del Santo 26, 35123 University of Padova, Italy

a r t i c l e

i n f o

Article history: Received 19 April 2010 Received in revised form 3 September 2010 Accepted 6 September 2010 Available online 22 September 2010 Keywords: Channel narrowing Island dynamics Human impact Piave River Italy

a b s t r a c t The timing and extent of the morphological changes that occurred in the last 200 years in a large gravel bed river (the Piave River, eastern Italian Alps) that was heavily impacted by human activities (training structures, hydropower schemes, and gravel mining) have been analyzed by historical maps, aerial photos, repeated topographic measurements, and geomorphological surveys. Results show that the channel underwent a strong narrowing during the twentieth century, but with a faster pace during the 1970s–1990s and with an associated shift from a dominant braided pattern to a wandering morphology. Bed incision up to 2 m — mostly from gravel mining — has been documented for this period. Large areas of the former active channel were colonized by riparian forests, both as islands and as marginal woodlands. The ceasing of gravel extraction in the late 1990s seems to have determined a reversal in the evolutionary trend, with evidence of vegetation erosion/channel widening even though a significant aggradation phase is not present. We conclude that alteration of sediment regime has played a major role on the long-term channel evolution. However, only relevant flood events (RI N 10–15 years) appear to determine substantial island erosion, and therefore the proportion of island vs. channel area fluctuates depending on flood history. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Braided rivers are characterized by a highly dynamic response to changes — both natural and human-induced — in their drainage basins. Several braided rivers (i.e., Brenta, Piave, Cellina, Tagliamento, and Torre) originate in the eastern Italian Alps, a tectonically active mountain range where large amounts of coarse sediment are supplied to the fluvial systems. These rivers, like most rivers in Italy (e.g., Pellegrini et al., 1979; Dutto and Maraga, 1994; Rinaldi, 2003; Surian and Rinaldi, 2003; Surian et al., 2009a) and in other European countries (e.g. Bravard, 1989; Wyzga, 1993; Garcia-Ruiz et al., 1997; Bravard et al., 1999; Liébault and Piégay, 2002; Keesstra et al., 2005; Rovira et al., 2005; Kondolf et al., 2007; Wyzga, 2008; Gurnell et al., 2009) underwent major transformations during the last century, mostly as a consequence of human impacts at the basin and channel scales. The general pattern of braided channel adjustment in Italian rivers (i.e., channel narrowing, bed incision, and shift toward a wandering/single thread pattern) as proposed by Surian and Rinaldi (2004) is shown in Fig. 1. Overall, subsequent studies (e.g. Surian et al., 2009a) confirmed that channel evolution model and analyzed more in detail the recent phase of adjustment that took place over the last 15–20 years. While

⁎ Corresponding author. Tel.: + 39 0471 017126; fax: +39 0471 017009. E-mail address: [email protected] (F. Comiti). 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.09.011

the previous phases of channel narrowing and bed incision were common to all the analyzed rivers, the third phase is more complex in terms of processes. Channel widening has become the dominant process in most of the study reaches but channel narrowing is still ongoing in some reaches. Widening is associated with aggradation in some reaches, but the relation between width and bed level changes is not as strong as during the previous phases of narrowing. In fact, channel widening has taken place without significant bed level variations in some reaches (Surian and Cisotto, 2007). Besides, it is still an open question if all the rivers underwent this recent phase of adjustment or, as proposed for rivers in France (Piégay et al., 2009), recent channel changes may be considered short-term fluctuations related to specific flood events, rather than real long-term adjustments. Different human interventions (i.e., sediment mining, channelization, dams, reforestation and control works in steep mountain streams) have been identified as the causes of channel adjustments in Italian rivers (Surian and Rinaldi, 2003; Surian et al., 2009a). Such interventions have caused a dramatic alteration of the sediment regime. Gravel mining was identified as the key driving factor of major adjustments in Italian rivers (Surian et al., 2009a), but it is worth noting that this is not the case for rivers in the French Alps. Hillslope and river corridor afforestation (Liébault and Piégay, 2001,2002; Kondolf et al., 2007) and channelization (Piégay et al., 2009) were considered the main factors controlling channel evolution of French rivers.

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Fig. 1. Channel evolution model for Italian braided rivers: “stage I” represents channel morphology in the early nineteenth or twentieth centuries, “stage IV” represents the present morphology. River channel is represented by dots, and abandoned areas in grey (after Surian and Rinaldi, 2004).

Overall, these recent changes led to the disruption of the very complex channel morphology typical of braided systems, which in turn strongly affected their ecological status (Ward et al., 1999) and their ability to contain flood flows (Wyzga, 1996). Besides channel narrowing and incision, vegetation encroachment took place within the former river channels, and thick riparian woodlands are now present in many locations within the river corridor of these Alpine rivers. Although these riparian forests may represent a “benefit” in terms of overall biological diversity within the river ecosystem, they generate several problems from a hydraulic perspective. The potential drawbacks can be grouped into three categories: (i) increased total roughness in the channel at high flows, possibly leading to more frequent flooding of adjacent lands; (ii) positive feedback on channel incision and scouring at bends from the stabilization of narrow sections; and (iii) delivery of wood elements into the channel, leading to potentially dangerous “plugs” downstream at critical sections (e.g., bridge piers, weirs, narrow cross sections). On the other hand, a higher presence of wood within the channel is known to produce positive effects to aquatic ecosystems (e.g., see Gregory et al., 2003). Across Italy, the increased presence of green patches within river channels as well as the higher presence of stranded wood on bars has been leading river managers — often pushed by local people and municipalities — to frequently cut riparian woodlands and remove trees and logs from the channel. These “river maintenance” activities are being justified as a measure to reduce hydraulic hazards but actually lack any sound scientific approach, and the economic rationale (cost–benefit balance) itself is arguable. Indeed, they can be paralleled to the classical, widespread river engineering practice of sediment bar removal, nowadays recognized to have overall negative effects (e.g., Kondolf, 1994,1997; Marston et al., 2003) and thus adopted only in fast-aggrading reaches posing compelling flood problems. On the contrary, a modern perspective on river management aims at restoring (where possible) natural processes occurring within the fluvial corridor, such as bank erosion and wood input (Piégay et al., 2005; Wohl et al., 2005; Habersack and Piégay, 2008). River restoration projects and environmental restoration as a whole most often tend to recreate some idealized past conditions, reckoned to be “more natural” than the current ones and thus considered worthy to be pursued. Nonetheless, a more pragmatic, objective- and processesoriented approach was recently advocated (Dufour and Piégay, 2009).

Within this context, understanding the extent to which the new forested areas in braided Alpine rivers — within (islands) and along (floodplains and recent terraces) the channel — are actually an “artefact” from recent human impacts (e.g., flow regulation, watershed erosion control projects, sediment trapped by dams, gravel mining) rather than a natural characteristic, and to develop a strategy for their management is of great relevance. However, natural (i.e., not affected by human presence) features in rivers of the European Alps may be almost impossible to retrace, due to the highly impacting millennia-long presence of humans in these regions. Nonetheless, a better understanding of the historical changes undergone by these rivers is much needed for evaluating potential and limitations for channel recovery and a range of different strategies of river restoration and management have been proposed (Surian et al., 2009b). This paper deals with the morphological evolution and the associated vegetation cover dynamics in the intermediate course of the Piave River (within the valley called “Vallone Bellunese”) over the past 200 years. Previous studies have analyzed in this river basin (i) morphological changes of the river channel (Surian, 1999; Da Canal et al., 2007; Surian et al., 2009b); (ii) land use changes within the fluvial corridor and abundance of in-channel wood (Dalla Fontana et al., 2003; Comiti et al., 2006; Pecorari et al., 2007; Comiti et al., 2008; and (iii) sediment budget and sediment transport in the headwater (Lenzi et al., 2006; Mao and Lenzi, 2007). The evolution of this stretch of the Piave River is quite interesting to study because the reach is affected by heavy pressures coming from a dense hydropower scheme and past gravel mining activity, and at the same time features several unregulated tributaries delivering large amount of sediment directly to the reach, such that its large-scale adjustments are of smaller magnitude compared to downstream reaches (Surian et al., 2009b). The novelty of the present work compared to previous papers for similar regulated Italian rivers (e.g., Surian et al., 2009a) mostly regards two points. First, the paper presents a combined analysis of lateral and vertical channel adjustment with vegetation cover and island dynamics; second, the varying channel response exhibited at the subreach scale is analyzed and linked to natural as well as humaninduced factors at this scale. As to the latter point, there is a need, not only in the Italian context (e.g., Piégay et al., 2009), of detailed examples where causes of channel adjustments can be accurately analyzed through detailed reconstruction of channel evolution. As mentioned above, an overall framework of causes of adjustments does exist but a better link between causes and adjustments needs to be established in most cases. The objectives of this paper are to (i) to quantify morphological changes, both in bed planform and in bed elevation; (ii) quantify the variation of vegetation cover, with particular emphasis on islands dynamics; and (iii) identify the driving factors of channel evolution and vegetation cover changes and thus to envisage the most likely future trends. Specifically, the following research hypotheses will be tested: (i) planform (narrowing/ widening) and vertical (incision/aggradation) processes are correlated; and (ii) gravel mining is the main factor driving recent channel and vegetation cover changes. 2. General setting of the study area 2.1. Climatic, geological and morphological setting of the Piave basin The Piave River basin (drainage area 3899 km2) lies in the eastern Italian Alps, and the main channel flows south for 220 km from its headwaters (at ~ 2000 m asl near the Italy–Austria border) to the outlet in the Adriatic Sea NE of Venice (Fig. 2). The climate is temperate-humid with an average annual precipitation of about 1350 mm. However, marked differences in precipitation (ranging from 1000 to 2000 mm) related to elevation and proximity to the sea exists within the basin. Considerable annual variations in the rainfall

