Geomorphology 125 (2011) 147–159

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Geomorphology

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

F. Comiti a,⁎, M. Da Canal b, N. Surian c, L. Mao b, L. Picco b, M.A. Lenzi b

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

⁎ Corresponding author. Tel.: +39 0471 017126; fax:E-mail address: francesco.comiti@unibz.it (F. Comiti

0169-555X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.geomorph.2010.09.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 April 2010Received in revised form 3 September 2010Accepted 6 September 2010Available online 22 September 2010

Keywords:Channel narrowingIsland dynamicsHuman impactPiave RiverItaly

The timing and extent of the morphological changes that occurred in the last 200 years in a large gravel bedriver (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, repeatedtopographic measurements, and geomorphological surveys. Results show that the channel underwent astrong narrowing during the twentieth century, but with a faster pace during the 1970s–1990s and with anassociated shift from a dominant braided pattern to awanderingmorphology. Bed incision up to 2 m—mostlyfrom gravel mining — has been documented for this period. Large areas of the former active channel werecolonized by riparian forests, both as islands and as marginal woodlands. The ceasing of gravel extraction inthe late 1990s seems to have determined a reversal in the evolutionary trend, with evidence of vegetationerosion/channel widening even though a significant aggradation phase is not present. We conclude thatalteration of sediment regime has played a major role on the long-term channel evolution. However, onlyrelevant flood events (RIN10–15 years) appear to determine substantial island erosion, and therefore theproportion of island vs. channel area fluctuates depending on flood history.

+39 0471 017009.).

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Braided rivers are characterized by a highly dynamic response tochanges — both natural and human-induced — in their drainagebasins. Several braided rivers (i.e., Brenta, Piave, Cellina, Tagliamento,and Torre) originate in the eastern Italian Alps, a tectonically activemountain range where large amounts of coarse sediment are suppliedto the fluvial systems. These rivers, like most rivers in Italy (e.g.,Pellegrini et al., 1979; Dutto and Maraga, 1994; Rinaldi, 2003; Surianand Rinaldi, 2003; Surian et al., 2009a) and in other Europeancountries (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, mostlyas a consequence of human impacts at the basin and channel scales. Thegeneral pattern of braided channel adjustment in Italian rivers (i.e.,channel narrowing, bed incision, and shift toward a wandering/singlethread pattern) as proposed by Surian and Rinaldi (2004) is shown inFig. 1. Overall, subsequent studies (e.g. Surian et al., 2009a) confirmedthat channel evolution model and analyzed more in detail the recentphase of adjustment that took place over the last 15–20 years. While

thepreviousphasesof channelnarrowingandbed incisionwere commonto all the analyzed rivers, the third phase is more complex in terms ofprocesses. Channel widening has become the dominant process in mostof the study reaches but channel narrowing is still ongoing in somereaches. Widening is associated with aggradation in some reaches, butthe relation between width and bed level changes is not as strong asduring the previous phases of narrowing. In fact, channel widening hastaken place without significant bed level variations in some reaches(Surian and Cisotto, 2007). Besides, it is still an open question if all therivers underwent this recent phase of adjustment or, as proposed forrivers in France (Piégay et al., 2009), recent channel changes may beconsidered short-term fluctuations related to specific flood events,rather than real long-term adjustments.

Different human interventions (i.e., sediment mining, channel-ization, dams, reforestation and control works in steep mountainstreams) have been identified as the causes of channel adjustments inItalian rivers (Surian and Rinaldi, 2003; Surian et al., 2009a). Suchinterventions have caused a dramatic alteration of the sedimentregime. Gravel mining was identified as the key driving factor ofmajor adjustments in Italian rivers (Surian et al., 2009a), but it isworth 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 themain factors controlling channel evolutionof French rivers.

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

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Overall, these recent changes led to the disruption of the verycomplex channel morphology typical of braided systems, which inturn strongly affected their ecological status (Ward et al., 1999) andtheir ability to contain flood flows (Wyzga, 1996).

Besides channel narrowing and incision, vegetation encroachmenttook place within the former river channels, and thick riparianwoodlands are now present in many locations within the rivercorridor of these Alpine rivers. Although these riparian forests mayrepresent a “benefit” in terms of overall biological diversity within theriver ecosystem, they generate several problems from a hydraulicperspective. The potential drawbacks can be grouped into threecategories: (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 bendsfrom the stabilization of narrow sections; and (iii) delivery of woodelements into the channel, leading to potentially dangerous “plugs”downstream at critical sections (e.g., bridge piers, weirs, narrow crosssections). On the other hand, a higher presence of wood within thechannel 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 riverchannels as well as the higher presence of stranded wood on bars hasbeen leading river managers — often pushed by local people andmunicipalities — to frequently cut riparian woodlands and removetrees and logs from the channel. These “river maintenance” activitiesare being justified as a measure to reduce hydraulic hazards butactually lack any sound scientific approach, and the economicrationale (cost–benefit balance) itself is arguable. Indeed, they canbe paralleled to the classical, widespread river engineering practice ofsediment bar removal, nowadays recognized to have overall negativeeffects (e.g., Kondolf, 1994,1997; Marston et al., 2003) and thusadopted only in fast-aggrading reaches posing compelling floodproblems.

On the contrary, a modern perspective on river management aimsat restoring (where possible) natural processes occurring within thefluvial corridor, such as bank erosion and wood input (Piégay et al.,2005; Wohl et al., 2005; Habersack and Piégay, 2008). Riverrestoration projects and environmental restoration as a whole mostoften tend to recreate some idealized past conditions, reckoned to be“more natural” than the current ones and thus considered worthy tobe pursued. Nonetheless, a more pragmatic, objective- and processes-oriented approach was recently advocated (Dufour and Piégay, 2009).

Within this context, understanding the extent to which the newforested 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, water-shed erosion control projects, sediment trapped by dams, gravelmining) rather than a natural characteristic, and to develop a strategyfor their management is of great relevance. However, natural (i.e., notaffected by human presence) features in rivers of the European Alpsmay be almost impossible to retrace, due to the highly impactingmillennia-long presence of humans in these regions. Nonetheless, abetter understanding of the historical changes undergone by theserivers is much needed for evaluating potential and limitations forchannel recovery and a range of different strategies of riverrestoration and management have been proposed (Surian et al.,2009b).

