water

Article What Discharge Is Required to Remove Silt and Sand Downstream from a Dam? An Adaptive Approach on the Selves River, France

Rémi Loire 1,2,* , Loïc Grosprêtre 3, Jean-René Malavoi 1, Olivier Ortiz 4 and Hervé Piégay 2 1 EDF, Hydro Engineering Center, Savoie Technolac, 73 370 Le Bourget du Lac CEDEX, France; [email protected] 2 University of Lyon, CNRS UMR5600, Site of ENS, 15 Parvis R. Descartes, 69 007 Lyon, France; [email protected] 3 Dynamique Hydro, 16 rue Masarik, 69 009 Lyon, France; [email protected] 4 EDF, Center Unit Production, 14 av. du Garric, 15 000 Aurillac, France; [email protected] * Correspondence: [email protected]; Tel.: +33-479-606-030



Received: 11 December 2018; Accepted: 20 February 2019; Published: 23 February 2019


Abstract: An increasing number of scientific studies are tackling the management of discharges downstream of dams for environmental objectives. Such management is generally complex, and experiments are required for proper implementation. This article present the main lessons from a silt sand removal experiment on a bypassed reach of a dam on the Selves River (164 km2), France. Three four-hour operational tests at maximum discharge (10, 15, and 20 m3/s) were carried out in September 2016 to determine the discharge required for transporting as much silt and sand as possible without remobilizing coarser sediments. In September 2017, an additional flow release was performed over 34 h at 15 m3/s. Suspended sediment concentration and water level were recorded throughout the releases. Monitoring at the reach scale was supplemented by morphological measurements. The results demonstrate that a discharge of approximately 10 m3/s enables significant transport of suspended sediments (SS), whereas a discharge of 15 m3/s enables significant sand transport. The results provide operational information on silt and sand transport applicable to other small rivers. This study represents an important contribution to the relatively sparse existing body of literature regarding the effects of water releases and sediment state. Our study also demonstrates that it is possible to successfully undertake water releases in small rivers with an adaptive management approach.

Keywords: flushing flow; channel maintenance flow; water release; sediment transport; river restoration; dam

1. Introduction For centuries, many rivers have been regulated to limit flooding and enable a variety of uses, including hydroelectricity, leisure, irrigation, and drinking water supply. In the northern third of the planet, 77% of rivers now have a dam or river diversion in place [1]. Environmental impacts of dams have been described by many authors [2–5]. These impacts include effects of water-storage dams on the full range of the flow regime. In absence of floods, (high) sediment supply from tributaries downstream of dams can lead to aggradation of the bypassed reach (reach immediately downstream of the dam receiving only minimum flows) [2]. In particular, sedimentation of fine sediments may significantly disrupt biological functions by bed clogging or sand deposits [2]. In the light of potential ecological alterations of dams on river systems, researchers have recommended restoring a more natural flow regime [6–8]. The natural river regime is variable and

Water 2019, 11, 392; doi:10.3390/w11020392 www.mdpi.com/journal/water Water 2019, 11, 392 2 of 19 this variability is key to maintaining indigenous species and biodiversity in general. The hydrological regime of regulated rivers can, therefore, be managed through the five components that characterize the variability of natural flow regime: flow magnitude, frequency, and duration, as well as variations in water levels at daily or seasonal scales [8]. High flow releases (or “flushing flows”) are a tool for discharge management that aims to act on the morphology of the channel downstream of dams to meet environmental objectives [8]. The scientific literature largely agrees on the need to perform such releases [8–13] in the interest of imitating the effects of natural floods as much as possible [14–17]. By revitalizing fluvial processes along the river, high flow releases can potentially improve physical habitats more efficiently than hydromorphological restoration work [18], but their widespread use is relatively recent [10]. Experiments have been conducted in a number of countries in recent years [19], and some have been presented in dedicated scientific publications in Spain [20], Switzerland [21], and the USA [22,23]. These flushing flows have varied and sometimes multiple objectives, including reducing clogging in spawning areas [24–26], improving general habitat conditions [17], controlling algae proliferation in the river channel [20,27,28], and riparian forest maintenance [17,29]. According to the scientific literature, displacing sediments delivered by tributaries is rarely the primary objective of flushing flows, with the exception of the Colorado River [22,30]. Although the study of sand transport is often an integral part of research on improving clogging conditions in spawning areas, releases most often target silts and not sand. Therefore, sand is generally not monitored in this type of operation, and there are only a few studies describing the influence of flushing flows on the sand fraction, such as those conducted on the Colorado River [31,32] and the Bill Williams River [23]. In most cases, flushing flows aim to imitate a period of natural high flows [8], and the selection of key parameters, such as magnitude and duration, raise a number of issues. In particular, these parameters need to be determined to account for the energy loss of releases as they travel downstream (i.e., the attenuation of peak depth magnitude), thereby resulting in reduced effectiveness along a river continuum (reduced transport capacity) [33]. The choice of the flow discharge depends on the velocities and bed shear stresses required to induce the desired morphological process [27,29]. The required duration of a release is difficult to determine a priori as it depends on a number of parameters, such as the project reach length, slope, and morphology (particularly mesohabitat units) [34]. The duration of the release also depends on the particle size targeted by the operation. Some authors have shown that the peak of the silt pulse travels slightly more slowly [35], at the same speed [24] or slightly more quickly [36] than the peak water pulse. The duration, therefore, needs to be calculated so that the release can simultaneously impact the entire reach with a discharge that falls relatively slowly. Sand travels more slowly than silt [12]; furthermore, the duration required to export sand corresponds to the length of transit desired and is more difficult to calculate than for the duration to export silt [37,38]. In the end, the intensity and the duration of a flow release is a compromise between the required sediment transport for an effective flushing operation and the water volume available according to the reservoir dam capacity and allocations for other water uses associated with the dam or the river [39]. These elements highlight the fact that a priori determination of flushing flow characteristics is complex and support recommendations for using adaptive management [10,40–42] and closely-monitored experiments [24,36,43]. This study aims to summarize findings and recommendations from an adaptive management plan of flushing flows on the Selves River. The plan concerns a large reservoir dam whose bypass reach is subject to significant sand and silt deposits, which are suspected of impairing reproductive habitat conditions for trout. Four experimental flushing flows were carried out in 2016 and 2017 to evaluate the required intensity and duration of flow releases. A set of hydraulic, geomorphological, and biological parameters were monitored to observe the effects of the flushing flows on sediment grain size distribution and physical habitats in the bypassed reach. The results illustrate the effects of different discharges under very similar time and weather conditions in 2016. The results also provide Water 2019, 11, 392 3 of 19 Water 2018, 10, x FOR PEER REVIEW 3 of 19 operationalto small rivers. information Furthermore, on silt this and study sand transportrepresents using an important an original contribution approach potentiallyto the relatively applicable sparse toexisting small rivers. body of Furthermore, literature regarding this study the represents effects of an water important releases contribution on streambed to the sediment relatively conditions sparse existing[44]. body of literature regarding the effects of water releases on streambed sediment conditions [44].

2.2. Site Site Description Description TheThe Selves Selves River River is is a a tributary tributary of of the the Truy Truyèreère River River (Garonne (Garonne Basin, Basin, France) France) that that flows flows from from east east toto west west for for a a distance distance of of 44.5 44.5 km km through through the the Massif Massif Central Central mountain mountain range range (Figure (Figure1). 1). Its Its source source is is atat 1300 1300 m m altitude, altitude, and and it it drains drains a a catchment catchment area area of of 189 189 km km2.2. It It crosses crosses the the Aubrac Aubrac plateau plateau up up to to the the MauryMaury dam, dam, then then flows flows into into gorges gorges over over the the last last 10 10 km. km. The The river river catchment catchment area area primarily primarily consists consists of littleof little fractured fractured and and highly highly impermeable impermeable volcanic volcanic rock. rock. It delivers It delivers large large quantities quantities of sand, of sand, which which are foundare found in the in bed the of bed the Selvesof the andSelves its and tributaries. its tribut Thearies. region The hasregion a montane has a montane climate with climate an average with an annualaverage precipitation annual precipitation of up to 1200 of up mm to 1200 in lower-lying mm in lower-lying areas and areas up to and 1800 up mm to 1800 in the mm higher-altitude in the higher- catchmentaltitude catchment area. The area. catchment The catchment area is mainly area is occupied mainly byoccupied meadows by meadows and woodlands. and woodlands.

Figure 1. Location of the Selves River, its catchment area and the bypassed reach downstream of the MauryFigure dam. 1. Location of the Selves River, its catchment area and the bypassed reach downstream of the Maury dam. The Maury dam was commissioned in 1947 by Electricity of France (EDF) to produce electricity and isThe also Maury used for dam tourism was commissioned purposes (swimming, in 1947 by fishing, Electricity etc.). of It France is 65 m (EDF) high andto produce has a catchment electricity areaand of is 164also km used2 to for create tourism a reservoir purpos ofes 166 (swimming, ha with a normalfishing, storageetc.). It capacityis 65 m high of 35 and hm 3has. The a catchment reservoir waterarea isof diverted164 km² to thecreate Lardit a reservoir powerplant of 166 located ha with downstream a normal storage of the Selves capacity catchment of 35 hm area,3. The bypassing reservoir thewater last 11.3is diverted km of the to Selvesthe Lardit River. powerplant located downstream of the Selves catchment area, bypassingThe natural the last characteristic 11.3 km of instantaneousthe Selves River. discharges at the Maury dam are 4.8 m3/s for the average 3 3 flow, 61The m natural/s for 2-year characteristic return flow, instantaneous and 110 m /s discharges for 10-year at return the Maury flow. The dam Maury are dam4.8 m has3/s greatlyfor the modifiedaverage theflow, hydrology 61 m3/s for of the2-year downstream return flow, river and course 110 m as3/s it for was 10-year designed return to store flow. a largeThe Maury volume dam of water.has greatly Since 1947,modified a number the hydrology of major floodsof the downstream have been observed river course upstream as it was of the designed dam (1965: to store 120 ma large3/s, 1981:volume 94 m of3/s, water. 2003: Since 110 m1947,3/s), a but number only one of major led to downstreamfloods have been overflows observed as the upstream rain event of hadthe notdam 3 been(1965: anticipated 120 m3/s, 1981: (1994: 94 74 m m3/s,/s). 2003: The 110 flows m3/s), observed but only forone over led to 70 downstream years in the overflows bypassed reachas the are,rain thus,event almost had not exclusively been anticipated at the minimum (1994: 74 flowm3/s). (240 The L/s), flows in observed addition for to theover small 70 years inflows in the from bypassed small tributariesreach are, inthus, the almost gorges. exclusively The many at tributaries the minimum of the flow bypassed (240 L/s), reach in measureaddition justto the a few small hundred inflows metersfrom small in length tributaries and have in the a catchment gorges. The area many representing tributaries 13% of the of thatbypassed of the reach Selves measure downstream just a thefew Mauryhundred dam. meters in length and have a catchment area representing 13% of that of the Selves downstreamThe transport the Maury capacity dam. downstream of the dam has consequently been considerably reduced as has theThe sediment transport supply capacity because downstream the reservoir of the trapsdam ha alls ofconsequently the incoming been sediments. considerably In the reduced bypass as reach,has the the sediment current sediment supply supplybecause comes the reservoir from the trap aforementioneds all of the incoming tributaries, sediments. mainly arriving In the during bypass reach, the current sediment supply comes from the aforementioned tributaries, mainly arriving