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Fig. 2. Location and aerial photo of the analyzed reach in the Piave River basin. The location of the nine considered subreaches is also showed. The lateral borders of the subreaches delimit the morphological fluvial corridor, defined by the presence of terraces or geological constraints.

amount are also present, but significant trends were not observed over the last century (Surian, 1999). The drainage basin is mainly composed of sedimentary rocks (predominantly limestone and dolomite), but volcanic and metamorphic rocks are also present. The river course can be divided into three main segments. The upper segment is mostly incised in the bedrock and presents a narrow single thread channel. In the middle course (which comprises the study reach described below) the gravel riverbed is very wide and characterized by a multithread channel pattern. The lower course features a sand-bed, meandering channel artificially straightened at places. The present physiographic setting of the river results mainly from drainage system evolution during the Late Glacial and the Holocene (Carton et al., 2009). Following the retreat of the Wurmian Glacier, which occurred before 15,000 years BP, a phase of valley aggradation took place in the Vallone Bellunese (the study reach). After this period of aggradation, which lasted up to 8000 ± 9000 years BP (Carton et al., 2009), the river began to incise into the deposits and to form a series of terraces. 2.2. Human impacts within the Piave basin The Piave River has suffered intense and multiple human impacts, which altered the basin and the river channel. The Piave basin has been inhabited since prehistoric times, but the population rose to a significant level — together with forest harvesting, crop cultivation, transport of logs in streams — after the Roman colonization (second century B.C.). After a short period of depopulation and consequent forest expansion after the end of the Roman Empire, a steady decline in forest cover occurred from the late Middle Ages through the Modern Era (Lazzarini, 2002). Forests have probably reached their

minimum extent between the eighteenth and the nineteenth century (Agnoletti, 2000). Natural (and to a minor extent artificial) reforestation has been taking place since World War I but most effectively after the 1950s because of rapid abandonment of traditional farming and cropping activities on the mountain slopes boosted by the development of industry and tourism (Del Favero and Lasen, 1993). Flows in the Piave River have been regulated for irrigation and hydroelectric power generation over a long period. During the 1930s– 1950s, dams were built in many parts of the drainage basin, intercepting sediments from more than 50% of the drainage area. Volumetric measurements of sediment trapped in the reservoirs indicate that the pre-dam total sediment yield was about 1,000,000 m3/year, in contrast to a present estimated value of 145,000 m3/year (Dipartimento Lavori Pubblici and PRASS, 1983; Surian et al., 2009b). The volume of water diverted has increased substantially since the early 1960s. The present regime of water regulation and diversion alters both the flow duration characteristics and volume of annual runoff in the river. Between the 1960s and 1990s, intense gravel mining was carried out in the main channel and in its main tributaries. Unfortunately, no reliable records of excavated gravel are available apart from a few (most likely underestimated) sparse figures. For example, 170,000 m3 of sediment were officially excavated in the upper basin in 1973, 303,000 m3 in 1993, and 348,000 m3 in 1995 (Surian, 1999). Finally, in contrast to the lowest river segment that has been modified and embanked since the early Middle Age, effective erosion and torrent control works (Conesa-Garcìa and Lenzi, 2010) started in the upper basin only in the 1930s, but massively only after the 1970s. However, the millennia-long practice of splash damming in the small tributaries and log rafting from the headwaters to Venice (Caniato, 1993) have likely affected channel morphology and sediment yield

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Table 1 Characteristics of channel subreaches. Subreach

Channel length (m)

Morphology (in 2006)

Max historical channel width (m)

Artificial structures affecting channel morphology

Gravel mining extent (ha) and periods

1

1470

Wandering

694

2 3 4

2650 1960 1840

Braided Wandering Wandering

604 366 744

Groynes built in 1940s; Right bank protection built in 1980s Groynes built in 1940s – Left bank protection

5

1120

252

–

6

3000

Single thread with alternate bars Braided

0.63 1980s and 1990s – – 4.65 1970s to 1990s –

743

–

7

2250

Braided

692

–

8

2050

Wandering

765

–

9

2950

Braided

810

–

through the removal of log jams, snags, and boulders that would have caused inconvenient obstructions to log flotation. 2.3. Study reach The study reach is ~30 km long and is located between Ponte nelle Alpi and Busche (Fig. 2), with a drainage area at Busche of 3174 km2. The morphology of the river in the study reach is dominated by braided and wandering channel patterns, but narrower reaches display an alternate bars channel pattern (Church, 1983). The slope of the study reach is (on average) 0.45%, and the median surface grain size ranges between 20 and 50 mm (Surian, 2002). The present (2006) active channel width ranges between 100 and 1000 m. However, in defining the lateral extent of the study reach, a morphological fluvial corridor has been identified that ranges between 100 and 2000 m, depending on the presence of Holocene fluvial terraces and other geological constraints such as hillslopes and alluvial fans (Surian, 1998). Within the study reach, nine subreaches have been delineated based on homogeneity in river corridor width, presence of artificial elements (i.e., groynes, longitudinal bank protections), historical as well as present morphological pattern (Fig. 2; Table 1). The two lowermost subreaches are located downstream of the largest tributary of the Piave, i.e., the Cordevole River. The subdivision into subreaches is mostly meant to infer how lateral constraints (natural and artificial) affect channel adjustments, whereas the detection of a possible longitudinal pattern of channel variations is hampered by the large natural variability (i.e., from hillslopes and tributaries) within the study reach and by the presence of the Busche weir at the downstream end of the reach, which has fixed the bed elevation since 1960. Because of the unavailability of a single flow data set, flow records for the present study are derived from two gauging stations depending on the period: the hydropower weir of Busche (the downstream end of the study reach, Fig. 2) for records from 1961 to 2007; and Segusino, a natural cross section located 16 km downstream of Busche, for records from 1926 to 1960. Because of the slightly different drainage area between Segusino and Busche (3333 and 3174 km2, respectively), flood peak discharges measured at the former station were reduced applying a corrective factor (0.962), as suggested by Villi and Bacchi (2001). A statistical magnitude–frequency analysis was carried out by using both Gumbel and lognormal distributions, the latter being the better in fitting the observed data. The largest flood event occurred in 1966 and reached almost 4000 m3 s−1, whereas “bankfull” discharge Q2 (RI = 2 years) was calculated to be about 700 m3 s−1, using the entire data set (Fig. 3; Table 2). Even though flow regulation capacity in the upper basin

2.36 1980s and 1990s 0.47 1980s and 1990s 3.57 1990s 3.48 1980s and 1990s

achieved its present level in the 1950s, the Q2 was found (Da Canal, 2006) not significantly different if calculated separately for pre- and post-regulation periods, i.e., using two subsets: 1926–1954 and 1954– 2007. However, higher frequency events (RI b 1.5 years) show a reduction of peak discharge in the post-regulation period (Da Canal, 2006). The selection of 1954 was somewhat arbitrary because of the complexity of the hydropower scheme, i.e., the dams possibly affecting the flow regime were built in different years. However, it was tested how changing the “turning point” year in the range 1950– 1955 does not substantially affect the results. Unfortunately, flow data to construct the entire flow duration curve are not available for the Busche weir and thus it is not possible to analyze the duration for which Q2 is passed each year, as in Piégay et al. (2004). 3. Materials and methods 3.1. Identification of geomorphological and vegetation features from maps and aerial photos Planform changes of river features over the last 200 years were analyzed on three historical maps (1805, 1890, and 1926) and seven aerial photos (1960, 1970, 1982, 1991, 1999, 2003, and 2006). The maps range in scale from 1:25,000 to 1:26,000, whereas the aerial photos range from 1:8000 to 1:33,000. Photos were scanned at a resolution of 600 dpi in order to obtain an average virtual resolution of 1 m or smaller. Digital maps and aerial photographs were rectified and coregistered to a common mapping base at 1:5000 by GIS

Fig. 3. Maximum annual peak discharge (1926–2007) measured at the downstream end of the study reach. Flow discharges featuring recurrence interval RI = 2 years (Q2) and RI = 10 years (Q10) are also shown.