This paper deals with the morphological evolution and theassociated vegetation cover dynamics in the intermediate course ofthe Piave River (within the valley called “Vallone Bellunese”) overthe past 200 years. Previous studies have analyzed in this river basin(i)morphological changes of the river channel (Surian, 1999; Da Canalet al., 2007; Surian et al., 2009b); (ii) land use changes within thefluvial corridor and abundance of in-channel wood (Dalla Fontana etal., 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 stretchof the Piave River is quite interesting to study because the reach isaffected by heavy pressures coming from a dense hydropower schemeand past gravel mining activity, and at the same time features severalunregulated tributaries delivering large amount of sediment directlyto the reach, such that its large-scale adjustments are of smallermagnitude compared to downstream reaches (Surian et al., 2009b).

The novelty of the present work compared to previous papers forsimilar regulated Italian rivers (e.g., Surian et al., 2009a) mostlyregards two points. First, the paper presents a combined analysis oflateral and vertical channel adjustment with vegetation cover andisland dynamics; second, the varying channel response exhibited atthe subreach scale is analyzed and linked to natural as well as human-induced factors at this scale. As to the latter point, there is a need, notonly in the Italian context (e.g., Piégay et al., 2009), of detailedexamples where causes of channel adjustments can be accuratelyanalyzed through detailed reconstruction of channel evolution. Asmentioned above, an overall framework of causes of adjustments doesexist but a better link between causes and adjustments needs to beestablished in most cases. The objectives of this paper are to (i) toquantify morphological changes, both in bed planform and in bedelevation; (ii) quantify the variation of vegetation cover, withparticular emphasis on islands dynamics; and (iii) identify the drivingfactors of channel evolution and vegetation cover changes and thus toenvisage the most likely future trends. Specifically, the followingresearch hypotheses will be tested: (i) planform (narrowing/widening) and vertical (incision/aggradation) processes are correlat-ed; and (ii) gravel mining is the main factor driving recent channeland 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 easternItalian Alps, and the main channel flows south for 220 km from itsheadwaters (at ~2000 m asl near the Italy–Austria border) to theoutlet in the Adriatic Sea NE of Venice (Fig. 2). The climate istemperate-humid with an average annual precipitation of about1350 mm. However, marked differences in precipitation (rangingfrom 1000 to 2000 mm) related to elevation and proximity to the seaexists within the basin. Considerable annual variations in the rainfall

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 subreachesdelimit the morphological fluvial corridor, defined by the presence of terraces or geological constraints.

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amount are also present, but significant trends were not observedover the last century (Surian, 1999).

The drainage basin is mainly composed of sedimentary rocks(predominantly limestone and dolomite), but volcanic and metamor-phic rocks are also present. The river course can be divided into threemain segments. The upper segment is mostly incised in the bedrockand presents a narrow single thread channel. In the middle course(which comprises the study reach described below) the gravelriverbed is very wide and characterized by a multithread channelpattern. The lower course features a sand-bed, meandering channelartificially straightened at places.

The present physiographic setting of the river results mainly fromdrainage 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 aggradationtook place in the Vallone Bellunese (the study reach). After this periodof 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 seriesof 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 hasbeen inhabited since prehistoric times, but the population rose to asignificant level — together with forest harvesting, crop cultivation,transport of logs in streams — after the Roman colonization (secondcentury B.C.). After a short period of depopulation and consequentforest expansion after the end of the Roman Empire, a steady declinein forest cover occurred from the late Middle Ages through theModern 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) refores-tation has been taking place since World War I but most effectivelyafter the 1950s because of rapid abandonment of traditional farmingand cropping activities on the mountain slopes boosted by thedevelopment of industry and tourism (Del Favero and Lasen, 1993).

Flows in the Piave River have been regulated for irrigation andhydroelectric 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 reservoirsindicate that the pre-dam total sediment yield was about1,000,000 m3/year, in contrast to a present estimated value of145,000 m3/year (Dipartimento Lavori Pubblici and PRASS, 1983;Surian et al., 2009b). The volume of water diverted has increasedsubstantially since the early 1960s. The present regime of waterregulation and diversion alters both the flow duration characteristicsand volume of annual runoff in the river.

Between the 1960s and 1990s, intense gravel mining was carriedout 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 beenmodified and embanked since the early Middle Age, effective erosionand torrent control works (Conesa-Garcìa and Lenzi, 2010) started inthe upper basin only in the 1930s, but massively only after the 1970s.However, the millennia-long practice of splash damming in the smalltributaries and log rafting from the headwaters to Venice (Caniato,1993) have likely affected channel morphology and sediment yield

Table 1Characteristics of channel subreaches.

Subreach Channel length(m)

Morphology(in 2006)

Max historical channelwidth (m)

Artificial structures affectingchannel morphology

Gravel mining extent (ha)and periods

1 1470 Wandering 694 Groynes built in 1940s; Right bankprotection built in 1980s

0.631980s and 1990s

2 2650 Braided 604 Groynes built in 1940s –

3 1960 Wandering 366 – –

4 1840 Wandering 744 Left bank protection 4.651970s to 1990s

5 1120 Single thread withalternate bars

252 – –

6 3000 Braided 743 – 2.361980s and 1990s

7 2250 Braided 692 – 0.471980s and 1990s

8 2050 Wandering 765 – 3.571990s

9 2950 Braided 810 – 3.481980s and 1990s

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

150 F. Comiti et al. / Geomorphology 125 (2011) 147–159

through the removal of log jams, snags, and boulders that would havecaused inconvenient obstructions to log flotation.

2.3. Study reach

The study reach is ~30 km long and is located between Ponte nelleAlpi and Busche (Fig. 2), with a drainage area at Busche of 3174 km2.The morphology of the river in the study reach is dominated bybraided and wandering channel patterns, but narrower reachesdisplay an alternate bars channel pattern (Church, 1983). The slopeof the study reach is (on average) 0.45%, and the median surface grainsize 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, amorphological fluvial corridor has been identified that rangesbetween 100 and 2000 m, depending on the presence of Holocenefluvial terraces and other geological constraints such as hillslopes andalluvial fans (Surian, 1998). Within the study reach, nine subreacheshave been delineated based on homogeneity in river corridor width,presence of artificial elements (i.e., groynes, longitudinal bankprotections), historical as well as present morphological pattern(Fig. 2; Table 1). The two lowermost subreaches are locateddownstream of the largest tributary of the Piave, i.e., the CordevoleRiver. The subdivision into subreaches is mostly meant to infer howlateral constraints (natural and artificial) affect channel adjustments,whereas the detection of a possible longitudinal pattern of channelvariations is hampered by the large natural variability (i.e., fromhillslopes and tributaries) within the study reach and by the presenceof the Busche weir at the downstream end of the reach, which hasfixed the bed elevation since 1960.