Water 2019, 11, 392 4 of 19

Water 2018, 10, x FOR PEER REVIEW 4 of 19 storms and is dominated by sand. It is not transported further once in the bypassed reach because the during storms and is dominated by sand. It is not transported further once in the bypassed reach minimum flow has a low transport capacity, resulting in fairly widespread sand deposits (Figure2). because the minimum flow has a low transport capacity, resulting in fairly widespread sand deposits Some of these deposits also contain silt due to soil erosion, decaying organic materials (mainly leaves), (Figure 2). Some of these deposits also contain silt due to soil erosion, decaying organic materials and(mainly dam maintenanceleaves), and dam operations maintenance (bottom operations gate opening (bottom and gate drainage). opening and drainage).

(a) (b) (c)

FigureFigure 2. 2.Sand Sand deposits deposits on on a a glide glide( a(a)) andand aa gorgegorge sectorsector ((bb).). Clogging of of a a lentic lentic channel channel (c (c).). In In (a) (a ,b), theand initial (b), the gravel initial substrate gravel substrate is no longer is no visiblelonger vi whereassible whereas in (c) in silt (c) deposits silt deposits spread spread across across the the entire riverentire bed. river bed.

TheThe bypassed bypassed reach reach isis locatedlocated in a series of of gorges gorges of of varying varying widths widths and and depths depths with with an anaverage average slopeslope of of 0.027 0.027 m/m. m/m. RockyRocky outcrops control control the the bed bed and and its its lateral lateral mobility. mobility. The The bed bed material material grain grain sizesize distribution distribution is bimodal,is bimodal, with with a coarse a coarse fraction fraction of bouldersof boulders and and cobbles cobbles (>64 (>64 mm) mm) and aand fine a fractionfine composedfraction composed of silts, sand, of silts, and sand, very fineand gravelvery fine (<4 gr mm).avel (<4 In themm). slower In the portions slower ofportions the river, of the widespread river, sandwidespread deposits sand can bedeposits observed can onbe observed the bed near on the the bed main near tributary the main mouths. tributary mouths. AlthoughAlthough the the aquaticaquatic invertebrateinvertebrate population population is is of of high high quality quality on on the the Selves Selves River River [45], [45 sand], sand aggradation and bed clogging have led to a reduction in fish populations compared to the natural aggradation and bed clogging have led to a reduction in fish populations compared to the natural capacity [46]. Electrofishing historically has demonstrated that there are larger trout populations at capacity [46]. Electrofishing historically has demonstrated that there are larger trout populations uninfluenced stations upstream of the dam than in the bypassed reach. A reduction in the frequency at uninfluenced stations upstream of the dam than in the bypassed reach. A reduction in the of small floods facilitates clogging of coarse alluvial substrates by fine sediments [24], which can frequency of small floods facilitates clogging of coarse alluvial substrates by fine sediments [24], significantly disrupt biological function, and reduce fish populations [47–49]. Clogging and sand whichdeposits can reduce significantly the number disrupt and biological size of spawning function, areas and as reducewell as fishthe amount populations of refuge [47 –and49]. nursery Clogging andhabitat, sand which deposits is especially reduce the problematic number andon the size Selves of spawning [46]. Salmonid areas fish as wellspecies as (brown the amount trout,of Salmo refuge andtrutta nursery fario) habitat,inhabiting which the Selves is especially River require problematic an unclogged on the Selves substrate [46]. for Salmonid reproduction. fish species Managers (brown trout,fromSalmo the local trutta fish fario agency) inhabiting and EDF the have Selves observed River silt require and sand an uncloggedaggradation substrate for many for years. reproduction. A joint Managersdecision fromwas, thetherefore, local fish taken agency to implement and EDF have sand-removal observed silt releases and sand downstream aggradation of forthe manydam years.to Aimprove joint decision conditions was, for therefore, the local taken troutto population. implement sand-removal releases downstream of the dam to improve conditions for the local trout population. 3. Material and Methods 3. Materials and Methods 3.1. Designing Experimental Flow Releases 3.1. Designing Experimental Flow Releases Two main morphological objectives were defined to improve biological functions: Two main morphological objectives were defined to improve biological functions: • Remove a maximum of silt, sand, and organic material (leaves and small woody debris) from • Removethe bypassed a maximum reach channel, of silt, sand, and organic material (leaves and small woody debris) from the • bypassedAvoid coarse reach sediment channel, transport to prevent substrate disruption that would impact aquatic • Avoidfauna coarse (as there sediment are no natural transport inputs to prevent of coarse substrate sediments). disruption that would impact aquatic fauna (asThis there approach are no naturalrequires inputs determining of coarse the sediments). most effective flow magnitude and duration for uncloggingThis approach and removing requires silt determiningand sand deposits the fr mostom the effective river, without flow magnitudeaffecting coarser and sediments. duration for Furthermore, the releases needed to be long enough to avoid fine sediment redeposition. unclogging and removing silt and sand deposits from the river, without affecting coarser sediments. Following the recommendations of [34,37,50], a range of potential discharges to be tested were Furthermore, the releases needed to be long enough to avoid fine sediment redeposition. determined empirically from flow statistics and calculation of at-a-station sediment transport Following the recommendations of [34,37,50], a range of potential discharges to be tested thresholds (flow velocity comparisons with Hjulström diagram, [51]). Hydraulic modelling was not were determined empirically from flow statistics and calculation of at-a-station sediment transport thresholds (flow velocity comparisons with Hjulström diagram, [51]). Hydraulic modelling was not

Water 2018, 10, x FOR PEER REVIEW 5 of 19

used as there are too many uncertainties regarding sand transport in rivers with large boulders and

Watersteep2019 slopes., 11, 392 5 of 19 An adaptive approach [7,18] was, therefore, used to determine the most effective flow magnitude, with three different discharges being tested in 2016 (10, 15, and 20 m3/s) before usedimplementing as there are a more too many ambitious uncertainties approach. regarding The peak sand flow transport duration, in rivers between with 4 large and boulders5 hours, andwas steepchosen slopes. based on our operational experience (transfer time observed when opening the bottom gate) (FigureAn 3). adaptive A gradual approach increase [7, 18in] discharge, was, therefore, from usedminimum to determine baseflow the up most to 10 effective m3/s, was flow implemented magnitude, withto limit three fish different drift, followed discharges by a being one-hour tested stable in 2016 flow (10, at 15,this and discharge, 20 m3/s) which before corresponds implementing to the a morealert ambitiouswave required approach. for the The safety peak flowof third duration, parties. between The hydraulic 4 and 5 h, wastransit chosen time based for these on our releases operational was experienceestimated at (transfer approximately time observed 2 hours when30 minutes. opening Howe thever, bottom we chose gate) (Figureto maintain3). A the gradual target increase discharges in discharge,for, at least, from 4 hours minimum to ensure baseflow that the up tooperatio 10 m3/s,n was was successful, implemented to enable to limit comparison fish drift, followed between by the a one-hourreleases, and stable to identify flow at this the discharge,impact of each which releas correspondse on the sand to the transport alert wave distance. required The for releases the safety were of thirdgenerated parties. by Thethe bottom hydraulic gate transit to retain time a fortemperature these releases similar was to estimated the minimum at approximately flow. 2 h 30 min. However,Following we chose these to maintaininitial experiments, the target discharges a fourth for,flow at least,release 4 hwas to ensure carried that out the from operation 19 to was 20 successful,September 2017, to enable with comparison a maximum between discharge the of releases, 15 m3/s. andThe toascending identify and the impactdescending of each phases release of the on thehydrograph sand transport were managed distance. Thein the releases same wereway generatedas previous by releases, the bottom but gate the toduration retain a at temperature maximum similardischarge to thewas minimum extended flow.to 34 hours to test the effects of duration.

Figure 3. 3. HydrographsHydrographs of successive of successive flow releases flow releases carriedcarried out on the out Selves on the on 6, Selves 13, and on 206, September 13, and 202016. September 2016.