F. Comiti et al. / Geomorphology 125 (2011) 147–159 Table 2 The 10 highest flood events estimated at Busche (downstream end of the study reach). Year

Peak discharge (m3 s−1)

Estimated RI (lognormal distribution, years)

1966 1951 1965 2002 1993 1953 1960 1980 1972 1976

3850 2406 2064 1775 1753 1749 1606 1565 1500 1456

200 30 20 13 12 12 10 9 8 7

software (Esri ArcGIS 9.2). Approximately 30 ground-control points were used to rectify each single frame, and second-order polynomial transformations were then applied, obtaining root mean square errors (RMSE) ranging from 2 to 4 m. The higher RMSE are for historical maps, particularly for the oldest map (1805, scale 1:26,000). Significant planform features were digitized on rectified maps and photos in order to derive planform characteristics for each image. Measurements are affected by errors from rectification and digitization (i.e. features' edge marking) processes. An error assessment was carried out based on (i) RMSE values, which can be an acceptable proxy of the average error of georectification (Hughes et al., 2006); (ii) previous studies that took into account both georectification and digitization errors (e.g., Gurnell, 1997; Winterbottom, 2000; Mount et al., 2003; Zanoni et al., 2008); and (iii) some field measurements with DGPS to assess the position of digitized features. This analysis revealed maximum errors of about 15–20 m and 6 m for measurements on maps and aerial photographs, respectively. Historical maps allowed us to distinguish the boundaries of three basic fluvial features lying within the fluvial corridor (i.e., the area bordered by hillslopes and ancient terraces and thus including active channel, floodplains, and recent terraces): unvegetated active channel, vegetated islands (i.e., shrubby/arboreal vegetation within the active channel), and marginal woody vegetation (i.e., shrubby/arboreal vegetation at the channel margins). Aerial photos allowed the identification of more vegetation classes: islands with arboreal vegetation, islands with shrubby vegetation, arboreal marginal vegetation, shrubby marginal vegetation, and herbaceous marginal vegetation. Furthermore, three additional classes related to human use of the river corridor were adopted: urban areas, cultivated areas, and gravel mines. All aerial photos were taken during low flow conditions. However, given the potential differences in discharge among photos, the distinction between main and secondary channels was not attempted. Instead, the entire unvegetated active channel class was used to describe areas occupied by flowing water during low flows (main and secondary channels) and exposed and unvegetated surfaces (i.e. bars) next to the channels, i.e., to represent the whole area inundated and subject to bed mobilization during frequent floods (RI =1–2 years). Bars covered with herbaceous vegetation were thus assigned to this active channel category. In the aerial photos, evidence of canopy texture, shape, and shadows were used to estimate vegetation height and thus to differentiate between arboreal and shrubby vegetation classes. A height of about 4–5 m was assumed to separate the two classes. An arboreal island was defined as a distinct vegetated area surrounded by the active channel having at least 60% of its surface occupied by arboreal vegetation (i.e., an arboreal island can include portions of herbaceous or shrubby vegetation). If the surface covered by trees is b60%, the area was classified as a shrubby island. 3.2. Topographical data: cross sections and LiDAR Nine cross sections surveyed in 1929 within the study reach by the “Magistrato alle Acque di Venezia” (the former management agency of the river) were acquired for the analysis of long-term bed level

151

changes (Fig. 2). These cross sections were resurveyed in 2007 using a DGPS system, with a GPS vertical error b2 cm. The 2007 ellipsoidal elevations (WGS-84 reference system) were then converted to orthometric elevations (i.e., to the same datum used in 1929 surveys) using the software VERTO2 provided by the Italian Military Geographical Institute (IGM), whose precision is claimed to be ±4 cm. In addition, the topographic surveys carried out by the “Genio Civile di Belluno” in 1985, 1991, and 1996 at 13 cross sections (Fig. 2) were also acquired and used to infer recent bed elevation variations. Unfortunately, only two of these cross sections correspond to those surveyed in 1929 and 2007 (Table 3), i.e., a complete set entailing both long- and short-term variations is available only for two sites. In order to determine how channel bed evolved after 1996 at these cross sections for which a DGPS survey of 2007 was not available, we used an airborne LiDAR survey (filtered point density of 1–2 m−2)) carried out by the “Autorità di Bacino dell'Alto Adriatico” during fall 2003 (adopting orthometric elevations, estimated vertical error ± 20 cm). The DTM was created at 0.5 m resolution using the tool “3D-analyst” of ArcGIS 9.2, and cross sections were then extracted from the DTM in correspondence to those of the 1980s–1990s. Even if LiDAR-derived cross sections may suffer from the inability to correctly represent inundated areas, the actual survey was carried out during low flow conditions such that only a minor fraction of each cross section is not correctly captured. For each available cross section dating from 1929, 1985, 1991, 1996, 2003, and 2007, the mean elevation of the active channel was calculated excluding floodplains and islands, identified either on survey point description (for 1929 and 2007) or on visual identification from aerial photos (for the others). In the subsequent analysis, cross section elevations from 2003 (LiDAR) and 2007 (DGPS) will be used as a reference to the “present day” conditions, because between these two surveys no relevant flood events occurred (Fig. 3) and the active channel width did not show marked variations (Fig. 4). Preference to 2007 elevation data (where available) will be granted because of the possible error associated to the missing sensing of the low flow channel thalweg. Table 3 summarizes all the cross sections used and the subreach they belong to. The longitudinal distance from the upstream end of the study reach (bridge in Ponte nelle Alpi, Fig. 2) is also reported. 3.3. Geomorphological field surveys Field surveys were carried out in 2006 and 2007 using standardized forms specifically designed to record measurements and Table 3 List of the cross sections used and their survey year. XS

Distance from upstream (km)

Year

1.26 2.84 3.99 5.21 8.00 9.74 10.54 10.98 11.68 12.96 14.56 15.19 17.45 19.45 21.98 23.35 28.12 28.69 29.01

1929, 1985, 1929, 1985, 1929, 1985, 1929, 1929, 1985, 1929, 1985, 1929, 1929, 1929, 1985, 1929, 1929, 1929, 1985,

Subreach

code S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9 S-10 S-11 S-12 S-13 S-14 S-15 S-16 S-17 S-18 S-19

2007 1991, 2007 1991, 2007 1991, 2007 2007 1991, 2007 1991, 2007 1985, 1985, 1991, 2003, 1985, 1985, 1991,

1996, 2003 1996, 2003 1996, 2003

1996, 2003 1996, 2003 1991, 1991, 1996, 2007 1991, 1991, 1996,

1996, 2007 1996, 2007 2003 1996, 2007 1996, 2003 2003

1 1 2 2 3 3 – 4 4 5 5 6 6 7 8 9 9 9

152

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4.1. Changes of active channel and vegetated areas in the whole study reach

being at its lower extent at the end of nineteenth century (about 20%) and afterwards extending until 1991 (Fig. 4), i.e., channel area no longer active was colonized by woody vegetation (mostly by Salix eleagnos, Salix alba, Populus nigra, Alnus incana, with occasional Pinus sylvestris). Similarly, the increase in active channel area that took place between 1991 and 2003 occurred mostly at the expense of vegetated areas located at the channel margins. Restricting the analysis to the period covered by aerial photos (i.e., only from 1960 onward), distinguishing the different variations of shrubby vegetation (i.e., pioneering stages comprising actual shrubs and young trees) vs. tree (i.e., established, older stages) extensions was possible. The extension of shrubby vegetation does not reveal any significant trend, whereas the arboreal vegetation area shows a clear increasing trend until 1999 and a subsequent reduction until 2006, whose extent is still twice that measured in 1960 (graphs not shown). Islands (including those covered by either shrubs or trees) were present in the study reach since the early 1800s, but their overall extension was always much smaller than marginal vegetation (Fig. 4). Also, they show a less pronounced temporal variation compared to the latter, with a maximum relative extension occurring in 1960 and a minimum in 1890. During the narrowing phase (1970–1991), the proportion of island area within the river corridor increased only slightly, as opposed to the remarkable expansion of surfaces covered in marginal vegetation. The channel expansion stage that occurred between 1991 and 2003 shows a similar reduction in island relative extension, reaching a final value similar to 1970. In contrast, marginal vegetation in 2003 still shows a much higher proportion compared to 1970. In order to analyze in greater detail the dynamics of islands, the ratio between islands and unvegetated active channel areas is considered (Fig. 5A). Apparently, the relative extension of islands calculated as such increased progressively during the whole first