Because of the unavailability of a single flow data set, flow recordsfor the present study are derived from two gauging stations dependingon the period: the hydropowerweir of Busche (the downstreamend ofthe study reach, Fig. 2) for records from 1961 to 2007; and Segusino, anatural cross section located 16 kmdownstream of Busche, for recordsfrom 1926 to 1960. Because of the slightly different drainage areabetween Segusino and Busche (3333 and 3174 km2, respectively),flood peak discharges measured at the former station were reducedapplying a corrective factor (0.962), as suggested by Villi and Bacchi(2001). A statistical magnitude–frequency analysis was carried out byusing both Gumbel and lognormal distributions, the latter being thebetter in fitting the observed data.

The largest flood event occurred in 1966 and reached almost4000 m3 s−1, whereas “bankfull” discharge Q2 (RI=2 years) wascalculated 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

achieved its present level in the 1950s, the Q2 was found (Da Canal,2006) not significantly different if calculated separately for pre- andpost-regulation periods, i.e., using two subsets: 1926–1954 and 1954–2007. However, higher frequency events (RIb1.5 years) show areduction of peak discharge in the post-regulation period (Da Canal,2006). The selection of 1954 was somewhat arbitrary because of thecomplexity of the hydropower scheme, i.e., the dams possiblyaffecting the flow regime were built in different years. However, itwas tested how changing the “turning point” year in the range 1950–1955 does not substantially affect the results. Unfortunately, flow datato construct the entire flow duration curve are not available for theBusche weir and thus it is not possible to analyze the duration forwhich Q2 is passed each year, as in Piégay et al. (2004).

3. Materials and methods

3.1. Identification of geomorphological and vegetation features frommaps and aerial photos

Planform changes of river features over the last 200 years wereanalyzed on three historical maps (1805, 1890, and 1926) and sevenaerial photos (1960, 1970, 1982, 1991, 1999, 2003, and 2006). Themaps range in scale from 1:25,000 to 1:26,000, whereas the aerialphotos range from 1:8000 to 1:33,000. Photos were scanned at aresolution of 600 dpi in order to obtain an average virtual resolution of1 m or smaller. Digital maps and aerial photographs were rectifiedand coregistered to a common mapping base at 1:5000 by GIS

Table 2The 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 3850 2001951 2406 301965 2064 202002 1775 131993 1753 121953 1749 121960 1606 101980 1565 91972 1500 81976 1456 7

Table 3List of the cross sections used and their survey year.

XS Distance from upstream(km)

Year Subreach

code

S-1 1.26 1929, 2007 1S-2 2.84 1985, 1991, 1996, 2003 1S-3 3.99 1929, 2007 2S-4 5.21 1985, 1991, 1996, 2003 2S-5 8.00 1929, 2007 -S-6 9.74 1985, 1991, 1996, 2003 3S-7 10.54 1929, 2007 3S-8 10.98 1929, 2007 –

S-9 11.68 1985, 1991, 1996, 2003 4S-10 12.96 1929, 2007 4S-11 14.56 1985, 1991, 1996, 2003 5S-12 15.19 1929, 2007 5S-13 17.45 1929, 1985, 1991, 1996, 2007 6S-14 19.45 1929, 1985, 1991, 1996, 2007 6S-15 21.98 1985, 1991, 1996, 2003 7S-16 23.35 1929, 2003, 2007 8S-17 28.12 1929, 1985, 1991, 1996, 2007 9S-18 28.69 1929, 1985, 1991, 1996, 2003 9S-19 29.01 1985, 1991, 1996, 2003 9

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software (Esri ArcGIS 9.2). Approximately 30 ground-control pointswere used to rectify each single frame, and second-order polynomialtransformations were then applied, obtaining root mean square errors(RMSE) ranging from 2 to 4 m. The higher RMSE are for historicalmaps, particularly for the oldest map (1805, scale 1:26,000).

Significant planform features were digitized on rectified maps andphotos in order to derive planform characteristics for each image.Measurements are affected by errors from rectification and digitiza-tion (i.e. features' edge marking) processes. An error assessment wascarried out based on (i) RMSE values, which can be an acceptableproxy of the average error of georectification (Hughes et al., 2006);(ii) previous studies that took into account both georectification anddigitization errors (e.g., Gurnell, 1997; Winterbottom, 2000; Mountet al., 2003; Zanoni et al., 2008); and (iii) some field measurementswith DGPS to assess the position of digitized features. This analysisrevealed maximum errors of about 15–20 m and 6 m for measure-ments on maps and aerial photographs, respectively.

Historical maps allowed us to distinguish the boundaries of threebasic fluvial features lying within the fluvial corridor (i.e., the areabordered by hillslopes and ancient terraces and thus including activechannel, floodplains, and recent terraces): unvegetated active channel,vegetated islands (i.e., shrubby/arboreal vegetation within the activechannel), and marginal woody vegetation (i.e., shrubby/arboreal vege-tation at the channel margins). Aerial photos allowed the identificationof more vegetation classes: islands with arboreal vegetation, islandswith shrubby vegetation, arboreal marginal vegetation, shrubbymarginal vegetation, andherbaceousmarginal vegetation. Furthermore,three additional classes related to human use of the river corridor wereadopted: 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 distinctionbetween main and secondary channels was not attempted. Instead, theentire unvegetated active channel class was used to describe areasoccupied by flowing water during low flows (main and secondarychannels) and exposed and unvegetated surfaces (i.e. bars) next to thechannels, i.e., to represent the whole area inundated and subject to bedmobilization during frequent floods (RI=1–2 years). Bars covered withherbaceous vegetationwere thus assigned to this active channel category.

