RESTORATION OF SPAWNING HABITATS OF BROWN TROUT (SALMO TRUTTA) IN A REGULATED CHALK STREAM

Gravel bed spawning grounds are essential for the reproduction of salmonids. Such spawning grounds have been severely degraded in many rivers of the world because of river regulation and erosive land use. To reduce its effects on salmonid reproduction rates, river managers have been restoring spawning grounds. However, measures of effectiveness are lacking for the restored spawning sites of brown trout (Salmo trutta). In this study, two methods were used to restore gravel bed spawning grounds in the Moosach River, a chalk stream in Southern Germany: the addition of gravel and the cleaning of colmated gravel. Seven test sites were monitored in the years 2004 to 2008, focussing on sediment conditions. Furthermore, brown trout egg survival and changes in the brown trout population structure were observed. Both gravel addition and gravel cleaning proved to be suitable for creating spawning grounds for brown trout. Brown trout reproduced successfully at all test sites. The relative number of young‐of‐the‐year brown trout increased clearly after the restoration. Sediment on the test sites colmated during the 4 years of the study. In the first 2 years, highly suitable conditions were maintained, with a potential egg survival of more than 50%. Afterwards, the sites offered moderate conditions, indicating an egg survival of less than 50%. Conditions unsuitable for reproduction were expected to be reached 5 to 6 years after restoration. Copyright © 2011 John Wiley & Sons, Ltd.     

EVALUATION OF SPAWNING SUBSTRATE ENHANCEMENT FOR WHITE STURGEON IN A REGULATED RIVER: EFFECTS ON LARVAL RETENTION AND DISPERSAL

Site‐specific habitat alterations have improved spawning success and early life stage survival of different fish species, including sturgeon, in regulated rivers. We modified the substrate within a section of river at the only known spawning site used by white sturgeon (Acipenser transmontanus) in the Mid Columbia River, Canada. Existing armoured riverbed conditions were modified using a mixture of larger and smaller angular rock with the assumption that the larger material would remain in place at higher discharges and help retain the smaller material. This increased substrate complexity and the amount of available interstitial spaces. We stocked 2‐day posthatch larvae over both the modified site and at an adjacent control site that represented existing substrate conditions. Our objectives were to determine (i) the extent that stocked larvae remained in both the modified and control sites immediately after release, (ii) the timing of subsequent dispersal of larvae from both sites and (iii) how total length of dispersing larvae changed over time and by site. Results from this work indicated that the modified section of riverbed retained significantly higher numbers of larvae after release compared with the control site. Larvae at the modified site were able to hide and remain within the substrate and initiated downstream drift 15 days after release. With the exception of the first day after release, dispersal from both sites occurred at night. There was a significant effect of time after release and site on the total length of dispersing larvae. The larger variation in total larval length observed at the control site compared with the modified site indicated greater difficulty in hiding within the control substrate. Larvae initiated dispersal from the modified site at a mean size of 17.5 mm, which may indicate an important growth threshold before drift. Results from this work are important for future mitigative efforts for sturgeon in regulated rivers where changes to spawning substrates have occurred. Copyright © 2012 John Wiley & Sons, Ltd.     

ASSESSING THE GEOMORPHIC EFFECTS OF A FLUSHING FLOW IN A LARGE REGULATED RIVER

The lower Ebro River experiences long‐term hydrological and sedimentary adjustments following major regulation. Alterations in water and sediment fluxes have enhanced a massive macrophyte colonization that, in turn, generates a series of ecological and socio‐economic problems. Controlled water releases, so‐called flushing flows (FFs), have been designed and implemented since 2002 in this part of the river with the objectives of controlling macrophyte populations and maintaining sediment transport in the channel. FFs may produce adverse geomorphic effects, such as bed incision driven by the increased sediment transport capacity and the lack of sediment replacement from upstream. It is thus important to evaluate the potential geomorphic responses to a specific FF design, and redesign FFs regularly to maximize macrophyte removal while minimizing the undesired consequences. Geomorphic responses associated with FFs can be assessed using hydraulic and sediment transport models. In this paper, we use the hydrodynamic model CCHE2D® to evaluate the role of a monitored FF on a river's geomorphology. The designed FF had a duration of 13 h, attaining a maximum discharge of 1350 m³ s^?1. A total of 3375 t of fine material were transported during that event. CCHE2D® model performance is evaluated in terms of hydraulics and sediment transport by comparing observed with modelled values (i.e. discharge, water surface elevation, sediment loads). Overall, objective functions indicate that simulations are in agreement with field observations. For instance, the Root Mean Square Error (RMSE) between the observed and modelled FF hydrograph was 93 m³ s^?1, whereas the RMSE of the total load was 71 t. The example modelled here shows that the FF design typically implemented in the lower Ebro does not cause severe geomorphic impacts. The model provides visualization of the spatial patterns of erosion and deposition for the first time, allowing identification of critical zones where degradation or aggradation may occur. Copyright © 2012 John Wiley & Sons, Ltd.     

THE COMBINED EFFECTS OF FLOW REGULATION AND AN ARTIFICIAL FLOW RELEASE ON A REGULATED RIVER

Damming and regulating the flow of rivers is a widespread issue and can have a significant impact on resident biota. The Tongariro River, central North Island, New Zealand, has a flow regime that is regulated by two hydroelectric dams along its length, and it has been suggested that ‘flushing flows’ would assist benthic communities by removing ‘nuisance’ periphyton growth forms that typically occur in autumn. We assessed whether (i) damming has altered periphyton and macroinvertebrate communities downstream of the Rangipo Dam and (ii) whether the release of a flow pulse equivalent to 50 times the baseflow is sufficient to (a) move the substrate in the section of river downstream of this dam and (b) impact benthic periphyton and macroinvertebrate communities. Downstream macroinvertebrate communities were impacted by the presence of the dam, but periphyton was not. No movement of substrate occurred downstream of the dam as a result of the flow release, which was likely because of naturally high embeddedness and armouring of substrate. Periphyton biomass and macroinvertebrate density were not affected by the release indicating that larger releases would be required to have any effect on benthic communities downstream of this dam. This study highlights the importance of considering natural bed structure and sediment dynamics when using flow releases downstream of dams to control periphyton. Copyright © 2013 John Wiley & Sons, Ltd.     