The analysis of the historical maps and aerial photographs shows that substantial changes took place in the Piave River within the investigated time interval. The extension of the unvegetated active channel was at its highest at the end of nineteenth century (1900 ha in 1890, i.e., about 80% of the entire fluvial corridor; Fig. 4). During the twentieth century, the active channel progressively reduced, reaching in 1991 an extension of approximately one-third of the corridor area. The active channel area reduced in two different stages (Fig. 4). A first phase of adjustment took place during the first half of the twentieth century and was characterized by a loss of about 35% of the initial active channel area, at a rate of about 11 ha/year. The mean active channel width, calculated as the ratio between channel area and reach length, narrowed at an average rate of 3.8 m/year. This trend was interrupted by the high magnitude/low frequency (RI ~ 200 year; see Table 2) flood event that occurred in 1966 (Fig. 4), with a possible contribution by the 1965 event (RI ~ 20 y; Table 2, characterized by a long duration) that determined an abrupt channel enlargement between 1960 and 1970 (205 ha, 68 m as average channel width). The subsequent narrowing phase (documented from 1970 to 1991) was more intense, occurring at a rate of about 32 ha/year (10.6 m/year in terms of channel width). In the following period (1991–2003), a reversal occurred with an evident sharp widening tendency that extended the active channel at a rate of 27 ha/year (9 m/year). Finally, no substantial variations occurred between 2003 and 2006. As to the morphological pattern of the entire study reach, this shifted from braided (still dominant until the 1960s) to single thread/ wandering in the 1990s. The expansion phase of the last decade is associated with a general recovery of at least a wandering style, with occasional braiding morphology. Complementary to the trends in the active channel, the proportion of the fluvial corridor covered by vegetation at the channel margins has experienced a significant change over the period 1805–2006,

Fig. 5. (A) Temporal variation of the ratio between the area of islands and active channel in the whole study reach. The arrows indicate major floods and their recurrence interval. (B) Variation of number and average area of islands.

Fig. 4. Proportion of the fluvial corridor occupied by the unvegetated active channel, marginal vegetation, and islands throughout the last two centuries. Flood events that exhibited relevant effects in terms of channel adjustments are marked.

observations of channel changes (Rinaldi, 2008). Data collected through such surveys integrate those coming from the previously described methods (planform changes and topographic surveys) and proved useful to infer direction and approximate magnitude of bed level changes. The geomorphological surveys allowed to infer both long- and short-term channel adjustments according to several morphological and sedimentological features, such as differences in elevation between higher bars and gravel in floodplains/recent terraces, presence/absence of sediment lobes, or presence/absence of bed armouring. 4. Results

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phase of channel narrowing until 1960. The occurrence of the major flood in 1966 (RI ~ 200 years) caused the ratio to drop significantly in 1970 (from 0.18 in 1960 to 0.06 in 1970). The ratio of islands over unvegetated active channel areas subsequently feature a fastincreasing trend until 1991 (i.e. during the most intense channel narrowing when it reached the maximum value of 0.24) and then a further drop until 2003 as a result of 1993 and 2002 floods (both RI ~ 10 years). Finally, the mentioned ratio remained constant around 0.07 between 2003 and 2006. Different and complementary information can be gained from the analysis of the variation of the number and average extension of islands (Fig. 5B). Both showed similar values in the 1800s and in 1926, with around 30 islands (1 island/km of channel length) and 4.5 ha, respectively. Subsequently, the first phase of channel narrowing (until 1960) exhibits a fragmented islands pattern, i.e., islands are numerous (about 240, ~ 8/km) but small (~0.8 ha). The 1966 flood eroded most of such relatively small islands, halving their number without substantially modifying their area. A different, abrupt variation characterized the beginning of the second phase of channel narrowing, when islands became relatively fewer but considerably larger (from 0.6 to 1.5 ha) in just one decade (1970–1980). The following decade of narrowing did not entail substantial changes for islands, until the

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floods of 1993 and 2002 caused a reduction of their number (to about 80, i.e., 2.7/km) and of their average area (0.8 ha) (Fig. 5B).

4.2. Changes of active channel and island area at the subreach scale An analysis of the surface variations reported above for the entire study segment was carried out individually for all nine subreaches shown in Fig. 2. As an example, Fig. 6 shows the vegetation cover evolution of subreach 9 over the last 200 years. In general, similar patterns can be observed for all subreaches (images not reported). Nonetheless, changes in the relative channel extension (i.e., compared to subreach corridor area) reveal that different portions of Piave River underwent adjustments of varying magnitude (Table 4). The temporal changes of the extent of active channel area are shown in Fig. 7A for subreaches 1, 3, and 8, selected as representative for their distinct trends of adjustment. The trends for all subreaches were analyzed, but they are no reported in Fig. 7 in order to make it more readable. Subreach 1 shows the overall lowest planimetric variations, for contraction (after subreach 7) and for expansion. In particular, this latter process is poorly observable here (2.6% after the 1991 photo); and also, the 1966 flood (see variation between 1960

Fig. 6. Planform evolution of subreach 9 from 1805 to 2006. The classification of fluvial features is simplified in 1805, 1890, and 1926 because it is derived from the analysis of historical maps. Aerial photographs, which allow more detailed interpretation of vegetation and land use, have been used in the subsequent years (1960 to 2006).

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Table 4 Summary of vertical and planform adjustments for the nine subreachesa. Vertical (m) Subreach

1 2 3 4 5 6 7 8 9

1929–1991

Planform (%) 1991–2007

XS surveys

Field indicators

XS surveys

Field indicators

−2.5 − 0.5 −1 − 1.2 −0.75 −0.75 − 0.6 −1.3 − 1.3

I I I I E E I I NA

−0.4 0.5 0.1 1.5 0.1 0.1 − 1.1 −0.6 − 0.2

E/A A E/A A A E/A A A E

1926– 1991

1991– 2006

−50.8 −60.9 − 70.4 − 59.2 −56.0 − 61.4 − 40.7 −58.4 − 64.3

2.6 59.6 130.7 46.4 59.3 58.2 56.6 80.4 35.7

a Negative values represent incision/narrowing; positive values widening/aggradation. % planform variations are calculated as channel width change divided by the former width (i.e., 1926 and 1991 in the two cases). Field indicators represent the synthetic results of the geomorphological surveys: (I) incision, (E) equilibrium, (E/A) equilibrium/aggradation, (A) aggradation, (NA) not available.

and 1970) caused only a limited widening. Notably, part of this reach was influenced by groynes built in the 1940s and by longitudinal bank protection on the right bank built in the 1980s (Table 2). In contrast, subreach 3 features the highest variations for narrowing (70%) and widening. During the recent expansion phase (1991–2003), the channel widened doubling its extent (from 88 to 203 m); and also, the widening between 1960 and 1970 is the largest experienced by a single subreach. An intermediate response to flood events is evident for subreach 8, where contraction and expansion are about 64% and 36%, respectively (Table 4).

Fig. 7. Patterns of adjustments occurred at the subreach scale: (A) active channel areas relative to the fluvial corridor extent; (B) island area relative to the active channel area. Only three representative subreaches are shown for clarity.

The variation of islands — in terms of island-to-active channel areas — among the different subreaches indicates (Fig. 7B) that their maximum relative cover did not coincide with the apex of narrowing (i.e., to 1991, as in Fig. 5A) in all the subreaches (see subreach 3 in Fig. 7B). Also, some subreaches, which apparently did not feature islands in the nineteenth century, reached considerable island cover in the first half of the twentieth century (e.g., subreaches 1 and 3), even though a direct comparison between maps and aerial photos should be considered just indicative at this small spatial scale. However, the dramatic island erosion effects of the 1966 flood is evident in all the subreaches, whereas the consequences of the 1993 and 2002 flood events in terms of island erosion vary (Fig. 5A). In fact, some subreaches (e.g., 3 and 8) even show a slight increase in island cover between 2000 and 2003. 4.3. Bed level changes Fig. 8 shows the variation of mean cross section elevations (as described in Fig. 3 and reported in Table 3) taking as a reference the most recent value, i.e., either the 2007 DGPS survey or the 2003 LiDAR-derived DTM. The use of either the 2003 and 2007 cross section to serve as a reference to present bed level has been proved to be accurate enough because of the lack of relevant flood events that occurred between the two years (see above) as well as of anthropic modifications of the bed (i.e., gravel mining). When both cross section measures are available, their mean elevation difference is always b20–30 cm, possibly because of the inability of LiDAR to detect waterfilled channels. As to the long-term bed changes, bed elevation in 1929 was about 1 m higher than present days; but the difference is N2.5 m at the very upstream limit of the reach, and it is negligible just upstream of the confluence with the Cordevole (cross section distance of 19 km). The recent bed level adjustment trends (i.e., after 1985) appear rather more complex. Unfortunately, the available data do not allow any precise analysis of incision trends until 1985. The 1966 flood events determined an abrupt channel expansion (see previous section) and most likely caused a moderate aggradation in the study reach, as chronicles suggest. A combined analysis of present cross sections and vegetation colonization, supported by a photogrammetry-derived DTM (even though quite rough) from the 1970 aerial photos (Susin, 1975) indicate that the mean bed elevation in the late 1960s was fairly similar to that surveyed in 1929, i.e., most of the incision measured by comparing cross sections in 1929 and 1985 started in the 1970s (see Discussion). However, we are not able to determine how the channel elevation varied from 1929 to 1966, i.e., if incision took place along

Fig. 8. Variation of average bed elevation as derived from the comparison of cross sections (Table 3) along the longitudinal distance of the study reach. Each cross section is plotted according to its distance from the upstream reach limit (Ponte nelle Alpi). Positive values indicate XS where the streambed at that time (i.e., 1929, 1985, 1991, or 1996) was higher than at present (i.e., where incision took place) and negative values the opposite. The dashed lines indicate the error that is associated with data comparison (± 0.4 m).