In the aerial photos, evidence of canopy texture, shape, andshadows were used to estimate vegetation height and thus todifferentiate between arboreal and shrubby vegetation classes. Aheight of about 4–5 m was assumed to separate the two classes. Anarboreal islandwas defined as a distinct vegetated area surrounded bythe active channel having at least 60% of its surface occupied byarboreal vegetation (i.e., an arboreal island can include portions ofherbaceous or shrubby vegetation). If the surface covered by trees isb60%, 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 agencyof the river) were acquired for the analysis of long-term bed level

changes (Fig. 2). These cross sections were resurveyed in 2007 using aDGPS system, with a GPS vertical error b2 cm. The 2007 ellipsoidalelevations (WGS-84 reference system) were then converted toorthometric elevations (i.e., to the same datum used in 1929 surveys)using the software VERTO2 provided by the Italian Military Geo-graphical Institute (IGM), whose precision is claimed to be ±4 cm.

In addition, the topographic surveys carried out by the “GenioCivile 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 thosesurveyed in 1929 and 2007 (Table 3), i.e., a complete set entailing bothlong- and short-term variations is available only for two sites.

In order to determine how channel bed evolved after 1996 at thesecross sections for which a DGPS survey of 2007 was not available, weused an airborne LiDAR survey (filtered point density of 1–2 m−2))carried out by the “Autorità di Bacino dell'Alto Adriatico” during fall2003 (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 extractedfrom the DTM in correspondence to those of the 1980s–1990s. Even ifLiDAR-derived cross sections may suffer from the inability to correctlyrepresent inundated areas, the actual survey was carried out duringlow flow conditions such that only a minor fraction of each crosssection is not correctly captured.

For each available cross section dating from 1929, 1985, 1991, 1996,2003, and 2007, themean elevation of the active channelwas calculatedexcluding floodplains and islands, identified either on survey pointdescription (for 1929 and 2007) or on visual identification from aerialphotos (for the others). In the subsequent analysis, cross sectionelevations from 2003 (LiDAR) and 2007 (DGPS) will be used as areference to the “present day” conditions, because between these twosurveysno relevantfloodevents occurred (Fig. 3) and the active channelwidth did not show marked variations (Fig. 4). Preference to 2007elevation data (where available)will be granted because of the possibleerror associated to themissing sensing of the low flow channel thalweg.Table 3 summarizes all the cross sections used and the subreach theybelong to. The longitudinal distance from the upstream end of the studyreach (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 standard-ized forms specifically designed to record measurements and

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

Fig. 5. (A) Temporal variation of the ratio between the area of islands and active channelin thewhole study reach. The arrows indicatemajor floods and their recurrence interval.(B) Variation of number and average area of islands.

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observations of channel changes (Rinaldi, 2008). Data collectedthrough such surveys integrate those coming from the previouslydescribed methods (planform changes and topographic surveys) andproved useful to infer direction and approximate magnitude of bedlevel changes. The geomorphological surveys allowed to infer bothlong- and short-term channel adjustments according to severalmorphological and sedimentological features, such as differences inelevation between higher bars and gravel in floodplains/recentterraces, presence/absence of sediment lobes, or presence/absenceof bed armouring.

4. Results

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

The analysis of the historical maps and aerial photographs showsthat substantial changes took place in the Piave River within theinvestigated time interval. The extension of the unvegetated activechannel was at its highest at the end of nineteenth century (1900 hain 1890, i.e., about 80% of the entire fluvial corridor; Fig. 4). During thetwentieth century, the active channel progressively reduced, reachingin 1991 an extension of approximately one-third of the corridor area.

The active channel area reduced in two different stages (Fig. 4). Afirst phase of adjustment took place during the first half of thetwentieth century and was characterized by a loss of about 35% of theinitial active channel area, at a rate of about 11 ha/year. The meanactive channel width, calculated as the ratio between channel areaand reach length, narrowed at an average rate of 3.8 m/year. Thistrend 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 abruptchannel enlargement between 1960 and 1970 (205 ha, 68 m asaverage channel width). The subsequent narrowing phase (docu-mented from 1970 to 1991) was more intense, occurring at a rate ofabout 32 ha/year (10.6 m/year in terms of channel width). In thefollowing period (1991–2003), a reversal occurred with an evidentsharp widening tendency that extended the active channel at a rate of27 ha/year (9 m/year). Finally, no substantial variations occurredbetween 2003 and 2006.

As to the morphological pattern of the entire study reach, thisshifted from braided (still dominant until the 1960s) to single thread/wandering in the 1990s. The expansion phase of the last decade isassociated with a general recovery of at least a wandering style, withoccasional braiding morphology.

Complementary to the trends in the active channel, the proportionof the fluvial corridor covered by vegetation at the channel marginshas experienced a significant change over the period 1805–2006,

being at its lower extent at the end of nineteenth century (about 20%)and afterwards extending until 1991 (Fig. 4), i.e., channel area nolonger active was colonized by woody vegetation (mostly by Salixeleagnos, Salix alba, Populus nigra, Alnus incana, with occasional Pinussylvestris). Similarly, the increase in active channel area that tookplace between 1991 and 2003 occurred mostly at the expense ofvegetated 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 ofshrubby vegetation (i.e., pioneering stages comprising actual shrubsand young trees) vs. tree (i.e., established, older stages) extensionswas possible. The extension of shrubby vegetation does not reveal anysignificant trend, whereas the arboreal vegetation area shows a clearincreasing 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) werepresent in the study reach since the early 1800s, but their overallextension was always much smaller thanmarginal vegetation (Fig. 4).Also, they show a less pronounced temporal variation compared tothe latter, with a maximum relative extension occurring in 1960 and aminimum in 1890. During the narrowing phase (1970–1991), theproportion of island area within the river corridor increased onlyslightly, as opposed to the remarkable expansion of surfaces coveredin marginal vegetation. The channel expansion stage that occurredbetween 1991 and 2003 shows a similar reduction in island relativeextension, reaching a final value similar to 1970. In contrast, marginalvegetation in 2003 still shows a much higher proportion compared to1970.

In order to analyze in greater detail the dynamics of islands, theratio between islands and unvegetated active channel areas isconsidered (Fig. 5A). Apparently, the relative extension of islandscalculated as such increased progressively during the whole first

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phase of channel narrowing until 1960. The occurrence of the majorflood in 1966 (RI~200 years) caused the ratio to drop significantly in1970 (from 0.18 in 1960 to 0.06 in 1970). The ratio of islands overunvegetated active channel areas subsequently feature a fast-increasing trend until 1991 (i.e. during the most intense channelnarrowing when it reached the maximum value of 0.24) and then afurther drop until 2003 as a result of 1993 and 2002 floods (bothRI~10 years). Finally, the mentioned ratio remained constant around0.07 between 2003 and 2006.