FollowingThe releases these were initial carried experiments, out on 6, a13, fourth and flow20 September release was 2016, carried to avoid out from conflicting 19 to 20 with September other 2017,uses or with temporal a maximum constraints: discharge (i) In of the 15 msummer,3/s. The the ascending reservoir and needs descending to be kept phases at a high of the level hydrograph to satisfy weretourism managed requirements, in the same and safety way as risks previous are at their releases, highest but because the duration of extensiv at maximume fishing discharge in the bypass was extendedreach; (ii) toheavy 34 h rain to test is likely the effects in autumn of duration. and could disrupt the experiment; (iii) salmonid spawning, incubationThe releases and growth were carriedphases take out onplace 6, 13,from and Nove 20 Septembermber to June. 2016, Various to avoid authors conflicting [22,40,52,53] with otherhave usesoutlined or temporal the biological constraints: risks (i)of Inthis the kind summer, of operation the reservoir on spawning, needs tobe incubation kept at a highor survival level to rate satisfy of tourismjuveniles. requirements, and safety risks are at their highest because of extensive fishing in the bypass reach; (ii) heavy rain is likely in autumn and could disrupt the experiment; (iii) salmonid spawning, incubation3.2. Continuous and Monitoring growth phases of the Experiment take place from November to June. Various authors [22,40,52,53] have outlined the biological risks of this kind of operation on spawning, incubation or survival rate Suspended sediment concentration (SSC) and water level were monitored at two stations. The of juveniles. first is located at the foot of the dam and the second is located 800 m upstream of the confluence with 3.2.the Truyère Continuous River Monitoring (Station of5 of the the Experiment geomorphological monitoring). SSC was monitored at 30-second time steps using a GGUN FL30/Albillia turbidimeter (Albillia, Neuchâtel, Switzerland) and a Tetraedre TRMCSuspended 5 autonomous sediment data concentration logger (Tetraedre, (SSC) andAuvernier, waterlevel Switzerland) were monitored. Twenty-four at two water stations. samples The firstwere is collected located atat the Stations foot of 1 theand dam 5 to andcorrelate the second SSC and is located turbidity 800 and m upstream then total of suspended the confluence sediment with theload Truy transportedère River during (Station each 5 of therelease geomorphological (SSL) by flux calculation monitoring). (TSS SSC concentration was monitored from at 30-sturbidity time steps using a GGUN FL30/Albillia turbidimeter (Albillia, Neuchâtel, Switzerland) and a Tetraedre

TRMC 5 autonomous data logger (Tetraedre, Auvernier, Switzerland). Twenty-four water samples Water 2019, 11, 392 6 of 19 were collected at Stations 1 and 5 to correlate SSC and turbidity and then total suspended sediment load transported during each release (SSL) by flux calculation (TSS concentration from turbidity measurement × water discharge). Water levels were measured using a probe and recorded every minute. Flow measurements were taken at Station 5 to verify the flow released by the dam with an ADCP RiverPro Teledyne RDI (Teledyne Marine, Stowe Drive Poway, CA, USA).

3.3. Geomorphological Monitoring Geomorphological monitoring at reach scale, i.e., from the Maury dam to the confluence with the Truyère River, was based on 116 equidistant 100-m transects. Each transect is a cross-section within the wetted perimeter. Data collection was performed on these transects at 4 periods: (i) during the summer preceding all releases (T0_16); (ii) in the autumn following the first three releases (T3_16); (iii) one week before the fourth release (T0_17); (iv) one week after the fourth release (T1_17). Measurements of the following parameters were conducted every meter within each transect: water depth at the minimum flow, dominant sediment coverage within a 50 cm radius from the point of measurement using Wentworth’s particle size classification [54], mesohabitats and thickness of fine sediments (silt, sand, and fine gravel). Similar geomorphological monitoring was undertaken at 5 stations (250 m average length), corresponding to dominant sandy bed-surface sub-reaches, and located at increasing distance downstream of the dam: 950 m (S1), 1800 m (S2), 4200 m (S3), 5200 m (S4), and 10,400 m (S5). They generally had much greater sand patches than other sub-reaches because of a lower bed slope or tributary proximity, with the exception of Station 5, which was selected for its location (downstream of the bypassed reach) and easier access. Each station was described by approximately thirty transects, and the distance between two transects corresponded to its average active channel width. The parameters measured on each transect were the same as for the reach scale monitoring. In addition, cobble and pebble sediment patches were painted on the riverbed at each of the stations (from 2 to 8 patches depending on the campaign). This station scale monitoring was carried out only in 2016 to be able to assess the effect of each of the trial discharges (10, 15, and 20 m3/s; Table1): before the first release (T0_16), after the first release (T1_16), after the second release (T2_16), and after the third release (T3_16).

Table 1. Morphological monitoring schedule (X: measurement performed).

Water Discharge Max Discharge Monitoring Reach Scale Station Scale Year Release m3/s Duration (h) Steps Monitoring Monitoring

2016 - -- T0_16 X X 2016 1 10 5 - 2016 - -- T1_16 X 2016 2 15 4 - 2016 - -- T2_16 X 2016 3 20 4 - 2016 - -- T3_16 X X 2017 - -- T0_17 X 2017 4 15 34 - 2017 - -- T1_17 X

Station 1 is immediately downstream of the upper tributary. This station was strategic as there was no sand supply during the releases upstream of this tributary: the upstream reach contained no sand, and the tributaries were at low flows. To monitor sand transport distance, 400 kg of fluorescent tracers (with sorted sand, 2–4 mm diameter) were injected into the river bed (Figure4) before the releases at Stations 3 (3 trials in 2016 and the 2017 trial), 2 (2017 trial only), and 5 (2017 trial only). Different colored tracers were used for each release. After each release, bulk samples (17 to 31) of approximately 5 kg of sediments were collected in sandy areas downstream from the injection points, continuing downstream until the colored sand had Water 2019, 11, 392 7 of 19 Water 2018, 10, x FOR PEER REVIEW 7 of 19 Water 2018, 10, x FOR PEER REVIEW 7 of 19 disappearedeffect (colored from tracers the samples.were found Discoloration 2 years after was in consideredjection). Tracer to be concentration a negligible effect of the (colored samples tracers was weredeterminedeffect found (colored 2by years image tracers after analysis were injection). found (automatic 2 Tracer years after concentrationcounting injection). of colored ofTracer the samplesconcentrationsand) to was report determinedof the their samples longitudinal by was image analysisdispersiondetermined (automatic and bydefine image counting the analysis median of colored(automatic distance sand) of counting transp to reportort. of theircoloredThis longitudinaltracing sand) technique to report dispersion istheir frequently longitudinal and define used the in mediancoastaldispersion environments distance and of define transport. [55,56], the median Thisbut few tracing distance studies technique of havetransp us isort. frequentlyed This it in tracing rivers used technique[57,58]. in coastal The is environmentsfrequently total sand used transport [55 in, 56], butvolumecoastal few was studies environments evaluated have used [55,56],by multiplying it in but rivers few studies [ 57the,58 active]. have The uslayer totaled it measured sandin rivers transport [57,58]. with volumeThescours total chains wassand evaluated transport[59] on the by multiplyingriverbedvolume at waseach the evaluated active of the layer stations by measuredmultiplying with the with the median scoursactive chainsdistancelayer measured [59 of] ontransport: the with riverbed scours (width) atchains each × (scour [59] of the on depth) stations the × riverbed at each of the stations with the median distance of transport: (width) × (scour depth) × with(median the mediandistance). distance of transport: (width) × (scour depth) × (median distance). (median distance).

FigureFigure 4. Photograph4. Photograph of of the the fluorescent fluorescent fluorescent orange orange sand tractrac tracersersers injected injected in in the the Selves Selves River River (Station (Station 3) 3) before the first release on September. before the firstfirst release on September.

4. Results 4. Results 4.1.4.1. Continuous Continuous Monitoring Monitoring of of Water Water Levels, Levels, WaterWater Velocities, and and SSC SSC 4.1. Continuous Monitoring of Water Levels, Water Velocities, and SSC TheThe various various discharges discharges tested tested generated generated substantiallysubstantially different different hydraulic hydraulic conditions conditions (Figure (Figure 5). 5). The various discharges tested generated substantially different hydraulic conditions (Figure 5). TheThe hydrograph hydrograph for for each each water water release release was was wellwell propagatedpropagated downstream. downstream. The The water water velocities velocities and and The hydrograph for each water release was well propagated downstream. The water velocities and depthsdepths measured measured by ADCPby ADCP at the at mostthe most downstream downstream station station for the for release the release show thatshow the that peak the discharge peak depthsdischarge measured measured by ADCPthere was at equalthe most to the downstream discharge generated station by for the the upstream release dam. show However, that the the peak measured there was equal to the discharge generated by the upstream dam. However, the gradual dischargegradual measured increase to there minimize was theequal impact to the on discharg aquatic faunae generated and provide by the a safetyupstream alert dam.was not However, effective the increase to minimize the impact on aquatic fauna and provide a safety alert was not effective at the gradualat the increase downstream to minimize station inthe either impact 2016 on or aquatic 2017: the fauna maximum and provide discharge a safety was alertreached was in not only effective 50 downstream station in either 2016 or 2017: the maximum discharge was reached in only 50 min. at theminutes. downstream station in either 2016 or 2017: the maximum discharge was reached in only 50 minutes.

(a) (b)

(a) (b)

(c) (d)

FigureFigure 5. View 5. View from from downstream downstream of Stationof Station 4 (see 4 (see Figure Figure1 for 1 for location) location) at ( ata) ( Minimuma) Minimum flow flow (b )(b 10) 10 m 3/s, 3 3 3 (c) 15m m/s,3 (/s,c) 15 (d m) 20/s,m (d3)/s. 20 m(c)/s. (d)

Figure 5. View from downstream of Station 4 (see Figure 1 for location) at (a) Minimum flow (b) 10 m3/s, (c) 15 m3/s, (d) 20 m3/s.

Water 2018, 10, x FOR PEER REVIEW 8 of 19

Water 2019SSC, 11at, the 392 most upstream station, i.e., immediately downstream of the dam, was always below8 of 19 0.1Water g/L 2018 for, 201610, x FOR releases PEER REVIEW and reached a maximum of 0.9 g/L in 2017. At the downstream station8 of (near19 the confluence of the Truyère River), SSC reached 2.1 g/L, 1.6 g/L, 1.0 g/L, and 2.0 g/L during the four SSC at the most upstream station, i.e., immediately downstream of the dam, was always below successiveSSC atreleases, the most respectively upstream station, (Figure i.e., 6). immediatelyThe differences downstream between ofthe the 2 stationsdam, was indicate always thatbelow each 0.1 g/L for 2016 releases and reached a maximum of 0.9 g/L in 2017. At the downstream station (near release0.1 g/L evacuated for 2016 releases fine sediment. and reached a maximum of 0.9 g/L in 2017. At the downstream station (near thethe confluence confluence of of the the TruyTruyèreère River),River), SSC SSC reached reached 2.1 2.1 g/L, g/L, 1.61.6 g/L, g/L, 1.0 g/L, 1.0 g/L,and 2.0 and g/L 2.0 during g/L duringthe four the foursuccessive successive releases, releases, respectively respectively (Figure (Figure 6). The6). differences The differences2500 between between the 2 stations the 2 stations indicate indicate that each that eachrelease release evacuated evacuated fine finesediment. sediment. 2000 2500 SSC water release 4 1500 2000 1000 SSC water release 4