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with channel narrowing. Besides, analyzing the trend at each cross section among the 1985, 1991, and 1996 surveys only, the year of lowest bed elevation turns out to be not the same along the study reach, generally varying between 1991 and 1996. Looking at the recent bed adjustments, most cross sections show 1990s bed elevation quite similar to present levels, estimating that an overall error of up to ±0.3–0.4 m (marked in Fig. 8 as dotted lines) could occur when comparing surveys of different sources and methods. However, three cross sections located upstream of the confluence with the Cordevole exhibit substantial lower elevations (up to 1.5 m, thus hinting to a recent phase of aggradation); whereas those just upstream and downstream of the Cordevole show higher elevations, up to 1.2 m, indicating a recent incision phase. However, the elevation of the three sections at the downstream end of the reach should be examined with caution because of their proximity to the Busche weir, which may have likely induced local aggradation at different times from backwater effects. Results from geomorphological surveys (Table 4) are in good agreement with those from cross section comparisons (one to three surveys were carried out for each subreach). As to long-term changes, incision occurred in all the subreaches, except for two of them (subreaches 5 and 6) that show no significant variations (i.e., bed level stability). In contrast, the very recent channel bed evolution is characterized by sedimentation or equilibrium. Notably, the two methods employed for assessing bed level changes may lead to slightly different results. For instance, short-term analysis from cross sections refers to the 1991–2003/2007 period, whereas results from geomorphological surveys may reflect channel response to very recent flood events (e.g., the 2002 flood as well as the subsequent smaller yet “formative” events). 5. Discussion 5.1. Is there any correlation between planform and vertical adjustments? As stated in the introduction, two goals of the present paper were to (i) establish whether planform and vertical adjustments are correlated and (ii) identify the driving factors of channel changes, that are the roles played by flows, sediments, and vegetation. Table 4 presents a summary of vertical (in terms of mean bed elevation) and planimetric (in terms of channel width) variations at the subreach scale divided into two periods: from the 1920s–1930s to 1991 (dominant narrowing) and between 1991 and 2006 (dominant expansion). Subreaches where actual cross sections data did not allow direct estimation of vertical variations were assigned a value based on the average of the two closer cross sections. The changes in mean bed level and active channel width (Table 4) were tested for statistically significant correlation using the Spearman ranking approach. Results show that vertical and lateral adjustments for the nine subreaches during the narrowing period are not correlated (nonsignificant R Spearman; p N 0.10), even though a direct relationship between narrowing and incision is weakly apparent (graph not shown). This means that subreaches that experienced the largest contraction did not necessarily undergo the largest incision (e.g. subreaches 2 and 6; Table 4). However, incision is associated with channel narrowing in all the subreaches. As mentioned before, there is evidence indicating that the great extent of the incision measured between 1930 and 1991 actually started only in the 1970s, after the likely aggradation of the 1966 flood event. Therefore, the correlation between incision and channel contraction in the period 1970–1991 was explored, but no significant results were obtained. As to the recent channel adjustments (1991–2006), the nonparametric correlation analysis shows an even weaker statistical link (p ≫ 0.10) between elevation changes and expansion. In fact, several subreaches (e.g., 3 and 8; Table 4) feature a considerable expansion of the active channel despite a very moderate aggradation or even in the

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presence of incision (e.g., subreaches 1, 3, 7, and 9). In contrast, subreach 4 features the largest aggradation value, but its widening is intermediate (Table 4). This could be partly due to the small magnitude of bed changes compared to the inherent measurement errors, but it might also indicate that the lateral channel mobility needed to re-enlarge the active channel by bank erosion can be restored by just stopping further incision — in the case of the Piave River by banning in-channel gravel extraction in the late 1990s — even without notable sediment deposition. As a consequence, the first hypothesis should be rejected. The only factor correlated (Spearman R = −0.65, p b 0.10) to the recent expansion rate is the “initial” (in 1926) channel width of subreaches, i.e., narrower reaches have expanded more intensely. No correlations were found instead for the narrowing rates. Overall, even though a general temporal correlation in the occurrence of narrowing and incision has been documented, their process intensities (i.e., rates) were not correlated (i.e., spatial analysis for the different subreaches). The lack of correlation might be partly caused by the different spatial sampling error associated to cross sectional and planimetric variations at the subreach scale, and more importantly by the time lag between the steadier vegetation growth (i.e., causing the reduction of active channel area) compared to bed incision processes that take place only during sedimenttransporting flood events. As to expansion–aggradation processes, not only do their intensities show no correlation, even the actual processes do not seem to take place simultaneously. 5.2. Which are the main drivers of channel changes ? 5.2.1. Bed incision or vegetation encroachment first ? Between the 1970s and 1990s, fast vegetation expansion, morphological shifts from braiding to wandering/single thread styles, and bed incision occurred together in the study reach. Now the question is: which came first? Vegetation or incision? And how did they interact? In other words, is the expansion of woody vegetation the driver of the important morphological changes that occurred in the Piave River ? One may in fact hypothesize that the flow regulation in the Piave River led to vegetation encroachment on gravel bars by reducing inundated areas, as it occurred in the Waitaki River (Hicks et al., 2008). There, the increased bank resistance caused by woody vegetation was deemed responsible for the reduction of braiding intensity, which leads to the concentration of flood flows in fewer channels with higher erosive power (Tal et al., 2004; Tal and Paola, 2010). For our study case, in order to provide a definite answer we would need much more frequent cross section surveys and aerial photos starting from the 1970s, which would allow us to determine the possible temporal shift between channel incision and vegetation establishment, or the opposite. In fact, vegetation reached its maximum (and channel its minimum) extent in the 1991 aerial photos in almost all the reaches (Fig. 7); whereas the year of survey (1991 or 1996) featuring the lowest bed elevation varies along the study reach as mentioned in the previous section. Nonetheless, what is relevant to observe is that bed incision in 1985 was generally already advanced and, in many cases, very similar to 1991–1996 values (Fig. 8). The solution to the question “vegetation vs. incision” can be tackled by a simplified calculation of the sediment eroded within the study reach between the early 1930s and late 1980s, i.e., the available years for which topographic surveys are available to document the incision phase. Considering an average bed incision of 1 m, and a mean active channel width of 300 m (value for the 1980s), this translates into a sediment erosion of 300 m3/m of channel length, i.e., 9 106 m3 for the 30 km-long-study reach. This value is more than two orders of magnitude larger than the annual bedload transport supplied to the reach under a “natural” regime (i.e., without dams

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in the upper basin as before the 1950s), which can be estimated to be about 6 104 m3. This value is based on mean total sediment yield (200 m3 km−2 year−1, Surian et al., 2009b), basin area (about 2000 km2 at the upstream end of the study reach), and assuming bedload to represent about 15% of the total sediment yield (Surian et al., 2009b). The comparison between eroded sediment and bedload annual yield implies that — assuming a total absence of sediment input to the reach from interception by dams and from possible reduction in the duration of transporting flows and an annual sediment transport capacity in the reach still equalling the estimated bedload supply — channel downcutting should have started N100 years before the 1980s. Instead, as previously described, evidence indicates that most of the incision only started in the 1970s, after the large 1966 flood which aggraded the channel and removed a great deal of vegetation. Because vegetation has re-established on gravel bars after the 1966 flood (as shown by the 1970 aerial images), and taking into account that shrubs and trees would take at least 5 years to impart significant effects on channel roughness and bank stability (e.g., Hicks et al., 2008), vegetation might have started to exert a morphological influence only in the late 1970s. Therefore, it is not possible that the bank strengthening effect of woody vegetation be responsible for the observed incision starting in the 1970s and peaking in the late 1980s– early 1990s,,because the time span between the early 1970s (vegetation establishment) and 1985 (incision already advanced) is too short, i.e., it would imply an unrealistically large annual sediment transport capacity to flush the “missing” sediment out of the study reach by fluvial processes only. Furthermore, the assumption of negligible sediment input from upstream is not actually true for this reach (see Surian et al., 2009b), thus an even longer time would be necessary to match the observed incision. 5.2.2. Gravel extraction as the main driver of channel changes in the Piave River Assuming an annual gravel extraction of 300,000 m3 for the period 1970–1990 (see the official gravel mining data reported in the “study site” section, very likely underestimated), the total extracted volume turns out 6 106 m3, i.e., of the same order of magnitude of the “lost” sediment associated to incision. Taking into account the considerations presented above about the unlikely role of vegetation, we argue that gravel extraction, which started in the 1970s and reached its peak during the 1980s, is most likely responsible for most of the bed incision in the study reach. This is also indicated by a significant correlation (Spearman R = 0.72, p b 0.10) between gravel mining areas (summed up from the 1970s to the 1990s, Table 1) and incision at the subreach scale, if subreach 1 (the one featuring the highest incision) is excluded. Its exclusion is justified by the relevant effect of artificial narrowing because of groynes and bank protection, which is likely co-responsible for such a strong incision. Apparently, bed incision did not propagate much from its original location, possibly because of the effect of sediment supplied by lateral tributaries along the reach along with the relatively small depths of gravel pits and large channel widths. Therefore, we believe that the second hypothesis stated in the introduction, i.e., gravel mining within the study reach is the key factor driving recent channel and vegetation changes, is true. The presence of hydropower dams in the upper Piave basin likely exerted only a secondary effect. However, dams may represent the most relevant impact and limiting factor for channel recovery in the long run (now that gravel extraction has virtually ceased), as argued by Surian et al. (2009b). 5.3. Summary of channel evolution and driving factors over the last 200 years The analyzed reach of the Piave River was characterized by remarkably wider extensions of its active channel area during the