Different and complementary information can be gained from theanalysis of the variation of the number and average extension ofislands (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 (until1960) exhibits a fragmented islands pattern, i.e., islands are numerous(about 240, ~8/km) but small (~0.8 ha). The 1966 flood eroded mostof such relatively small islands, halving their number without sub-stantially modifying their area. A different, abrupt variation charac-terized the beginning of the second phase of channel narrowing, whenislands became relatively fewer but considerably larger (from 0.6 to1.5 ha) in just one decade (1970–1980). The following decade ofnarrowing did not entail substantial changes for islands, until the

Fig. 6. Planform evolution of subreach 9 from 1805 to 2006. The classification of fluvial fehistorical maps. Aerial photographs, which allow more detailed interpretation of vegetation

floods of 1993 and 2002 caused a reduction of their number (to about80, 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 entirestudy segment was carried out individually for all nine subreachesshown in Fig. 2. As an example, Fig. 6 shows the vegetation coverevolution of subreach 9 over the last 200 years. In general, similarpatterns can be observed for all subreaches (images not reported).Nonetheless, changes in the relative channel extension (i.e., comparedto subreach corridor area) reveal that different portions of Piave Riverunderwent adjustments of varying magnitude (Table 4).

The temporal changes of the extent of active channel area areshown in Fig. 7A for subreaches 1, 3, and 8, selected as representativefor their distinct trends of adjustment. The trends for all subreacheswere analyzed, but they are no reported in Fig. 7 in order to make itmore readable. Subreach 1 shows the overall lowest planimetricvariations, for contraction (after subreach 7) and for expansion. Inparticular, this latter process is poorly observable here (2.6% after the1991 photo); and also, the 1966 flood (see variation between 1960

atures is simplified in 1805, 1890, and 1926 because it is derived from the analysis ofand land use, have been used in the subsequent years (1960 to 2006).

Table 4Summary of vertical and planform adjustments for the nine subreachesa.

Vertical (m) Planform (%)

Subreach 1929–1991 1991–2007 1926–1991

1991–2006

XSsurveys

Fieldindicators

XSsurveys

Fieldindicators

1 −2.5 I −0.4 E/A −50.8 2.62 −0.5 I 0.5 A −60.9 59.63 −1 I 0.1 E/A −70.4 130.74 −1.2 I 1.5 A −59.2 46.45 −0.75 E 0.1 A −56.0 59.36 −0.75 E 0.1 E/A −61.4 58.27 −0.6 I −1.1 A −40.7 56.68 −1.3 I −0.6 A −58.4 80.49 −1.3 NA −0.2 E −64.3 35.7

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

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and 1970) caused only a limited widening. Notably, part of this reachwas influenced by groynes built in the 1940s and by longitudinal bankprotection on the right bank built in the 1980s (Table 2). In contrast,subreach 3 features the highest variations for narrowing (70%) andwidening. During the recent expansion phase (1991–2003), thechannel widened doubling its extent (from 88 to 203 m); and also, thewidening between 1960 and 1970 is the largest experienced by asingle subreach. An intermediate response to flood events is evidentfor subreach 8, where contraction and expansion are about 64% and36%, respectively (Table 4).

Fig. 7. Patterns of adjustments occurred at the subreach scale: (A) active channel areasrelative 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 channelareas — among the different subreaches indicates (Fig. 7B) that theirmaximum 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 inFig. 7B). Also, some subreaches, which apparently did not featureislands in the nineteenth century, reached considerable island coverin the first half of the twentieth century (e.g., subreaches 1 and 3),even though a direct comparison between maps and aerial photosshould be considered just indicative at this small spatial scale.However, the dramatic island erosion effects of the 1966 flood isevident in all the subreaches, whereas the consequences of the 1993and 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 islandcover between 2000 and 2003.

4.3. Bed level changes

Fig. 8 shows the variation of mean cross section elevations (asdescribed in Fig. 3 and reported in Table 3) taking as a reference themost recent value, i.e., either the 2007 DGPS survey or the 2003LiDAR-derived DTM. The use of either the 2003 and 2007 cross sectionto serve as a reference to present bed level has been proved to beaccurate enough because of the lack of relevant flood events thatoccurred between the two years (see above) as well as of anthropicmodifications of the bed (i.e., gravel mining). When both cross sectionmeasures are available, their mean elevation difference is alwaysb20–30 cm, possibly because of the inability of LiDAR to detect water-filled channels.

As to the long-term bed changes, bed elevation in 1929 was about1 m higher than present days; but the difference is N2.5 m at the veryupstream limit of the reach, and it is negligible just upstream of theconfluence with the Cordevole (cross section distance of 19 km). Therecent bed level adjustment trends (i.e., after 1985) appear rathermore complex. Unfortunately, the available data do not allow anyprecise analysis of incision trends until 1985. The 1966 flood eventsdetermined an abrupt channel expansion (see previous section) andmost likely caused a moderate aggradation in the study reach, aschronicles suggest. A combined analysis of present cross sections andvegetation colonization, supported by a photogrammetry-derivedDTM (even though quite rough) from the 1970 aerial photos (Susin,1975) indicate that themean bed elevation in the late 1960swas fairlysimilar to that surveyed in 1929, i.e., most of the incision measured bycomparing cross sections in 1929 and 1985 started in the 1970s (seeDiscussion). However, we are not able to determine how the channelelevation 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 crosssections (Table 3) along the longitudinal distance of the study reach. Each cross sectionis 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, or1996) was higher than at present (i.e., where incision took place) and negative valuesthe opposite. The dashed lines indicate the error that is associated with datacomparison (±0.4 m).

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with channel narrowing. Besides, analyzing the trend at each crosssection among the 1985, 1991, and 1996 surveys only, the year oflowest bed elevation turns out to be not the same along the studyreach, generally varying between 1991 and 1996.

Looking at the recent bed adjustments, most cross sections show1990s bed elevation quite similar to present levels, estimating that anoverall error of up to ±0.3–0.4 m (marked in Fig. 8 as dotted lines)could occur when comparing surveys of different sources andmethods. However, three cross sections located upstream of theconfluence with the Cordevole exhibit substantial lower elevations(up to 1.5 m, thus hinting to a recent phase of aggradation); whereasthose just upstream and downstream of the Cordevole show higherelevations, up to 1.2 m, indicating a recent incision phase. However,the elevation of the three sections at the downstream end of the reachshould be examined with caution because of their proximity to theBusche weir, which may have likely induced local aggradation atdifferent times from backwater effects.