1500SSC (mg/l 500 1000 SSC (mg/l 0 500 0 5000 10000 Duration (min) 0 (a) 0(b 5000) 10000 Duration (min) Figure 6. Suspended sediment concentration (SSC) observed at the downstream station (Station 5, (a) (b) Figure 1) for each flow release in (a) 2016 and (b) 2017. FigureFigure 6. 6.Suspended Suspended sedimentsediment concentration (SSC) (SSC) observed observed at at the the downstream downstream station station (Station (Station 5, 5, FigureTheFigure total1) 1) for suspended for each each flow flow releasesediment release in in ( (loadaa)) 20162016 (SSL) andand transported (b) 2017. at the downstream station by the first three releases, of equal duration but increasing intensity, are 140, 190, and 176 tonnes (±5%, [60]) respectively.TheThe total total The suspended suspended 2017 water sediment sediment release loadload was (SSL) much transported longer and at at theremoved the downstream downstream an intermediate station station by by the volume the first first three of three 180 releases, of equal duration but increasing intensity, are 140, 190, and 176 tonnes (±5%, [60]) releases,tonnes. In of 2017, equal the duration SSL was but slightly increasing lower intensity, than that are of 140, the 190, second and 176release tonnes in 2016, (±5%, despite [60]) respectively. having the respectively. The 2017 water release was much longer and removed an intermediate volume of 180 Thesame 2017 discharge water releaseand a much was much longer longer duration, and removedalthough anthe intermediate reach was already volume well-declogged of 180 tonnes. Inin 2017,2017 tonnes. In 2017, the SSL was slightly lower than that of the second release in 2016, despite having the thebefore SSL the was release. slightly lower than that of the second release in 2016, despite having the same discharge same discharge and a much longer duration, although the reach was already well-declogged in 2017 and aFor much the longer three duration,water releases although in the2016 reach with was a already4-hour well-decloggedpeak discharge, in 2017the beforeSSC levels the release. were beforeFor the release. three water releases in 2016 with a 4-h peak discharge, the SSC levels were substantially substantiallyFor the reduced three water just 2.5releases hours in after 2016 flooding, with a 4-byhour a factor peak of discharge, 4, 3, and the2 after SSC the levels three were water reduced just 2.5 h after flooding, by a factor of 4, 3, and 2 after the three water releases, respectively. releases,substantially respectively. reduced Thejust 2.5cumulative hours after SSL flooding, curves by (Figure a factor 7) of also 4, 3, demons and 2 aftertrate the that three 50% water of the The cumulative SSL curves (Figure7) also demonstrate that 50% of the sediment was transported in sedimentreleases, was respectively. transported The in cumulativeless than 2 hoursSSL curves and around (Figure 90% 7) also in 5 demonshours. Thetrate duration that 50% of of the the 2016 less than 2 h and around 90% in 5 h. The duration of the 2016 releases was, therefore, sufficient to releasessediment was, was therefore, transported sufficient in less thanto export 2 hours most and of around the fine 90% sediment in 5 hours. that The could duration be transported of the 2016 in export most of the fine sediment that could be transported in suspension. suspension.releases was, therefore, sufficient to export most of the fine sediment that could be transported in suspension.

Figure 7.7.Cumulative Cumulative percentage percentage of suspendedof suspended sediment sedi loadment (SSL) load of (SSL) 2016 releasesof 2016 at thereleases downstream at the Figure 7. Cumulative percentage of suspended sediment load (SSL) of 2016 releases at the downstream station (Station 5). stationdownstream (Station station 5). (Station 5). 4.2. Morphological Monitoring across the Bypassed Reach 4.2.4.2. Morphological Morphological Monitoring Monitoring across across thethe Bypassed ReachReach Before the releases, large accumulations of silt and sand were observed at the confluences of BeforeBefore the the releases, releases, large large accumuaccumulations ofof siltsilt andand sand sand were were observed observed at atthe the confluences confluences of of tributaries and in sub-reaches where the slope was low (up to 50 cm, Figure8). Between T 0_16 and tributariestributaries and and in in sub-reaches sub-reaches wherewhere the slope waswas lowlow (up (up to to 50 50 cm, cm, Figure Figure 8). 8). Between Between T0_16 T 0_16and and T3_16, the majority of large fine sediment deposits observed before the releases (Figure9a) disappeared, T3_16T3_16, ,the the majority majority ofof largelarge finefine sedimentsediment deposidepositsts observedobserved beforebefore the the releases releases (Figure (Figure 9a) 9a) which resulted in a decrease of average fine sediment thickness and an increase of average water depth.

Water 2019, 11, 392 9 of 19 Water 2018, 10, x FOR PEER REVIEW 9 of 19 Water 2018, 10, x FOR PEER REVIEW 9 of 19 After these three releases, no significant resedimentation was observed on the bypassed reach except disappeared,disappeared, whichwhich resultedresulted inin aa decreasedecrease ofof averageaverage finefine sedimentsediment thicknessthickness andand anan increaseincrease ofof at Cross-sectionaverageaverage waterwater 32. depth.depth. Between AfterAfter theseTthese3_16 threeandthree T releases,0_17releases,, a period nono significantsignificant of almost resedimentationresedimentation one year without waswas flow observedobserved releases, onon thethe some newbypassed sedimentation reach except patches at Cross-section were observed 32. Between around T the3_16 mainand T0_17 tributaries,, a period of particularly almost one downstreamyear without of bypassed reach except at Cross-section 32. Between T3_16 and T0_17, a period of almost one year without flow releases, some new sedimentation patches were observed around the main tributaries, Stationflow 1releases, and Station some 4 (Figurenew sedime9b). Betweenntation patches T 0_17 and were T1_17 observed(Figure 9aroundc), sediments the main transported tributaries, from particularly downstream of Station 1 and Station 4 (Figure 9b). Between T0_17 and T1_17 (Figure 9c), Stationparticularly 1 to Station downstream 2 were of moved Station and 1 and deposited Station 4 again. (Figure Around 9b). Between Station T0_17 1, waterand T1_17 depths (Figure increased. 9c), Downstreamsedimentssediments transportedtransported of Station 2, fromfrom sedimentation StationStation 1 1 toto StationStation patches 2 2 were alternatedwere movedmoved with andand deposited scoureddeposited patches. again. again. AroundAround Overall StationStation changes 1, water depths increased. Downstream of Station 2, sedimentation patches alternated with scoured were1, water less marked depths thanincreased. between Downstream T0_16 and of T 3_16Station, showing 2, sedimentation some redistribution patches alternated of fine sedimentswith scoured along patches. Overall changes were less marked than between T0_16 and T3_16, showing some redistribution thepatches. bypassed Overall reach. changes Comparing were less T0_16 markedand T than1_17, between shows that T0_16 the and large T3_16, sand showing deposits some near redistribution the tributary of fine sediments along the bypassed reach. Comparing T0_16 and T1_17, shows that the large sand confluencesof fine sediments before the along releases the bypassed (T0_16) were reach. generally Comparing removed T0_16 andand thatT1_17, water shows depths that the usually large droppedsand deposits near the tributary confluences before the releases (T0_16) were generally removed and that consequentlydeposits near in the tributary same sub-reaches confluences (Figure before9d). the releases (T0_16) were generally removed and that waterwater depths depths usually usually dropped dropped consequently consequently in in the the same same sub-reaches sub-reaches (Figure (Figure 9d). 9d).

Figure 8. Longitudinal distribution of average water depth and fine sediment thickness at T0_16 in the FigureFigure 8. Longitudinal8. Longitudinal distribution distributionof of averageaverage water depth depth and and fine fine sediment sediment thickness thickness at T at0_16 T 0_16in thein the bypassed reach and position of main geomorphic units with their respective slopes. bypassedbypassed reach reach and and position position of of main main geomorphicgeomorphic units with with their their respective respective slopes. slopes.

((aa)) ((bb))

((cc)) ((dd))

Figure 9. Change in average water depth and fine sediment thickness on each of the cross-sections in

the bypassed reach. (a)T0_16–T3_16,(b)T3_16–T0_17,(c)T0_17–T1_17 and (d)T0_16–T1_17. Water 2018, 10, x FOR PEER REVIEW 10 of 19

Figure 9. Change in average water depth and fine sediment thickness on each of the cross-sections in Water 2019, 11, 392 10 of 19 the bypassed reach. (a) T0_16–T3_16, (b) T3_16–T0_17, (c) T0_17–T1_17 and (d) T0_16–T1_17.

AverageAverage fine fine sediment sediment thickness thickness clearly clearly dropped followingfollowing the the first first releases releases (− 1.8(−1.8 cm cm between between T0_16T0_16–T3_16–T)3_16 and) and the the last last release release (− (0.4−0.4 cm cm between between T0_17–T–T1_171_17) )(Table (Table 22).). TheThe transient transient period period between between releasesreleases slightly slightly increased increased the the average average sand sand deposit. deposit. The The average average water water depth depth increased increased between between T0_16 andT 0_16T3_16and (+0.7cm). T3_16 (+0.7 It then cm). Itdropped then dropped during during the year the yearbetween between releases, releases, presumably presumably in in relation relation to tributaryto tributary inflows inflows (−0.5 (cm−0.5 between cm between T3_16–T T0_173_16).–T It0_17 continued). It continued to drop to between drop between T0_17 and T0_17 T1_17and (−0.8cm). T1_17 (−0.8 cm). Table 2. Average water depth and fine sediment thickness in the bypassed reach for each of the surveys.Table 2. Average water depth and fine sediment thickness in the bypassed reach for each of the surveys. Parameters T T T T Parameters 0_16 T0_163_16 T3_16 0_17 T0_17 1_17 T1_17 AverageAverage water water depth depth (cm) (cm) 26.0 26.026.7 26.7 26.2 26.2 25.4 25.4 Average fine sediment 4.8 3.0 3.1 2.7 Average thicknessfine sediment (cm) thickness (cm) 4.8 3.0 3.1 2.7

The change in cumulated fine sediment thickness between successive releases clearly showed a The change in cumulated fine sediment thickness between successive releases clearly showed a drop in the sedimentary stock of fine materials in the river (Figure 10). The first 3 releases performed drop in the sedimentary stock of fine materials in the river (Figure 10). The first 3 releases performed between T0_16 and T3_16 had a significant effect. The cumulated average fine sediment thickness between T0_16 and T3_16 had a significant effect. The cumulated average fine sediment thickness reduced by a little more than 29% following the three releases. However, only a small difference is reduced by a little more than 29% following the three releases. However, only a small difference is observed between T3_16 and T0_17. A slight trend for sand to aggrade is again observed at the first observed between T3_16 and T0_17. A slight trend for sand to aggrade is again observed at the first tributarytributary confluence confluence for for Station Station 11 andand after after the the river rive gorges.r gorges. The The final final release release also contributed also contributed to a drop to a dropin sedimentaryin sedimentary stock, stock, but tobut a lesserto a lesser extent. extent. Downstream Downstream of Station of 1,Station progressive 1, progressive erosion is erosion clearly is clearlyevident evident between between T0_16 and T0_16 T3_16 andand T3_16 between and between T3_16 and T T3_161_17 and. It isT the1_17. clearestIt is the erosion clearest effect erosion observed effect observedon the reach.on the The reach. downstream The downstream V-shape narrow V-shape valley narrow reach valley (between reach Station (between 2 and Station 32 orand even Station 4) 3 orgenerally even 4) seemsgenerally to be seems in sediment to be in balance, sediment since ba sedimentlance, since inflows sediment and outflows inflows do and not outflows seem to have do not seemany to repercussions have any repercussions on the changes on to the fine changes sediment to thicknessesfine sediment between thicknesses releases. between This is, therefore,releases. This a is, sedimenttherefore, transfer a sediment reach. transfer Erosion processesreach. Erosion then pick processes up again then downstream pick up ofagain the narrowdownstream valley, andof the narrowthe balance valley, stabilizes and the afterbalance the stabilizes valley widened after th (downstreame valley widened from Station (downstream 4). from Station 4).