nineteenth century (Fig. 4) and this evidence can be taken as representative of conditions of unregulated basins in terms of hydrology and sediment transport. However, this wider active channel may also reflect a higher human pressure on basin land use because at that time the forest cover was at its historical minimum (see Section 2.2) and sediment supply from the basin was likely at its maximum. Possibly, a larger sediment supply during the nineteenth century was partly from the effect of the Little Ice Age, i.e., lower timberline and higher glacial activity (Bravard, 1989). During the twentieth century, two phases of channel narrowing were identified in the study reach as in other Italian rivers (Rinaldi, 2003; Surian and Cisotto, 2007; Surian et al., 2009a). The channel narrowing that occurred during the first half of the 1900s was caused by an array of factors whose single relevance is difficult to estimate, such as river training structures (i.e., the perpendicular groynes built in the 1940s, likely the dominant factors in the associated subreaches) and land use variation at catchment scale (i.e., increase in forest cover on slopes). However, also the milder climate following the Little Ice Age might have contributed to a natural reduction of sediment supply and thus, partly, to channel narrowing. We do not have evidences that this phase of narrowing was associated with bed incision, therefore it is possible that narrowing occurred without incision, as shown in several French rivers (Liébault and Piégay, 2002; Rinaldi et al., 2010), or, alternatively, that very slight incision occurred, as documented in some Italian rivers (Surian and Rinaldi, 2003, 2004). The second phase of narrowing was relatively short (approximately from 1970 to 1990) but very intense and associated with channel degradation. As discussed above, gravel mining is the main driver of incision and narrowing during this phase, and vegetation encroachment is a consequence of incision. This confirms the key role of sediment mining in Italian rivers (Surian et al., 2009a) and highlights some differences with other Alpine regions where afforestation in the floodplains (following changes in the land use) is considered, along with sediment mining, a major cause of channel incision (Liébault and Piégay, 2002; Rinaldi et al., 2010). The fact that bed incision in the study reach is about 1 m, i.e., smaller than in other alluvial rivers (Surian and Rinaldi, 2003; Surian et al., 2009a) may depend on two aspects. First, the channel width is quite large, such that even a minor bed change corresponds to large sediment volumes (see Section 5.2.1). It is also common that width changes are much more intense than depth changes in rivers with high width/depth ratio (e.g., Surian and Rinaldi, 2003). Second, the sediment supply from the lateral tributaries within the reach — being the upper basin mostly dammed — likely contributed to prevent more substantial bed incision. As to the most recent channel evolution, Surian and Rinaldi (2004) identified a phase of channel widening in several Italian rivers and Surian et al. (2009a) pointed out that such phase is often associated with aggradation, even if it can also occur without significant bed level changes. In our study site, recent channel adjustments are complex and spatially variable, such that a clear trend is less obvious. Even though a general channel widening is apparent in the study reach between the 1990s and 2006, no clear indications of a widespread, concomitant aggradation phase are observed, confirming that widening is taking place without any aggradation in some reaches (see Section 5.1). Taking into account the magnitude and duration of this widening phase, we suggest that it should represent a real phase of adjustment rather than short-term morphological changes from the occurrence of episodic, larger flood events. Again, the comparison with alpine rivers in southeastern France is of great value. In French rivers a widespread channel recovery was not observed, probably because after the main phase of incision (peaking in the 1970s) the river channels are still adjusting to the decrease of sediment supply from catchment afforestation (Liébault and Piégay, 2002; Rinaldi et al., 2010). On the contrary, the Piave River in the study reach underwent a remarkable morphological recovery (mainly by widening) thanks to

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the sediment input from the tributaries once the major degrading factor, i.e., gravel mining, ceased. However, this recover will likely be constrained in the near future by the present condition of the catchment (dams, increased forest cover, torrent control works). 5.4. Variation of island cover In the study reach of the Piave River, islands extent relative to channel area apparently increased (Fig. 5A) from the end of the nineteenth century to the 1960s, possibly as a consequence of reduced sediment transport from land use variations (see previous section) and reduced pressure of animal grazing and fuel wood removal (as described by Liébault and Piégay, 2002 for French rivers). The extreme 1966 event (RI ~ 200 years) dramatically reduced the island-tochannel area ratio to its lowest values, which then soared in concomitance with channel narrowing and bed incision. Subsequently, floods in 1993 and 2002 (RI = 10–15 years) determined relevant drops in island cover as well as erosion of marginal vegetation. These observations match with those of Bertoldi et al. (2009) in the Tagliamento River, a braided river similar for many aspects to the Piave River, but featuring virtually unregulated flow and sediment regimes. In fact, in the Tagliamento River the island dynamics were found to be strictly associated to the occurrence of major floods (RI N 10–15 years), which are the only ones able to determine substantial island erosion. As to the relationship between island size and number, a sharp difference is evident when comparing different periods in the Piave reach (Fig. 5B). The increase in island area observed between 1926 and 1960 took place by the establishment of many small islands, whereas total islands area increased by coalescence of formerly separated island during the most intense narrowing phase accompanied by bed incision (1970–1991). The process of island coalescence was described also for natural island-braided systems (Gurnell and Petts, 2002, 2006) as well as in flume experiments simulating vegetation growth in braided channels under altered flow regimes (Tal and Paola, 2010). Unfortunately, we believe that the interpretation of island dynamics (in terms of islands number and mean area) can be reliably carried out only for the trend starting in 1960, i.e., topographical maps very likely did not represent small islands and thus these parameters (nonetheless reported in Fig. 5B for the 1805 and 1926 maps) feature a large uncertainty. Using the 1960 as the best proxy for unregulated river conditions (i.e., before the major alterations from gravel mining and dams), we infer that the Piave River was (under the conditions of basin land use existing at that time) characterized by many, small islands. Notably, a large (RI ~ 30 years, Table 2) flood event occurred in 1951, and many islands in the 1960 aerial photo appear to be rather young and thus successive to such event. Caution must be used in picking such a configuration of island number/size as representative of “natural” conditions, because island dynamics in similar rivers follow a complex temporal evolutions (Gurnell and Petts, 2002, 2006). Therefore, the outcome of the present study is that islands — as well as patches of arboreal vegetation at channel margins — were present in the study reach also before any human-induced channel alteration, thus their presence should be viewed as “natural” there. However, the large extent of single islands and the large total island area during the 1990s was a consequence of an altered sediment regime, because it was determined by gravel mining leading to bed incision, in turn causing vegetation establishment and accelerated island coalescence. The envisaged differences in island dynamics between unaltered and incised (from active mining) large gravel bed rivers are depicted in Fig. 9, which builds on the previous conceptual model put forward by Gurnell and Petts (2002) and on evidence from the present study reach (e.g., evolution maps such as that reported in Fig. 6 as well as results in Fig. 7). However, the graph is speculative as to how the

Fig. 9. Conceptual models of island dynamics in natural (A) and incised from gravel mining (B) large gravel bed rivers. Vertical bars represent the magnitude of annual flood peak discharge, solid lines the average island area and dashed lines the number of islands within a certain reach. The pattern depicted in (A) is based on that proposed by Gurnell and Petts (2002) and intends to represent all types of tree generation (i.e., living wood and seeds). The pattern for braided rivers subject to incision from gravel mining (B) partly derives from observation in the study reach (see Figs. 6 and 7) but is also speculative because here gravel extraction is imagined to continue throughout the analyzed period, such that a wandering/single thread morphology becomes established. In (B), phase “a” represents an accelerated formation of islands from vegetation encroachment on higher bars, phase “b” corresponds to the merging of these islands into fewer larger ones. The moderate flood event towards the end of “b” is able to cause only some minor island erosion. Phase “c” represents the incorporation of some large islands within the floodplain, thereby only few island may still persist. However, the occurrence of a major flood event (end of “c”) causes the dissection of part of the floodplain creating newer large dissection islands. Overall, the number of islands is lower, and the average island area is higher, for an incised braided river than for one under a natural sediment regime.