Results from geomorphological surveys (Table 4) are in goodagreement with those from cross section comparisons (one to threesurveys 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 levelstability). In contrast, the very recent channel bed evolution ischaracterized by sedimentation or equilibrium. Notably, the twomethods employed for assessing bed level changes may lead toslightly different results. For instance, short-term analysis from crosssections refers to the 1991–2003/2007 period, whereas results fromgeomorphological surveys may reflect channel response to veryrecent flood events (e.g., the 2002 flood as well as the subsequentsmaller 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 wereto (i) establish whether planform and vertical adjustments arecorrelated and (ii) identify the driving factors of channel changes,that are the roles played by flows, sediments, and vegetation. Table 4presents a summary of vertical (in terms of mean bed elevation) andplanimetric (in terms of channel width) variations at the subreachscale divided into two periods: from the 1920s–1930s to 1991(dominant narrowing) and between 1991 and 2006 (dominantexpansion). Subreaches where actual cross sections data did notallow direct estimation of vertical variations were assigned a valuebased 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 Spearmanranking approach. Results show that vertical and lateral adjustmentsfor the nine subreaches during the narrowing period are notcorrelated (nonsignificant R Spearman; pN0.10), even though a directrelationship between narrowing and incision is weakly apparent(graph not shown). This means that subreaches that experienced thelargest contraction did not necessarily undergo the largest incision(e.g. subreaches 2 and 6; Table 4). However, incision is associatedwith channel narrowing in all the subreaches. As mentioned before,there is evidence indicating that the great extent of the incisionmeasured between 1930 and 1991 actually started only in the 1970s,after the likely aggradation of the 1966 flood event. Therefore, thecorrelation between incision and channel contraction in the period1970–1991 was explored, but no significant results were obtained.

As to the recent channel adjustments (1991–2006), the nonpara-metric correlation analysis shows an even weaker statistical link(p≫0.10) between elevation changes and expansion. In fact, severalsubreaches (e.g., 3 and 8; Table 4) feature a considerable expansion ofthe active channel despite a very moderate aggradation or even in the

presence of incision (e.g., subreaches 1, 3, 7, and 9). In contrast,subreach 4 features the largest aggradation value, but its widening isintermediate (Table 4).

This could be partly due to the small magnitude of bed changescompared to the inherent measurement errors, but it might alsoindicate that the lateral channel mobility needed to re-enlarge theactive channel by bank erosion can be restored by just stoppingfurther incision— in the case of the Piave River by banning in-channelgravel extraction in the late 1990s — even without notable sedimentdeposition. As a consequence, the first hypothesis should be rejected.The only factor correlated (Spearman R=−0.65, pb0.10) to therecent expansion rate is the “initial” (in 1926) channel width ofsubreaches, i.e., narrower reaches have expanded more intensely. Nocorrelations were found instead for the narrowing rates.

Overall, even though a general temporal correlation in theoccurrence of narrowing and incision has been documented, theirprocess intensities (i.e., rates) were not correlated (i.e., spatialanalysis for the different subreaches). The lack of correlation mightbe partly caused by the different spatial sampling error associated tocross sectional and planimetric variations at the subreach scale, andmore importantly by the time lag between the steadier vegetationgrowth (i.e., causing the reduction of active channel area) comparedto bed incision processes that take place only during sediment-transporting flood events. As to expansion–aggradation processes, notonly do their intensities show no correlation, even the actualprocesses 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 thequestion is: which came first? Vegetation or incision? And how didthey interact? In other words, is the expansion of woody vegetationthe driver of the important morphological changes that occurred inthe Piave River ? One may in fact hypothesize that the flow regulationin the Piave River led to vegetation encroachment on gravel bars byreducing inundated areas, as it occurred in the Waitaki River (Hickset al., 2008). There, the increased bank resistance caused by woodyvegetation was deemed responsible for the reduction of braidingintensity, which leads to the concentration of flood flows in fewerchannels with higher erosive power (Tal et al., 2004; Tal and Paola,2010).

For our study case, in order to provide a definite answer we wouldneed much more frequent cross section surveys and aerial photosstarting from the 1970s, which would allow us to determine thepossible temporal shift between channel incision and vegetationestablishment, or the opposite. In fact, vegetation reached itsmaximum (and channel its minimum) extent in the 1991 aerialphotos in almost all the reaches (Fig. 7); whereas the year of survey(1991 or 1996) featuring the lowest bed elevation varies along thestudy reach as mentioned in the previous section. Nonetheless, whatis relevant to observe is that bed incision in 1985 was generallyalready advanced and, in many cases, very similar to 1991–1996values (Fig. 8).

The solution to the question “vegetation vs. incision” can betackled by a simplified calculation of the sediment eroded within thestudy reach between the early 1930s and late 1980s, i.e., the availableyears for which topographic surveys are available to document theincision phase. Considering an average bed incision of 1 m, and amean active channel width of 300 m (value for the 1980s), thistranslates 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 twoorders of magnitude larger than the annual bedload transportsupplied to the reach under a “natural” regime (i.e., without dams

156 F. Comiti et al. / Geomorphology 125 (2011) 147–159

in the upper basin as before the 1950s), which can be estimated to beabout 6 104 m3. This value is based on mean total sediment yield(200 m3 km−2 year−1, Surian et al., 2009b), basin area (about2000 km2 at the upstream end of the study reach), and assumingbedload to represent about 15% of the total sediment yield (Surianet al., 2009b).