FigureFigure 10. 10. CumulativeCumulative fine fine sediment sediment thicknessthickness in in the the bypass bypass reach. reach.

ChangesChanges in infine fine sediment sediment thickness thickness are are significan significantlytly tiedtied toto thethe various various mesohabitats mesohabitats units units [61 [61].]. In particular,In particular, changes changes were were particularly particularly clear clear for for glides (Figure(Figure 11 11),), where where median median sediment sediment thickness thickness droppeddropped considerably considerably after after each each flow flow release release (reduc (reductiontion byby aa factor factor of of 2 2 or or more). more). Faster Faster flowing flowing mesohabitatsmesohabitats units units (riffles, (riffles, cascad cascades,es, and and rapids) rapids) showedshowed nono change. change.

Water 2019, 11, 392 11 of 19 Water 2018,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 11 of 19

45 Glide 40 35 30 25 20 15 10 5 0 T0_16 T3-16 T0-17 T1-17

Figure 11. DistributionDistributionDistribution ofof of averageaverage average finefine fine sedimentsediment sediment thicknessthickness thickness onon on glideglide glide atat atTT0_16 T0_16,, TT3_16,T,,3_16 TT0_17,T,, andand0_17 ,TT and1_17 (cross(cross T1_17 (cross= average, = average, bars = barsmedian, = median, 25th and 25th 75th and percentiles 75th percentiles and min and and min max). and max).

Bed-sediment composition became progressively coarsercoarser during the experiments, as determined by measured grain-size grain-size distributions distributions (Figure (Figure 12). 12). The The proportion proportion of ofsilt silt was was reduced reduced by bya factor a factor of 2.5 of 2.5after after the three the three short short releases releases of 2016 of (from 2016 (from10% to 10% 4% between to 4% between T0_16 to T0_163_16) andto T became3_16) and null became (1%) after null (1%)the long after release the long of 2017 release (T0_17 of to 2017 T1_17 (T). 0_17The tointermediate T1_17). The period intermediate between period the third between and fourth the third releases and fourthled to slight releases silt led deposits to slight (+2%, silt deposits T3_16 to (+2%,T0_17). TThe3_16 proportionto T0_17). The of proportionsand in the of sediment sand in the distribution sediment distributiondecreased very decreased slightly very after slightly each release, after each although release, a althoughportion of a the portion sand of was the exported sand was to exported the edges to the(observed edges (observedbut not calculated). but not calculated). As a result As of athe result overall of the decrease overall of decrease fine sediments, of fine sediments, coarse particles, coarse particles,such as boulders, such as boulders,rocks, and rocks, flagstones, and flagstones, and prim andarily primarily coarse cobbles coarse were cobbles more were common more commonafter the afterreleases. the releases.

Silt Sand Fine gravel Coarse gravel Fine pebble Coarse pebble Fine cobble Coarse cobble Boulder / Rock / flagstone

38 42 47 44

10 6 17 4 22 4 20 4 7 3 4 7 14 24 1 14 4 13 2 2 12 9 3 10 12 9 8 10 7 4 6 1 T0_16 T3_16 T0_17 T1_17

Figure 12. DistributionDistribution (%)(%) ofof particleparticle sizessizessizes observed obseobserved for forthe the four four monitoring monitoring campaigns campaigns (T (T0_160_16to toto T TT1_171_17)) in thethe bypassedbypassed channel.channel.

4.3. Station-Based Morphological Monitoring At Station 1 (where the incoming sediment suppl supplyy is low), fine fine sediment thickness dropped by approximately 60% following the first first flow flow release and around 90% after the the second second one one (Figure (Figure 13a).13a). This translates into a sharpsharp increase in waterwater depth,depth, whichwhich almostalmost doubleddoubled after the secondsecond release (Figure(Figure 1313b).b). The third release was less effective because the remaining finefine sediments were trapped behind boulders. At the other stations, fewer changes were observed because of the incoming fine sediment supply from upstream sub-reaches. At Station 2, fine sediment thickness and water depth remained relatively constant during all of the 2016 experiments because of a strong sand supply coming from Station 1. At Station 5, water depth did not change significantly despite a decrease of fine sediment thickness after two releases. In fact, fine sediment thickness was already very low in this station before the experiments so that its evolution in terms of absolute value was limited. At Station 3 and

Water 2018, 10, x FOR PEER REVIEW 12 of 19

Water4, the2019 first, 11, 392release led to insignificant changes, but average water depth increased progressively12 as of 19 fine sediment thickness decreased after the second release.

(a) (b)

FigureFigure 13. 13.( a(a))Evolution Evolution ofof averageaverage fine fine sediment sediment thickness thickness and and (b (b) )average average water water level level before before and and after each 2016 release at the 5 stations (100 base level at T0_16). after each 2016 release at the 5 stations (100 base level at T0_16).

4.4. AtBedload the other Monitoring stations, fewer changes were observed because of the incoming fine sediment supply from upstream sub-reaches. At Station 2, fine sediment thickness and water depth remained relatively constant4.4.1. Pebble during and all Cobble of the Entrainment 2016 experiments because of a strong sand supply coming from Station 1. At StationAccording 5, water to the depth painted did not particles change survey, significantly the largest despite particles a decrease mobilized of fineby all sediment releases thicknessin 2016 afterwere two small releases. cobbles In (64–128 fact, fine mm), sediment but they thickness were few was in alreadynumber veryand transported low in this stationonly over before short the experimentsdistances (less so than that its5 meters). evolution Most in termsof the painted of absolute particles value transported was limited. were At medium Station 3 gravels and 4, the(8–16 first releasemm) during led to insignificantthe 1st release, changes, coarse gravels but average (16–32mm) water during depth increasedthe 2nd release progressively and very coarse as fine gravels sediment thickness(32–64 mm) decreased during afterthe 3rd the release. second The release. discharge values of the first two releases (10 and 15 m3/s) resulted in limited entrainment of medium-sized particles (required for fish spawning and to 4.4.preserve Bedload the Monitoring diversity of aquatic habitats). In contrast, the discharge value of the 3rd release (20 m3/s) resulted in much greater mobilization of coarser sediments, which is contrary to the initial objectives 4.4.1.agreed Pebble upon. and Cobble Entrainment According to the painted particles survey, the largest particles mobilized by all releases in 2016 were4.4.2. small Sand cobblesExport (64–128 mm), but they were few in number and transported only over short distancesAt Station (less than 3, the 5 m). transport Most of distances the painted of fluore particlesscent transported tracers (2–4 were mm) medium increased gravels linearly (8–16 with mm) duringdischarge the with 1st release, a proportionality coarse gravels ratio (16–32greater mm) than during1 (Table the 3). 2ndCompared release to and the very first coarserelease, gravels the (32–64median mm) transport during distance the 3rd was release. increased The dischargeby a factor values of 2.4 for of thethe firstsecond two release releases and (10 by and a factor 15 m of3/s) resulted3.6 for the in limitedthird release. entrainment For a constant of medium-sized volume of particles water, the (required transport for efficiency fish spawning of a release and to should preserve thebe diversityimproved of if aquatic one maximizes habitats). its In peak contrast, flow therather discharge than its value duration. of the However, 3rd release the (20 previous m3/s) resulted results in muchshowed greater that mobilization20 m3/s release of coarser can be sediments,detrimental which to coarse is contrary sediments to the necessary initial objectives for fish agreedspawning upon. patches. 4.4.2. SandThe fourth Export release was dimensioned in light of this last observation, keeping a flow rate of 15 m3/s but increasing its duration. At Station 3, the median transport distance was only slightly more At Station 3, the transport distances of fluorescent tracers (2–4 mm) increased linearly with than twice that obtained for the second release despite a duration five times greater. discharge with a proportionality ratio greater than 1 (Table3). Compared to the first release, the median For the water releases in 2016, the total sand distance transport increased with the discharge transport distance was increased by a factor of 2.4 for the second release and by a factor of 3.6 for the from 30 m to 111 m. In fact, based on a similar active layer depth as measured by scour chains, the third release. For a constant volume of water, the transport efficiency of a release should be improved volume transported is proportional to the particle transport distance: The increase in discharge from if one maximizes its peak flow rather than its duration. However, the previous results showed that 10 to 15 m3/s transported a volume 2.8 times greater; when the discharge doubles, i.e., for the 20 m3/s 3 20release, m /s releasethe transported can be detrimental volumes increase to coarse by sedimentsa factor of 3.9. necessary for fish spawning patches. The fourth release was dimensioned in light of this last observation, keeping a flow rate of 15 m3/s but increasing its duration.Table At Station3. Transport 3, the distances median of transportthe fluorescent distance tracers. was only slightly more than twice that obtained for the second release despite a duration five times greater. Flow Release 1 Flow Release 2 Flow Release 3 Flow Release 4 Flow Release 4 Flow Release 4 ForParameters the water releases2016 in 2016, the2016 total sand distance2016 transport2017 increased2017 with the discharge2017 from 30 m to 111 m. In fact,Station based 3 on a similarStation 3 active layerStation depth 3 asStation measured 3 byStation scour 2 chains,Station the volume 5 Discharge (m3/s) 10 15 20 15 15 15 transportedDuration (h) is proportional5 to the particle 4 transport 4 distance: The 34 increase in34 discharge from34 10 to 15 m3/s transported a volume 2.8 times greater; when the discharge doubles, i.e., for the 20 m3/s release, the transported volumes increase by a factor of 3.9.