variation in island number/area would have been in the study reach should gravel extraction have persisted. In fact, in the Piave River the cease of gravel mining in the 1990s led to the channel recovery described above, which is likely the cause for the reduction in relative island cover and in both island number and mean area, suggesting a phase of net island removal. 6. Conclusions The natural interplay among water flow, sediment transport, and vegetation in natural, large, gravel bed rivers is highly complex. When fluvial systems are affected by human impacts, as often happens, it becomes even more challenging to disentangle the causes of channel evolution over time. This study in the Piave River provides some insights into both the long-term and short-term morphological dynamics of a typical Alpine river subjected to heavy anthropic influence. The main findings are: (i) alteration of sediment supply rather than flow regime appears to be the key factor of channel adjustments; and (ii) high rates of channel narrowing and expansion are not necessarily linked to substantial bed incision or aggradation. However, bed incision from gravel mining does imply an extraordinary vegetation encroachment (and thus channel narrowing) and only intense, infrequent flood events (RI N 10–15 years) are able to determine substantial island erosion. However, once gravel mining stops, channel widening starts again, as observed in several Italian rivers. As put forward in Surian et al. (2009b), this reach of the Piaver

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River seems to have the potential to recover further its morphology over the next future (i.e., next 30–50 years), even though no restoration actions are carried out, provided that gravel mining remains banned. However, we expect the reach cannot achieve channel widths and morphologies comparable to the pre-1950s because the present sediment supply is greatly limited by dams in the upper basin and by higher forest cover and denser presence of torrent control works. The channel expansion at the expense of forested margins and islands poses several river management issues because, in the near future, bank erosion and in-channel wood are likely to increase. Management strategies should take into account the negative effects that may be due to those processes, but also the benefits brought about by bank erosion (e.g., sediment and wood supply). Acknowledgements This research was partially funded by the EU project INCO-CT2004-510735 “Epic Force” (Evidence-based policy for integrated control of forested river catchments in extreme rainfall and snowmelt) and PRIN 2007 project, “Present evolutionary trends and possible future dynamics of alluvial channels in northern and central Italy”. Support for field surveys were also provided by the University of Padova project, “Channel adjustment and restoration in response to human alterations, wood and sediment fluxes in gravel bed rivers”, No. 60A081729/08 and by the University of Padova Strategic Project “GEORISKS, Geological, morphological and hydrological processes: monitoring, modelling and impact in the north-eastern Italy” Research Unit STPD08RWBY-004. The “Autorità di Bacino dei fiumi dell'Alto Adriatico” is kindly thanked for providing LiDAR data. All colleagues and students who helped in the field are greatly thanked. Finally, we greatly thank the four reviewers whose suggestions greatly improved the original manuscript. References Agnoletti, M., 2000. Il bosco in età veneziana. In: Bondesan, A., Caniato, G., Vallerani, F., Zanetti, M. (Eds.), Il Piave. Cierre Edizioni, Verona, pp. 259–272. Bertoldi, W., Gurnell, A., Surian, N., Tockner, K., Zanoni, L., Ziliani, L., Zolezzi, G., 2009. Understanding reference processes: linkages between river flows, sediment dynamics and vegetated landforms along the Tagliamento River, Italy. River Research and Applications 25, 501–516. Bravard, J.P., 1989. La metamorphose des rivieres des Alpes francaises a la fin du Moyen-Age et a l'epoque moderne. Bulletin de la Societe Geographie de Liege 25, 145–157. Bravard, J.P., Kondolf, G.M., Piégay, H., 1999. Environmental and societal effects of channel incision and remedial strategies. In: Darby, S.E., Simon, A. (Eds.), Incised River Channels: Processes, Forms, Engineering and Management. Wiley, New York, pp. 303–341. Caniato, G. (Ed.), 1993. La via del fiume. Cierre, Verona. Carton, A., Bondesan, A., Fontana, A., Meneghel, M., Miola, A., Mozzi, P., Primon, S., Surian, N., 2009. Geomorphological evolution and sediment transfer in the Piave River system (northeastern Italy) since the Last Glacial Maximum. Géomorphologie: Relief, Processus, Environnement 3, 155–174. Church, M., 1983. Pattern of instability in a wandering gravel bed channel. In: Collinson, J.D., Lewin, J. (Eds.), Modern and Ancient Fluvial Systems. : Special Publication, 6. IAS, pp. 169–180. Comiti, F., Andreoli, A., Lenzi, M.A., Mao, L., 2006. Spatial density and characteristics of woody debris in five mountain rivers of the Dolomites (Italian Alps). Geomorphology 78, 44–63. Comiti, F., Pecorari, E., Mao, L., Picco, L., Rigon, E., Lenzi, M.A., 2008. New Methods for Determining Wood Storage and Mobility in Large Gravel-bed Rivers. EPIC FORCE project report (D20bis). University of Padova, Padova, Italy. (http://www.tesaf. unipd.it/epicforce/Download.asp). Conesa-Garcìa, C., Lenzi, M.A. (Eds.), 2010. Check Dams, Morphological Adjustments and Erosion Control in Torrential Streams. Nova Science Publishers, New York, p. 298. Da Canal, M., 2006. Studio delle variazioni morfologiche del F. Piave nel Vallone Bellunese durante gli ultimi duecento anni. MSc Thesis, University of Padova, Padova, Italy. Da Canal, M., Comiti, F., Surian, N., Mao, L., Lenzi, M.A., 2007. Studio delle variazioni morfologiche del F. Piave nel Vallone Bellunese durante gli ultimi 200 anni. Quaderni di Idronomia Montana 27, 259–271. Dalla Fontana, G., Marchi, L., Crivello, F., 2003. Studio multitemporale sulla vegetazione del Fiume Piave tra Belluno e Fener. Genio rurale, estimo e territorio, LXVI, p. 3. Del Favero, R., Lasen, C., 1993. La Vegetazione Forestale del Veneto. Progetto Editore, Padova, Italy, p. 312.