The comparison between eroded sediment and bedload annualyield implies that— assuming a total absence of sediment input to thereach from interception by dams and from possible reduction in theduration of transporting flows and an annual sediment transportcapacity in the reach still equalling the estimated bedload supply —

channel downcutting should have started N100 years before the1980s. Instead, as previously described, evidence indicates that mostof the incision only started in the 1970s, after the large 1966 floodwhich aggraded the channel and removed a great deal of vegetation.Because vegetation has re-established on gravel bars after the 1966flood (as shown by the 1970 aerial images), and taking into accountthat shrubs and trees would take at least 5 years to impart significanteffects on channel roughness and bank stability (e.g., Hicks et al.,2008), vegetation might have started to exert a morphologicalinfluence only in the late 1970s. Therefore, it is not possible that thebank strengthening effect of woody vegetation be responsible for theobserved 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) istoo short, i.e., it would imply an unrealistically large annual sedimenttransport capacity to flush the “missing” sediment out of the studyreach by fluvial processes only. Furthermore, the assumption ofnegligible sediment input from upstream is not actually true for thisreach (see Surian et al., 2009b), thus an even longer time would benecessary to match the observed incision.

5.2.2. Gravel extraction as the main driver of channel changes in thePiave River

Assuming an annual gravel extraction of 300,000 m3 for the period1970–1990 (see the official gravel mining data reported in the “studysite” section, very likely underestimated), the total extracted volumeturns out 6 106 m3, i.e., of the same order of magnitude of the “lost”sediment associated to incision. Taking into account the considera-tions presented above about the unlikely role of vegetation, we arguethat gravel extraction, which started in the 1970s and reached its peakduring the 1980s, ismost likely responsible formost of the bed incisionin the study reach. This is also indicated by a significant correlation(Spearman R=0.72, pb0.10) between gravel mining areas (summedup from the 1970s to the 1990s, Table 1) and incision at the subreachscale, if subreach 1 (the one featuring the highest incision) is excluded.Its exclusion is justified by the relevant effect of artificial narrowingbecause of groynes and bank protection, which is likely co-responsiblefor such a strong incision. Apparently, bed incision did not propagatemuch from its original location, possibly because of the effect ofsediment supplied by lateral tributaries along the reach alongwith therelatively small depths of gravel pits and large channel widths.

Therefore, we believe that the second hypothesis stated in theintroduction, i.e., gravel mining within the study reach is the keyfactor driving recent channel and vegetation changes, is true. Thepresence of hydropower dams in the upper Piave basin likely exertedonly a secondary effect. However, dams may represent the mostrelevant impact and limiting factor for channel recovery in the longrun (now that gravel extraction has virtually ceased), as argued bySurian et al. (2009b).

5.3. Summary of channel evolution and driving factors over the last200 years

The analyzed reach of the Piave River was characterized byremarkably wider extensions of its active channel area during the

nineteenth century (Fig. 4) and this evidence can be taken asrepresentative of conditions of unregulated basins in terms ofhydrology and sediment transport. However, this wider activechannel may also reflect a higher human pressure on basin land usebecause at that time the forest cover was at its historical minimum(see Section 2.2) and sediment supply from the basin was likely at itsmaximum. Possibly, a larger sediment supply during the nineteenthcentury was partly from the effect of the Little Ice Age, i.e., lowertimberline and higher glacial activity (Bravard, 1989).

During the twentieth century, two phases of channel narrowingwere identified in the study reach as in other Italian rivers (Rinaldi,2003; Surian and Cisotto, 2007; Surian et al., 2009a). The channelnarrowing that occurred during the first half of the 1900s was causedby an array of factors whose single relevance is difficult to estimate,such as river training structures (i.e., the perpendicular groynes builtin the 1940s, likely the dominant factors in the associated subreaches)and land use variation at catchment scale (i.e., increase in forest coveron slopes). However, also the milder climate following the Little IceAge might have contributed to a natural reduction of sediment supplyand thus, partly, to channel narrowing. We do not have evidences thatthis phase of narrowing was associated with bed incision, therefore itis possible that narrowing occurred without incision, as shown inseveral French rivers (Liébault and Piégay, 2002; Rinaldi et al., 2010),or, alternatively, that very slight incision occurred, as documented insome Italian rivers (Surian and Rinaldi, 2003, 2004).

The second phase of narrowing was relatively short (approxi-mately from 1970 to 1990) but very intense and associated withchannel degradation. As discussed above, gravel mining is the maindriver of incision and narrowing during this phase, and vegetationencroachment is a consequence of incision. This confirms the key roleof sediment mining in Italian rivers (Surian et al., 2009a) andhighlights some differences with other Alpine regions whereafforestation in the floodplains (following changes in the land use)is considered, along with sediment mining, a major cause of channelincision (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; Surianet al., 2009a) may depend on two aspects. First, the channel width isquite large, such that even a minor bed change corresponds to largesediment volumes (see Section 5.2.1). It is also common that widthchanges are much more intense than depth changes in rivers withhigh width/depth ratio (e.g., Surian and Rinaldi, 2003). Second, thesediment supply from the lateral tributaries within the reach — beingthe upper basin mostly dammed— likely contributed to prevent moresubstantial bed incision.

As to the most recent channel evolution, Surian and Rinaldi (2004)identified a phase of channel widening in several Italian rivers andSurian et al. (2009a) pointed out that such phase is often associatedwith aggradation, even if it can also occur without significant bed levelchanges. In our study site, recent channel adjustments are complexand spatially variable, such that a clear trend is less obvious. Eventhough a general channel widening is apparent in the study reachbetween the 1990s and 2006, no clear indications of a widespread,concomitant aggradation phase are observed, confirming thatwidening is taking place without any aggradation in some reaches(see Section 5.1). Taking into account the magnitude and duration ofthis widening phase, we suggest that it should represent a real phaseof adjustment rather than short-termmorphological changes from theoccurrence of episodic, larger flood events. Again, the comparisonwith alpine rivers in southeastern France is of great value. In Frenchrivers a widespread channel recovery was not observed, probablybecause after the main phase of incision (peaking in the 1970s) theriver channels are still adjusting to the decrease of sediment supplyfrom catchment afforestation (Liébault and Piégay, 2002; Rinaldi et al.,2010). On the contrary, the Piave River in the study reach underwent aremarkable morphological recovery (mainly by widening) thanks to

Fig. 9. Conceptual models of island dynamics in natural (A) and incised from gravelmining (B) large gravel bed rivers. Vertical bars represent the magnitude of annualflood peak discharge, solid lines the average island area and dashed lines the number ofislands within a certain reach. The pattern depicted in (A) is based on that proposed byGurnell 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 gravelmining (B) partly derives from observation in the study reach (see Figs. 6 and 7) but isalso speculative because here gravel extraction is imagined to continue throughout theanalyzed period, such that a wandering/single thread morphology becomes estab-lished. In (B), phase “a” represents an accelerated formation of islands from vegetationencroachment on higher bars, phase “b” corresponds to the merging of these islandsinto fewer larger ones. The moderate flood event towards the end of “b” is able to causeonly some minor island erosion. Phase “c” represents the incorporation of some largeislands within the floodplain, thereby only few island may still persist. However, theoccurrence of a major flood event (end of “c”) causes the dissection of part of thefloodplain creating newer large dissection islands. Overall, the number of islands islower, and the average island area is higher, for an incised braided river than for oneunder a natural sediment regime.