Ultimately, the estimated transported volumes ranged from approximately 100 m3 for the 2016 releases, to several hundred m3 for the 2017 releases. Water 2019, 11, 392 13 of 19

Table 3. Transport distances of the fluorescent tracers.

Flow Flow Flow Flow Flow Flow Release 1 Release 2 Release 3 Release 4 Release 4 Release 4 Parameters 2016 2016 2016 2017 2017 2017 Station 3 Station 3 Station 3 Station 3 Station 2 Station 5 Discharge (m3/s) 10 15 20 15 15 15 Duration (h) 5 4 4 34 34 34 Median transport 30 71 111 164 879 223 distance (m) Total sand 24 68 93 162 550 368 transport (m3)

5. Discussion and Recommendations

5.1. Design of the Flow Release and Impacts on SSC The attenuation of peak depth magnitude sometimes observed in the flow diffusion downstream of dams [33,35] did not occur on the Selves at maximum flow, even during the short 2016 releases: The nominal flow rate was the same from the Maury dam to the output of the bypassed reach. However, attenuation of peak magnitude was observed for the lowest discharges, corresponding to the alert wave (for safety and fish protection). In the absence of flow measurements in the middle part of the bypassed reach, it is impossible to define where the energy loss occurred in 2016. Thanks to the installation of a water level monitoring station in 2017 at S2, it seems that this loss occurred between Stations 2 and 3. We attribute a large portion of this loss to the pronounced step-pool morphology of the bed channel between Stations 2 and 3 where the slope of the valley bottom is higher. Longitudinal variations in channel morphology must, therefore, be taken into account to ensure that the alert wave (for fish protection and safety) are effective along the entire reach. SSC was closely monitored to meet regulatory requirements and to evaluate the success of the releases with respect to the export of fine sediments. The SSC peaks coincided with the flood waves and their peak values decreased for each successive release, despite their increasing intensity. Similar decreases in SSC have already been observed in other rivers following successive releases which have the same peak discharge value [36,62,63]. The increase in volumes between the first and second releases can be attributed to higher specific stream powers and water levels: Besides the obvious influence of flow intensity on transport rate, the higher water level of the second release has remobilized more overbank fine materials. The lower SSL of the third release, despite an even greater discharge, shows that the environments were already well-leached after the second one. The example of the Selves in 2016 illustrates that the volume of fine sediment transported may increase if the flow discharge of a following release is higher than the first, even when it is undertaken relatively soon (i.e., 1 week) after the previous one. It is difficult to compare the 2017 release with the previous releases because it took place in a reach that was already well-declogged, demonstrating flow release success is highly contingent on initial conditions. However, the volume of sediment transported in 2017 shows that the environment tends to gradually reclog, requiring frequent flow releases. Thus, even if the SSC level achieved can be an indicator of release success in itself, the volumes transported should also be calculated in such cases to evaluate and compare the effects of successive releases. The percentage of silt in the channel bed was reduced from 10% to 4% after the first release. This first result shows that regular flow releases of 10 m3/s during 5 h is efficient to entrain silt in the bypassed reach of the Selves River. This flow is approximately twice the pre-dam average annual flow. Tests could also be performed at slightly lower discharges to observe their efficacy. Water 2019, 11, 392 14 of 19

5.2. Meso-Habitat Improvement Unlike for silt, the four flow releases were not long enough to export all of the sand from the bypassed reach, however, they did result in a significant decrease in the amount of sand stored in the bed channel. This decrease can be explained by a partial export to the Truyère River and by sand accumulation along the river edges (Figure 14), as also observed on the Colorado River [31]. The latter phenomenon, which has also been documented for larger rivers [23,29,32], ultimately results in improved channel mesohabitat conditions without requiring complete sand transport over the entire bypassedWater 2018 reach,, 10, x whichFOR PEER would REVIEW require a much longer release. Lateral sand export allows for14 achieving of 19 habitat objectives while also minimizing gravel transport and economizing water volumes used during habitat objectives while also minimizing gravel transport and economizing water volumes used the releases. However, the lateral accumulation of sand could result in raising and progressively during the releases. However, the lateral accumulation of sand could result in raising and disconnectsprogressively the adjacent disconnects floodplain. the adjacent floodplain.

(a) (b)

FigureFigure 14. 14.(a )(a Creation) Creation of of new new sand sand barsbars and ( (bb)) sand sand deposition deposition on on the the river river banks. banks.

TheThe 10 m 103/s m discharge3/s discharge is likely is likely to promote to promote sedimentation sedimentation on reaches on reaches sensitive sensitive to sand aggradation.to sand Theaggradation. critical shear The stress critical is shear undoubtedly stress is undoubtedly not reached not for reached this discharge for this discharge in such in areas. such areas. The 15 The m 3/s 3 release15 m seems/s release the mostseems effective the most aseffective it generates as it generate a clears reduction a clear reduction in the percentagein the percentage of silt of and silt sandand for sand for all stations, with no exceptions, and without significant coarse sediment transport. The last all stations, with no exceptions, and without significant coarse sediment transport. The last 20 m3/s 20 m3/s release had contrasting effects on sand deposits at the station scale. As the discharge was release had contrasting effects on sand deposits at the station scale. As the discharge was theoretically theoretically more effective than for the second release, this observation is only due to the irregularity more effective than for the second release, this observation is only due to the irregularity of inflowing of inflowing sediments. Diverging results between stations have already been reported by other sediments.investigators Diverging during water results releases, between despite stations those have stations already having been a similar reported gradient by [23]. other This investigators diverse duringbehavior water shows releases, the despiteimportance those of monitoring stations having sediment a similar transfer gradient at the reach [23]. Thisscale diverseto learn behaviorfrom showsexperiments the importance and choosing of monitoring the most sediment effective transfer discharg ate. theIn addition, reach scale the to rele learnase fromresulted experiments in greater and choosingmobility the of most coarse effective sediments. discharge. However, In as addition, there is theno natural release replacement resulted in greaterof this sediment mobility due of coarseto sediments.upstream However, trapping asby therethe dam, is no mobilizing natural replacementthe coarse sediment of this could sediment work due contrary to upstream to the objectives, trapping by the dam,in that mobilizing these sediments the coarse are one sediment of the major could components work contrary of desired to the fish objectives, habitat [37,39]. in that these sediments are one ofThe the average major componentsthickness of silt of desiredand sand fish was habitat reduced [37 ,by39 ].a factor of 1.8 in the bypassed reach channelThe average after thicknessthe four releases, of silt and which sand is was largely reduced due byto athe factor export of 1.8of insilt. the The bypassed reduction reach in channelfine aftersediments the four releases,below the which dam had is largely positive due effects to the on substrates, export of silt. and Themore reduction generally inon fine physical sediments habitats, below by increasing the water depth at low flow. This last effect, which is not obvious at the bypassed reach the dam had positive effects on substrates, and more generally on physical habitats, by increasing the scale, is clearly positive for the monitoring stations most sensitive to fine sediment deposits. Other water depth at low flow. This last effect, which is not obvious at the bypassed reach scale, is clearly authors have previously reported an increase in water depth following water releases [12], creating positivenew forhabitats the monitoringfor fish, however, stations for mostthe Selves sensitive River, to this fine is sediment a localized deposits. effect (only Other very authors clogged have previouslystations reportedwere affected) an increase and does in waternot apply depth to the following whole reach. water releases [12], creating new habitats for fish, however, for the Selves River, this is a localized effect (only very clogged stations were affected) and5.3. does Sand not Export apply to the whole reach. The fluorescent tracing of sediment particles was motivated by the need to evaluate the release 5.3. Sand Export duration necessary to evacuate sand from the bypassed reach. According to the results from the 2016 experimentThe fluorescent on a single tracing station, of sediment the transport particles distan wasce is motivatedlinearly related by the to discharge need to evaluate magnitude the with release durationa proportionality necessary to ratio evacuate greater sand than from one the(see bypassed Table 3). reach.The 2017 According experimental to the results results from from three the 2016 stations demonstrate that transport distances differ strongly from one site to another, even when these sites did not initially appear to have substantial morphological differences. At Station 3, this result could be explained by heterogeneous hydraulic conditions within the reach, probably less favorable for transport in its downstream part. In the same way, the results obtained downstream from the two other stations are significantly different because of specific hydraulic conditions including bed slope, bankfull width, and hydraulic roughness. These results confirm the need to equip several sites because responses are site-specific [18]. Three hypotheses explain these differences

Water 2019, 11, 392 15 of 19 experiment on a single station, the transport distance is linearly related to discharge magnitude with a proportionality ratio greater than one (see Table3). The 2017 experimental results from three stations demonstrate that transport distances differ strongly from one site to another, even when these sites did not initially appear to have substantial morphological differences. At Station 3, this result could be explained by heterogeneous hydraulic conditions within the reach, probably less favorable for transport in its downstream part. In the same way, the results obtained downstream from the two other stations are significantly different because of specific hydraulic conditions including bed slope, bankfull width, and hydraulic roughness. These results confirm the need to equip several sites because responses are site-specific [18]. Three hypotheses explain these differences among sites. The first hypothesis is that there is an attenuation of the peak depth magnitude further downstream [33,35]. However, this hypothesis is highly unlikely since the range of water depths at the most downstream station were the same as those further upstream. The second hypothesis is related to site-specific morphological and hydraulic conditions. Previous investigators have shown that the results obtained from water releases cannot necessarily be applied from one site to another [18]. Local flow conditions are likely to be key in determining morphodynamic responses [64], which create different responses for each site. The main difference between the sites is that downstream of Station 1, the released water is already completely saturated with sand, thereby reducing its capacity for erosion. Finally, the last hypothesis corresponds to a measurement bias with the fluorescent tracers. For example, at Station 2, some of the tracer material settled on the river banks and was not taken into account in the analyses, which leads to overestimation of the distances travelled. Given all of these uncertainties, for a 15 m3/s release, we adopted a median transport velocity of around 15 m/hour in areas conducive to sand transport and slightly less in more lentic reaches. Based on these assumptions, it would take at least 30 days and up to 45 days to fully export sand from the entire bypassed reach at a constant discharge of 15 m3/s. For all of the 2016 water releases, which are quite similar in terms of duration, the transported volume increased with increasing discharge. An increase in discharge from 10 to 15 m3/s resulted in a transported volume around 2.8 times higher. Likewise, an increase in discharge from 15 to 20 m3/s transported a volume around 3.9 times greater. These results are similar to those made by Collier et al. [22], where the volumes of sand transported also increased significantly with discharge. However, the rate of increase given by Collier et al. [22] specifically an 8-fold increase in transported volume when discharge was doubled, was not observed in our case. This difference can be attributed to the difference in size and morphology of the rivers studied. Finally, these results show that even relatively low peak discharges can influence the physical habitat of the flow channel in rivers with a high sand load [23]. In our case, a flow equal to three times the pre-dam mean annual flow is effective.