Dipartimento Lavori Pubblici and PRASS, 1983. Indagini e studi per la disciplina delle attività estrattive e per la definizione di un piano sperimentale di escavazione di materiale litoide dall'alveo del Fiume Piave. Regione del Veneto, Venezia, Italy. Dufour, S., Piégay, H., 2009. From the myth of a lost paradise to targeted river restoration: forget natural references and focus on human benefits. River Research and Applications 25 (5), 568–581. Dutto, F., Maraga, F., 1994. Variazioni idrografiche e condizionamento antropico, Esempi in pianura padana. Il Quaternario 7, 381–390. Garcia-Ruiz, J.M., White, S.M., Lasanta, T., Marti, C., Gonzalez, C., Errea, M.P., Valero, B., Ortigosa, L., 1997. Assessing the effects of land-use changes on sediment yield and channel dynamics in the central Spanish Pyrenees. In: Walling, D.E., Prost, J.L. (Eds.), Human Impact on Erosion and Sedimentation. Proceedings of Rabat Symposium S6. IAHS Press, Institute of Hydrology, Wallingford, UK, pp. 151–158. Gregory, S.V., Boyer, K.L., Gurnell, A.M. (Eds.), 2003. The Ecology and Management of Wood in World Rivers. American Fisheries Society Publication, Bethesda, MD, USA. Gurnell, A.M., 1997. Channel changes of the river Dee meanders, 1946–1992, from the analysis of air photographs. Regulated Rivers: Research and Management 13, 13–26. Gurnell, A.M., Petts, G.E., 2002. Island-dominated landscapes of large floodplain rivers, a European perspective. Freshwater Biology 47, 581–600. Gurnell, A.M., Petts, G.E., 2006. Trees as riparian engineers: the Tagliamento River, Italy. Earth Surface Processes and Landforms 31, 1558–1574. Gurnell, A.M., Surian, N., Zanoni, L., 2009. Multi-thread river channels: a perspective on changing European Alpine river systems. Aquatic Sciences 71, 253–265. Habersack, H.M., Piégay, H., 2008. River restoration in the Alps and their surroundings: past experience and future challenges. In: Habersack, H.M., Piégay, H., Rinaldi, M. (Eds.), Gravel-Bed Rivers VI: From Processes Understanding to River Restoration. : Developments in Earth Surface Processes, 11. Elsevier, Amsterdam, The Netherlands, pp. 703–737. Hicks, D.M., Duncan, M.J., Lane, S.T., Tal, M., Westway, R., 2008. Contemporary morphological changes in braided gravel-bed rivers: new developments from field and laboratory studies, with particular references to the influence of riparian vegetation. In: Habersack, H.M., Piégay, H., Rinaldi, M. (Eds.), Gravel-bed Rivers VI: From Processes Understanding to River Restoration. : Developments in Earth Surface Processes, 11. Elsevier, Amsterdam, The Netherlands, pp. 557–584. Hughes, M.L., McDowell, P.F., Marcus, W.A., 2006. Accuracy assessment of georectified aerial photographs: implications for measuring lateral channel movement in a GIS. Geomorphology 74, 1–16. Keesstra, S.D., van Huissteden, J., Vandenberghe, J., Van Dam, O., de Gier, J., Pleizier, I.D., 2005. Evolution of the morphology of the river Dragonja (SW Slovenia) due to landuse changes. Geomorphology 69 (1–4), 191–207. Kondolf, G.M., 1994. Geomorphic and environmental effects of instream gravel mining. Landscape and Urban Planning 28, 225–243. Kondolf, G.M., 1997. Hungry water: effects of dams and gravel mining on river channel. Environmental Management 21 (4), 533–551. Kondolf, G.M., Piégay, H., Landon, N., 2007. Changes in the riparian zone of the lower Eygues River, France, since 1830. Landscape Ecology 22, 367–384. Lazzarini, A., 2002. Il dibattito sul diboscamento montano nel Veneto fra Sette e Ottocento. In: Lazzarini, A. (Ed.), Diboscamento montano e politiche territoriali. FrancoAngeli, Milan, Italy, pp. 57–97. Lenzi, M.A., Mao, L., Comiti, F., 2006. Effective discharge for sediment transport in a mountain river: computational approaches and geomorphic effectiveness. Journal of Hydrology 326, 257–276. Liébault, F., Piégay, H., 2001. Assessment of channel changes due to long-term bedload supply decrease, Roubion River, France. Geomorphology 36, 167–186. Liébault, F., Piégay, H., 2002. Causes of 20th century channel narrowing in mountain and piedmont rivers of southeastern France. Earth Surface Processes and Landforms 27, 425–444. Mao, L., Lenzi, M.A., 2007. Sediment mobility and bedload transport conditions in an Alpine stream. Hydrological Processes 21, 1882–1891. Marston, R.A., Bravard, J.P., Green, T., 2003. Impacts of reforestation and gravel mining on the Malnant River, Haute-Savoie, French Alps. Geomorphology 55, 65–74. Mount, N.J., Louis, J., Teeuw, R.M., Zukowskyj, P.M., Stott, T., 2003. Estimation of error in bankfull width comparison from temporally sequenced and corrected aerial photographs. Geomorphology 56, 65–77. Pecorari, E., Comiti, F., Rigon, E., Picco, L., Lenzi, M.A., 2007. Caratteristiche e quantificazione del legname in alveo in corsi d'acqua di grandi dimensioni: risultati preliminari sul fiume Piave. Quaderni di Idronomia Montana 27, 477–488. Pellegrini, M., Perego, S., Tagliavini, S., Toni, G., 1979. La situazione morfologica degli alvei dei corsi d'acqua emiliano-romagnoli: stato di fatto, cause ed effetti. Proc. Conf. “La programmazione per la difesa attiva del suolo e la tutela delle sue risorse: i piani di bacino idrografico”, 28–29 June, Modena, Italy, pp. 169–195. Piégay, H., Walling, D.E., Landon, N., He, Q., Liébault, F., Petiot, R., 2004. Contemporary changes in sediment yield in an alpine mountain basin due to afforestation (the upper Drôme in France). Catena 55, 183–212. Piégay, H., Darby, S., Mosselman, E., Surian, N., 2005. A review of techniques available for delimiting the erodible river corridor: a sustainable approach to managing bank erosion. River Research and Applications 21, 773–789. Piégay, H., Alber, A., Slater, L., Bourdin, L., 2009. Census and typology of braided rivers in the French Alps. Aquatic Sciences 71, 371–388. Rinaldi, M., 2003. Recent channel adjustments in alluvial rivers of Tuscany, central Italy. Earth Surf. Process. and Landforms 28, 587–608. Rinaldi, M., 2008. Schede di rilevamento geomorfologico di alvei fluviali. Il Quaternario 21, 353–366. Rinaldi, M., Piégay, H., Surian, N., 2010. Geomorphological approaches for river management and restoration in Italian and French rivers. In: Simon, A., Bennett, S.,

F. Comiti et al. / Geomorphology 125 (2011) 147–159 Castro, J., Thorne, C.R. (Eds), The Scientific Basis for Stream Restoration in Dynamic Fluvial Systems: Deterministic Approaches, Analyses and Tools. AGU, in press. Rovira, A., Batalla, R.J., Sala, M., 2005. Response of a river sediment budget after historical gravel mining (the lower Tordera, NE Spain). River Research and Applications 21, 829–847. Surian, N., 1998. Studio finalizzato alla definizione geomorfologica della fascia di pertinenza fluviale del Fiume Piave tra Perarolo e Falzè e del torrente Cordevole tra Mas e Santa Giustina. Autorità di bacino dei fiumi Isonzo, Tagliamento, Livenza, Piave, BrentaBacchiglione. Studi finalizzati alla redazione del piano di bacino del Fiume Piave. 38 pp. Surian, N., 1999. Channel changes due to river regulation: the case of the Piave River, Italy. Earth Surface Processes and Landforms 24, 1135–1151. Surian, N., 2002. Downstream variation in grain size along an Alpine river: analysis of controls and processes. Geomorphology 43, 137–149. Surian, N., Cisotto, A., 2007. Channel adjustments, bedload transport and sediment sources in a gravel-bed river, Brenta River, Italy. Earth Surface Processes and Landforms 32, 1641–1656. Surian, N., Rinaldi, M., 2003. Morphological response to river engineering and management in alluvial channels in Italy. Geomorphology 50, 307–326. Surian, N., Rinaldi, M., 2004. Channel adjustments in response to human alteration of sediment fluxes: examples from Italian rivers. In: Golosov, V., Belyaev, V., Walling, D.E. (Eds.), Sediment Transfer Through the Fluvial System, Publication 288. IAHS, pp. 276–282. Surian, N., Rinaldi, M., Pellegrini, L., Audisio, C., Maraga, F., Teruggi, L., Turitto, O., Ziliani, L., 2009a. Channel adjustments in northern and central Italy over the last 200 years. In: James, L.A., Rathburn, S.L., Whittecar, G.R. (Eds.), Management and Restoration of Fluvial Systems with Broad Historical Changes and Human Impacts: Geological Society of America Special Paper, 451, pp. 83–95. Surian, N., Ziliani, L., Comiti, F., Lenzi, M.A., Mao, L., 2009b. Channel adjustments and alteration of sediment fluxes in gravel-bed rivers of northeastern Italy: potentials and limitations for channel recovery. River Research and Applications 25, 551–567.

159

Susin, G.M., 1975. Fiume Piave tronco Soverzene-Fener. Aspetti fluviali principali. Studio per la difesa del suolo della Provincia di Belluno. Provincia di Belluno, pp. 59–68. Tal, M., Paola, C., 2010. Effects of vegetation on channel morphodynamics: results and insights from laboratory experiments. Earth Surface Processes and Landforms 35, 1014–1028. Tal, M., Gran, K., Murray, A.B., Paola, C., Hicks, D.M., 2004. Riparian vegetation as a primary control on channel characteristics in multi-thread rivers. In: Bennett, S.J., Simon, A. (Eds.), Riparian Vegetation and Fluvial Geomorphology. American Geophysical Union, Washington DC, USA, pp. 43–58. Villi, V., Bacchi, B., 2001. Valutazione delle piene nel triveneto. CNR-GNDCI, Gruppo nazionale per la difesa dalle catastrofi idrogeologiche, Italy. Ward, J.V., Tockner, K., Schiemer, F., 1999. Biodiversity of flooplain river ecosystems: ecotones and connectivity. Reg. Riv. 15, 125–139. Winterbottom, S.J., 2000. Medium and short-term channel planform changes on the Rivers Tay and Tummel, Scotland. Geormorphology 34, 195–208. Wohl, E.E., Angermeier, P.L., Bledsoe, B., Kondolf, G.M., MacDonnell, D., Merritt, M., Palmer, M.A., Poff, N.L., Tarboton, D., 2005. River restoration. Water Resources Research 41, 1–12. Wyzga, B., 1993. River response to channel regulation: case study of the Raba River, Carpathians, Poland. Earth Surface Processes and Landforms 18, 541–556. Wyzga, B., 1996. Changes in the magnitude and transformation of flood waves subsequent to the channelization of the Raba River, Polish Carpathians. Earth Surface Processes and Landforms 21 (8), 749–763. Wyzga, B., 2008. A review on channel incision in the Polish Carpathian rivers during the 20th century. In: Habersack, H.M., Piégay, H., Rinaldi, M. (Eds.), Gravel-bed Rivers VI: From Processes Understanding to River Restoration. : Developments in Earth Surface Processes, 11. Elsevier, Amsterdam, The Netherlands, pp. 525–555. Zanoni, L., Gurnell, A., Drake, N., Surian, N., 2008. Island dynamics in a braided river from analysis of historical maps and air photographs. River Research and Applications 24 (8), 1141–1159.