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the sediment input from the tributaries once the major degradingfactor, i.e., gravel mining, ceased. However, this recover will likely beconstrained in the near future by the present condition of thecatchment (dams, increased forest cover, torrent control works).

5.4. Variation of island cover

In the study reach of the Piave River, islands extent relative tochannel area apparently increased (Fig. 5A) from the end of thenineteenth century to the 1960s, possibly as a consequence of reducedsediment transport from land use variations (see previous section)and reduced pressure of animal grazing and fuel wood removal (asdescribed by Liébault and Piégay, 2002 for French rivers). The extreme1966 event (RI~200 years) dramatically reduced the island-to-channel area ratio to its lowest values, which then soared inconcomitance with channel narrowing and bed incision. Subsequent-ly, floods in 1993 and 2002 (RI=10–15 years) determined relevantdrops in island cover as well as erosion of marginal vegetation. Theseobservations match with those of Bertoldi et al. (2009) in theTagliamento River, a braided river similar for many aspects to thePiave River, but featuring virtually unregulated flow and sedimentregimes. In fact, in the Tagliamento River the island dynamics werefound to be strictly associated to the occurrence of major floods(RIN10–15 years), which are the only ones able to determinesubstantial island erosion.

As to the relationship between island size and number, a sharpdifference is evident when comparing different periods in the Piavereach (Fig. 5B). The increase in island area observed between 1926and 1960 took place by the establishment of many small islands,whereas total islands area increased by coalescence of formerlyseparated island during the most intense narrowing phase accompa-nied by bed incision (1970–1991). The process of island coalescencewas described also for natural island-braided systems (Gurnell andPetts, 2002, 2006) as well as in flume experiments simulatingvegetation growth in braided channels under altered flow regimes(Tal and Paola, 2010).

Unfortunately, we believe that the interpretation of islanddynamics (in terms of islands number and mean area) can be reliablycarried out only for the trend starting in 1960, i.e., topographical mapsvery likely did not represent small islands and thus these parameters(nonetheless reported in Fig. 5B for the 1805 and 1926 maps) featurea large uncertainty. Using the 1960 as the best proxy for unregulatedriver conditions (i.e., before the major alterations from gravel miningand dams), we infer that the Piave River was (under the conditions ofbasin land use existing at that time) characterized by many, smallislands. Notably, a large (RI~30 years, Table 2) flood event occurred in1951, and many islands in the 1960 aerial photo appear to be ratheryoung and thus successive to such event. Caution must be used inpicking such a configuration of island number/size as representativeof “natural” conditions, because island dynamics in similar riversfollow a complex temporal evolutions (Gurnell and Petts, 2002, 2006).

Therefore, the outcome of the present study is that islands — aswell as patches of arboreal vegetation at channel margins — werepresent in the study reach also before any human-induced channelalteration, thus their presence should be viewed as “natural” there.However, the large extent of single islands and the large total islandarea during the 1990s was a consequence of an altered sedimentregime, because it was determined by gravel mining leading to bedincision, in turn causing vegetation establishment and acceleratedisland coalescence.

The envisaged differences in island dynamics between unalteredand incised (from active mining) large gravel bed rivers are depictedin Fig. 9, which builds on the previous conceptual model put forwardby Gurnell and Petts (2002) and on evidence from the present studyreach (e.g., evolution maps such as that reported in Fig. 6 as well asresults in Fig. 7). However, the graph is speculative as to how the

variation in island number/area would have been in the study reachshould gravel extraction have persisted. In fact, in the Piave River thecease of gravel mining in the 1990s led to the channel recoverydescribed above, which is likely the cause for the reduction in relativeisland cover and in both island number and mean area, suggesting aphase of net island removal.

6. Conclusions

The natural interplay among water flow, sediment transport, andvegetation in natural, large, gravel bed rivers is highly complex. Whenfluvial systems are affected by human impacts, as often happens, itbecomes even more challenging to disentangle the causes of channelevolution over time. This study in the Piave River provides someinsights into both the long-term and short-term morphologicaldynamics of a typical Alpine river subjected to heavy anthropicinfluence. The main findings are: (i) alteration of sediment supplyrather than flow regime appears to be the key factor of channeladjustments; and (ii) high rates of channel narrowing and expansionare not necessarily linked to substantial bed incision or aggradation.However, bed incision from gravel mining does imply an extraordi-nary vegetation encroachment (and thus channel narrowing) andonly intense, infrequent flood events (RIN10–15 years) are able todetermine substantial island erosion. However, once gravel miningstops, channel widening starts again, as observed in several Italianrivers. 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 morphologyover the next future (i.e., next 30–50 years), even though no resto-ration actions are carried out, provided that gravel mining remainsbanned. However, we expect the reach cannot achieve channel widthsand morphologies comparable to the pre-1950s because the presentsediment supply is greatly limited by dams in the upper basin and byhigher forest cover and denser presence of torrent control works.

The channel expansion at the expense of forested margins andislands poses several river management issues because, in the nearfuture, bank erosion and in-channel wood are likely to increase.Management strategies should take into account the negative effectsthat may be due to those processes, but also the benefits broughtabout by bank erosion (e.g., sediment and wood supply).

Acknowledgements

This research was partially funded by the EU project INCO-CT-2004-510735 “Epic Force” (Evidence-based policy for integratedcontrol of forested river catchments in extreme rainfall andsnowmelt) and PRIN 2007 project, “Present evolutionary trends andpossible future dynamics of alluvial channels in northern and centralItaly”. Support for field surveys were also provided by the Universityof Padova project, “Channel adjustment and restoration in response tohuman 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 fiumidell'Alto Adriatico” is kindly thanked for providing LiDAR data. Allcolleagues and students who helped in the field are greatly thanked.Finally, we greatly thank the four reviewers whose suggestionsgreatly improved the original manuscript.

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