5.4. Perspectives Our study demonstrates that it is possible to successfully undertake water releases in small rivers with an adaptive management approach. Specifically, we have shown that hydraulic modelling is not always necessary for this type of operation and that simple geomorphological monitoring is able to provide the necessary information to evaluate the effectiveness. Although the movement of materials larger than those targeted is inevitable, it should be limited as far as possible in the absence of coarse sediment supply downstream from the dam. It is, therefore, strongly recommended to continue to use an iterative approach to determine this critical discharge in terms of magnitude, duration, and frequency, starting with low discharges for a short duration. For the Selves River, two discharge levels are now recommended: 10 m3/s for silts and 15 m3/s for sands. To export most silt out of the bypassed reach, the release time required is only 5 h. For sands, the duration is not yet defined because it should be adapted to the quantity of tributary inputs. These contributions are not yet known but could be quantified at stations to be established immediately downstream of the tributaries, using the same measurement protocol presented in this study. Water 2019, 11, 392 16 of 19

Initial results show that silt aggradation can be observed between two releases one year apart. This annual silt deposit is not necessarily problematic for the environment, but a target threshold should be defined beyond which a water release should be carried out. We suggest continuing the full geomorphological monitoring of the reach and to carry out new water releases when the silts reach more than 7% of the overall grain size distribution corresponding to a “good” situation for the stakeholder and also equivalent to conditions observed at T0_2017. This value should be reassessed with the forthcoming biological monitoring results. For sand, monitoring events (storms) likely to transfer sediments into the Selves River from tributaries would provide useful information to quantify the effects of major storms on the volume of sediment stored into the Selves. It would then be possible to conduct a release to ensure a well-balanced sediment budget. To evaluate the biological effects of these releases (and ultimately their ecological efficacy), biological monitoring (fish, spawning areas, macroinvertebrates) should be continued for at least five more years to ensure that the conditions defined by these thresholds are adequate. There is also a need to compare monitoring results (both sediment and biological responses) with reference conditions on a nearby river to objectively evaluate release success. It is difficult to use data from other publications without at least some basic information and meta-analysis of such experiments are needed to improve release design and determine a priori potential channel responses. Therefore, to make these types of studies comparable, we strongly recommend that experimenters systematically state at least the following information in their publications: catchment area, historical data on flood and average discharges, average bed slope and particle size distribution of the channel and of the sediment load.

6. Conclusions Conducting iterative water releases on the Selves River paid off with a reduction of surficial clogging, effective sand export, and water depth increases while avoiding mobility of coarser sediments, all while requiring only limited human and financial resources. Although the discharges were chosen empirically, they ultimately were in the necessary range for the management objectives and allowed us to identify the optimum discharge for future operations. We now know that a discharge slightly below or equal to 10 m3/s enables significant transport of SSCs but not of the sand, whereas a discharge of 15 m3/s provides much greater movement of sand sediments. Higher discharges are not cost-effective because of the risk of entrainment of coarse particles and because the gain in distance traveled relative to the increased discharge is smaller. The measurements performed at station scale and at the overall bypassed reach scale provided complementary information. For example, we saw that water levels increased significantly in clogged areas but did not vary at the reach scale. The station-scale monitoring allowed us to precisely describe the effects of each release and, thus, to validate each step according to an adaptive management process. The status of the watercourse and the state of its streambed sediment has been improved by transporting silt and sand out of the bypassed reach, through over bank deposition, and by increasing the water depth in sandy sectors, thereby meeting stakeholder expectations. Managers and users are now convinced of the value of this kind of operation because of visual habitat improvement with no evidence of potential ecological damage during the experiment. Water releases are, therefore, a management option that is now locally accepted.

Author Contributions: R.L. and J.-R.M. conceived and designed the study. L.G. assisted with the measurements. R.L. analyzed the data and wrote the MS as leader-author. H.P. analyzed the data and reviewed the paper. O.O. assisted with the relation between stakeholders. Funding: This research was funded by Electricity of France (EDF) and by Adour Garonne Water Agency (AEAG). Acknowledgments: The authors would like to thank the teams from the research firms (Dynamique Hydro and Hydrogéosphère) that assisted with the measurements and also the EDF teams who worked before and during the water releases (Sébastien Zanker, Pierre Oustrière, Thomas Geay, Valentin Lemaire, Yann Lemoine, Stéphane Chataigner, Luc Tabary, Gilette Guidet, Lucas Bascoul), Sebastien Menu for his expertise on the state of the river after the releases and Leah Bêche for English proofreading. The authors would also like to thank the Olt Valley Water 2019, 11, 392 17 of 19 hydropower operational group (Julien Julhes and Jean François Lesade) and the operating technicians who helped carry out the releases under optimal conditions. Finally, the authors would like to thank the study steering committee for their constructive suggestions and involvement in the various water releases: Aubrac Regional Nature Park, Adour Garonne Water Agency, fishing associations, the fishing federation, the Aveyron department territories division and the municipalities in the catchment area. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Dynesius, M.; Nilsson, C. Fragmentation and flow regulation of river systems in the northen third of the world. Science 1994, 266, 753–762. [CrossRef][PubMed] 2. Kondolf, G.M. Hungry water: Effects of dams and gravel mining on river channels. Environ. Manag. 1997, 21, 533–551. [CrossRef] 3. Brandt, S.A. Classification of geomorphological effects downstream of dams. Catena 2000, 40, 375–401. [CrossRef] 4. Petts, G.E.; Gurnell, A.M. Dams and geomorphology: Research progress and future directions. Geomorphology 2005, 71, 27–47. [CrossRef] 5. Williams, G.P.; Wolman, M.G. Downstream Effects of Dams on Alluvial Rivers; U.S. Geological Survey, Professional Paper 1286; United States Government Priting Office: Washiton, DC, USA, 1984. 6. Arthington, A.H.; Zalucki, J.M. Comparative Evaluation of Environmental Flow Assessment Techniques: Review of Methods; Land and Water Resources Research and Development Corporation: Canberra, Australia, 1998. 7. Stanford, J.A.; Ward, J.V.; Liss, W.J.; Frissell, C.A.; Williams, R.N.; Lichatowich, J.A.; Coutant, C.C. A general protocol for restoration of regulated rivers. River Res. Appl. 1996, 12, 391–413. [CrossRef] 8. Poff, N.L.; Allan, J.D.; Bain, M.B.; Karr, J.R.; Prestegaard, K.L.; Richter, B.D.; Sparks, R.E.; Stromberg, J.C. The natural flow regime: A paradigm for river conservation and restoration N. Bioscience 1997, 47, 769–784. [CrossRef] 9. Montgomery, D.R.; Bolton, S. Hydrogeomorphic variability and river restoration. In Strategies for Restoring River Ecosystems: Sources of Variability and Uncertainty in Natural and Managed Systems; American Fisheries Society: Bethesda, MD, USA, 2003; pp. 39–80. 10. Robinson, C.T.; Uehlinger, U. Using artificial floods for restoring river integrity. Aquat. Sci. 2003, 65, 181–182. [CrossRef] 11. Zahar, Y.; Ghorbel, A.; Albergel, J. Impacts of large dams on downstream flow conditions of rivers: Aggradation and reduction of the Medjerda channel capacity downstream of the Sidi Salem dam (Tunisia). J. Hydrol. 2008, 351, 318–330. [CrossRef] 12. Mürle, U.; Ortlepp, J.; Zahner, M. Effects of experimental flooding on riverine morphology, structure and riparian vegetation: The River Spöl, Swiss National Park. Aquat. Sci. 2003, 65, 191–198. [CrossRef] 13. Konrad, C.P.; Olden, J.D.; Lytle, D.A.; Melis, T.S.; Schmidt, J.C.; Bray, E.N.; Freeman, M.C.; Gido, K.B.; Hemphill, N.P.; Kennard, M.J.; et al. Large-scale flow experiments for managing river systems. Bioscience 2011, 61, 948–959. [CrossRef] 14. Kondolf, G.M.; Williams, J.G. Flushing Flows: A Review of Concepts Relevant to Clear Creek, California; US Fish and Wildlife Service: Red Bluff, CA, USA, 1999. 15. Rivaes, R.; Rodriguez-Gonzalez, P.M.; Albuquerque, A.; Pinheiro, A.N.; Egger, G.; Ferreira, M.T. Reducing river regulation effects on riparian vegetation using flushing flow regimes. Ecol. Eng. 2015, 81, 428–438. [CrossRef] 16. Wu, F.C.; Chou, Y.J. Tradeoffs associated with sediment-maintenance flushing flows: A simulation approach to exploring non-inferior options. River Res. Appl. 2004, 20, 591–604. [CrossRef] 17. Konrad, C.P.; Warner, A.; Higgins, J.V. Evaluating dam re-operation for freshwater conservation in the sustainable rivers project. River Res. Appl. 2012, 28, 777–792. [CrossRef] 18. Malcolm, I.A.; Gibbins, C.N.; Soulsby, C.; Tetzlaff, D.; Moir, H.J. The influence of hydrology and hydraulics on salmonids between spawning and emergence: Implications for the management of flows in regulated rivers. Fish. Manag. Ecol. 2012, 19, 464–474. [CrossRef] 19. Olden, J.D.; Konrad, C.P.; Melis, T.S.; Kennard, M.J.; Freeman, M.C.; Mims, M.C.; Bray, E.N.; Gido, K.B.; Hemphill, N.P.; Lytle, D.A.; et al. Are large-scale flow experiments informing the science and management of freshwater ecosystems? Front. Ecol. Environ. 2014, 12, 176–185. [CrossRef] Water 2019, 11, 392 18 of 19

20. Batalla, R.; Vericat, D.; Tena, A. The fluvial geomorphology of the lower Ebro (2002–2013): Bridging gaps between management and research. Cuad. Investig. Geogr. 2014, 40, 29–51. [CrossRef] 21. Robinson, C.T. Long-term changes in community assembly, resistance and resilience following experimental floods. Ecol. Appl. 2012, 22, 1949–1961. [CrossRef][PubMed] 22. Collier, M.P.; Webb, R.H.; Andrews, E.D. Experimental flooding in grand canyon. Sci. Am. 1997, 276, 82–89. [CrossRef] 23. Wilcox, A.C.; Shafroth, P.B. Coupled hydrogeomorphic and woody-seedling responses to controlled flood releases in a dryland river. Water Resour. Res. 2013, 49, 2843–2860. [CrossRef] 24. Beschta, R.L.; Jackson, W.L.; Knoop, K.D. Sediment transport during a controlled reservoir release. J. Am. Water Resour. Assoc. 1981, 17, 635–641. [CrossRef] 25. King, J.; Cambray, J.A.; Impson, N.D. Linked effects of dam-released floods and water temperature on spawning of the Clanwilliam yellowfish Barbus capensis. Hydrobiologia 1998, 384, 245–265. [CrossRef] 26. Robinson, C.T.; Uehlinger, U.; Monaghan, M.T. Stream ecosystem response to multiple experimental floods from a reservoir. River Res. Appl. 2004, 20, 359–377. [CrossRef] 27. Flinders, C.A.; Hart, D.D. Effects of pulsed flows on nuisance periphyton growths in rivers: A mesocosm study. River Res. Appl. 2009, 25, 1320–1330. [CrossRef] 28. Tonkin, J.D.; Death, R.G. The combined effects of flow regulation and an artificial flow release on a regulated river. River Res. Appl. 2014, 30, 329–337. [CrossRef] 29. Cooper, D.J.; Andersen, D.C. Novel plant communities limit the effects of a managed flood to restore riparian forests along a large regulated river. River Res. Appl. 2012, 28, 204–215. [CrossRef] 30. Daesslé, L.W.; van Geldern, R.; Orozco-Durán, A.; Barth, J.A.C. The 2014 water release into the arid Colorado River delta and associated water losses by evaporation. Sci. Total Environ. 2016, 542, 586–590. [CrossRef] [PubMed] 31. Schmidt, J.C.; Parnell, R.A.; Grams, P.E.; Hazel, J.E.; Kaplinski, M.A.; Stevens, L.E.; Hoffnagle, T.L. The 1996 controlled flood in grand canyon: Flow, sediment transport, and geomorphic change. Ecol. Appl. 2001, 11, 657–671. [CrossRef] 32. Hazel, J.E., Jr.; Grams, P.E.; Schmidt, J.C.; Kaplinski, M. Sandbar Response Following the 2008 High-Flow Experiment on the Colorado River in Marble and Grand Canyons; U.S. Geological Survey Science Investigation Report 2010-5015; U.S. Geological Survey: Reston, VA, USA, 2010. 33. Henson, S.S.; Ahearn, D.S.; Dahlgren, R.A.; Van Nieuwenhuyse, E.; Tate, K.W.; Fleenor, W.E. Water quality response to a pulsed-flow event on the Mokelumne River, California. River Res. Appl. 2007, 23, 185–200. [CrossRef] 34. Reiser, D.W.; Ramey, M.P.; Beck, S.; Lambert, T.R.; Geary, R.E. Flushing flow recommendations for maintenance of salmonid spawning gravels in a steep, regulated stream. Rivers Res. Appl. 1989, 3, 267–275. [CrossRef] 35. Petts, G.E.; Foulger, T.R.; Gilvear, D.J.; Pratts, J.D.; Thoms, M.C. Wave-movement and water-quality variations during a controlled release from Kielder reservoir, North Tyne River, U.K. J. Hydrol. 1985, 80, 371–389. [CrossRef] 36. Jakob, C.; Robinson, C.T.; Uehlinger, U. Longitudinal effects of experimental floods on stream benthos downstream from a large dam. Aquat. Sci. 2003, 65, 223–231. [CrossRef] 37. Dollar, E.S.J. Fluvial geomorphology. Prog. Phys. Geogr. 2000, 24, 385–406. [CrossRef] 38. Wilcock, P.R.; Barta, A.F.; Shea, C.C.; Kondolf, G.M.; Matthews, W.V.G.; Pitlick, J. Observations of flow and sediment entrainment on a large gravel-bed river. Water Resour. Res. 1996, 32, 2897–2909. [CrossRef] 39. Wilcock, P.R.; Kondolf, G.M.; Matthews, W.V.G.; Barta, A.F. Specification of sediment maintenance flows for a large gravel-bed river. Water Resour. Res. 1996, 32, 2911–2921. [CrossRef] 40. Melis, T.S.; Walters, C.J.; Korman, J. Surprise and opportunity for learning in grand canyon: The glen canyon dam adaptive management program. Ecol. Soc. 2015, 20, 22. [CrossRef] 41. Scheurer, T.; Molinari, P. Experimental floods in the River Spol, Swiss National Park: Framework, objectives and design. Aquat. Sci. 2003, 65, 183–190. [CrossRef] 42. Patten, D.T.; Harpman, D.A.; Voita, M.I.; Randle, T.J. A managed flood on the Colorado River: Background, objectives, design, and implementation. Ecol. Appl. 2001, 11, 635–643. [CrossRef] 43. Cambray, J.A. The effects on fish spawning and management implications of impoundment water releases in an intermittent South African river. Rivers Res. Appl. 1991, 6, 39–52. [CrossRef] Water 2019, 11, 392 19 of 19

44. Ellis, L.M.; Molles, M.C.; Crawford, C.S. Influence of experimental flooding on litter dynamics in a Rio Grande riparian forest, New Mexico. Restor. Ecol. 1999, 7, 193–204. [CrossRef] 45. Jouvet, C.; Delavaud, J.P. Campagne d’inventaires piscicoles sur la Selves aval et inventaire des macro-invertébrés. Etat des lieux avant la mise en œuvre du protocole de désensablement de la Selves en aval du barrage de Maury; Aygua: Rodez, France, 2016. (In French) 46. Laures, J.L. Regional fish agency. Personal communication, 2016. 47. Descloux, S. Le colmatage mineral du lit des cours d’eau: méthode d’estimation et effets sur la composition et la structure des communautes d’invertebres benthiques et hyporheiques. Ph.D. Thesis, Université Claude Bernard Lyon 1, Lyon, France, 2011. (In French) 48. Cazaubon, A.; Giudicelli, J. Impact of the residual flow on the physical characteristics and benthic community (algae, onvertebrates) of a regulated mediterranean river: The Durance, France. Rivers Res. Appl. 1999, 15, 441–461. [CrossRef] 49. Corse, E.; Pech, N.; Sinama, M.; Costedoat, C.; Chappaz, R.; Gilles, A. When anthropogenic river disturbance decreases hybridisation between non-native and endemic cyprinids and drives an ecomorphological displacement towards juvenile state in both species. PLoS ONE 2015, 10, e0142592. [CrossRef][PubMed] 50. Kondolf, G.M.; Wilcock, P.R.The flushing flow problem: Defining and evaluating objectives. Water Resour. Res. 1996, 32, 2589–2599. [CrossRef] 51. Hjulstrom, F. Studies of the morphological activity of rivers as illustrated by the River Fyris. Geol. Inst. Upsalsa 1935, 25, 221–527. 52. May, C.L.; Pryor, B.; Lisle, T.E.; Lang, M. Coupling hydrodynamic modeling and empirical measures of bed mobility to predict the risk of scour and fill of salmon redds in a large regulated river. Water Resour. Res. 2009, 45, W05402. [CrossRef] 53. Nelson, R.W.; Dwyer, J.R.; Greenberg, W.E. Regulated flushing in a gravel-bed river for channel habitat maintenance: A Trinity River fisheries case study. Environ. Manag. 1987, 11, 479–493. [CrossRef] 54. Wentworth, C.K. A scale of grade and class terms for clastic sediments. J. Geol. 1922, 30, 377–392. [CrossRef] 55. Ingle, J.C.J. The movement of beach sand: An analysis using fluorescent grains. In Developments in Sedimentology; Elsevier Publishing Company: New York, NY, USA, 1966; Volume 5. 56. Teleki, P.G. Fluorescent sand tracers. J. Sediment. Petrol. 1966, 36, 468–485. [CrossRef] 57. Grospretre, L. Etude et gestion des impacts hydrogéomorphologiques de la périurbanisation. L’exemple du bassin de l’Yzeron dans l’Ouest lyonnais. Ph.D. Thesis, Lumière University Lyon 2, Lyon, France, 2011. (In French) 58. Kennedy, V.C.; Kouba, D.L. Fluorescent Sand as a Tracer of Fluvial Sediment; United States Government Priting Office: Washington, DC, USA, 1970. 59. Laronne, J.B.; Outhet, D.N.; Duckham, J.L.; McCabe, T.J. Determining event bedload volumes for evaluation of potential degradation sites due to gravel extraction, N.S.W., Australia. In Erosion and Sediment Transport Monitoring Programmes in River Basins, Proceedings of the Oslo Symposium, IAHS, Oslo, Norway, 24–28 August 1992; International Association of Hydrological Sciences: Oxfordshire, UK, 1992; pp. 87–94. 60. Oustrière, P.; (EDF-DTG, Grenoble, France). Personal communication, 2017. 61. European Commission. Ecological Flows in the Implementation of the Water Framework Directive; Guidance Document No. 31. Technical Report-2015-086; European Commission: Luxembourg City, Luxembourg, 2015; ISBN 978-92-79-45758-6. 62. Eder, A.; Exner-Kittridge, M.; Strauss, P.; Blöschl, G. Re-suspension of bed sediment in a small stream—Results from two flushing experiments. Hydrol. Earth Syst. Sci. 2014, 18, 1043–1052. [CrossRef] 63. Davies-colley, R.; Nagels, J. Flood flushing of bugs in agricultural streams. Water Atmos. 2004, 12, 18–20. 64. Grams, P.E. A Sand Budget for Marble Canyon, Arizona—Implications for Long-Term Monitoring of Sand Storage Change; U.S. Geological Survey Fact Sheet 2013–3074; U.S. Geological Survey: Reston, VA, USA, 2013.

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