CLASSIFICATION OF FLOW REGIMES FOR ENVIRONMENTAL FLOW ASSESSMENT IN REGULATED RIVERS: THE HUAI RIVER BASIN,CHINA

Flow regime characteristics (magnitude, frequency, duration, seasonal timing and rates of change) play a primary role in regulating the biodiversity and ecological processes in rivers. River classification provides the foundation for comparing the hydrologic regimes of rivers and development of hydro‐ecological relationships to inform environmental flow management and river restoration. This paper presents a classification of natural flow regimes and hydrologic changes due to dams and floodgates in the Huai River Basin, China, in preparation for an environmental flow assessment. The monthly natural flow regime of 45 stations in the upper and middle Huai River Basin were simulated for the period 1963–2000, based on the hydrological model SWAT (Soil and Water Assessment Tool). Six classes of flow patterns (low or high discharge, stable or variable, perennial or intermittent, predictable or unpredictable) were identified based on 80 hydrologic metrics, analysed by hierarchical clustering algorithms. The ecologically relevant climatic and geographic characteristics of these flow classes were tested for concordance with, and to strengthen, the hydro‐ecological classification. The regulation of natural flow patterns by dams and floodgates changed flows at some locations within each flow class and caused some gauges to shift into another class. The research reported here is expected to provide a foundation for development of hydro‐ecological relationships and environmental flow methods for wider use in China, as well as setting a new scientific direction for integrated river basin management in the Huai River Basin. Copyright © 2011 John Wiley & Sons, Ltd.     

SPATIAL VARIABILITY IN SPAWNING HABITAT SELECTION BY CHINOOK SALMON (ONCORHYNCHUS TSHAWYTSCHA) IN A WILDERNESS RIVER

Chinook salmon (Oncorhynchus tshawytscha) survival during early life stages depends largely on spawning habitat selection by adults, which has been linked to biophysical stream variables (e.g. stream flow, velocity and substrate composition) as well as hyporheic exchange associated with riffle/pool and run/pool transitions. To examine how physical habitat variables influenced spawning habitat choice in one central Idaho (USA) wilderness stream, we used remote sensing techniques to classify and quantify the total amount of each aquatic habitat type present to assess how habitat quantity changed as stream order increased. Additionally, we measured physical habitat variables at each redd throughout the entire stream length for one spawning season to assess whether Chinook salmon selected for the same habitat parameters at varying spatial scales. Run, riffle and pool habitat types contributed similar proportions to the total area in both the upper and lower basins. However, ‘transitional zones’ (i.e. pool‐riffle and pool‐run transitions) accounted for 16% of the total area in the upper basin and only 4% in the lower. Redds were built in multiple habitat types in each of the three primary spawning locations, but transitional zones were chosen most frequently only in the upper basin. Significant differences in habitat variables were seen between spawning groups, with stream wetted width and velocity accounting for the majority of the variation. The techniques described here could be used to locate features that serve as indicators of potential spawning habitat, although caution should be exercised when extrapolating spawning habitat needs over large spatial extents. Copyright © 2013 John Wiley & Sons, Ltd.     

EFFECTS OF FLOW DYNAMICS ON THE AQUATIC‐TERRESTRIAL TRANSITION ZONE (ATTZ) OF LOWER MISSOURI RIVER SANDBARS WITH IMPLICATIONS FOR SELECTED BIOTA

Sandbars are an important aquatic terrestrial transition zone (ATTZ) in the active channel of rivers that provide a variety of habitat conditions for riverine biota. Channelization and flow regulation in many large rivers have diminished sandbar habitats and their rehabilitation is a priority. We developed sandbar‐specific models of discharge‐area relationships to determine how changes in flow regime affect the area of different habitat types within the submerged sandbar ATTZ (depth) and exposed sandbar ATTZ (elevation) for a representative sample of Lower Missouri River sandbars. We defined six different structural habitat types within the sandbar ATTZ based on depth or exposed elevation ranges that are important to different biota during at least part of their annual cycle for either survival or reproduction. Scenarios included the modelled natural flow regime, current managed flow regime and two environmental flow options, all modelled within the contemporary river active channel. Thirteen point and wing‐dike sandbars were evaluated under four different flow scenarios to explore the effects of flow regime on seasonal habitat availability for foraging of migratory shorebirds and wading birds, nesting of softshell turtles and nursery of riverine fishes. Managed flows provided more foraging habitat for shorebirds and wading birds and more nursery habitat for riverine fishes within the channelized reach sandbar ATTZ than the natural flow regime or modelled environmental flows. Reduced summer flows occurring under natural and environmental flow alternatives increased exposed sandbar nesting habitat for softshell turtle hatchling emergence. Results reveal how management of channelized and flow regulated large rivers could benefit from a modelling framework that couples hydrologic and geomorphic characteristics to predict habitat conditions for a variety of biota. Published in 2011 by John Wiley & Sons, Ltd.     

MANAGING THE FLOOD PULSE FOR OPTIMAL FISHERIES PRODUCTION IN THE ATCHAFALAYA RIVER BASIN,LOUISIANA (USA)

The Atchafalaya River Basin in south‐central Louisiana (USA) comprises the largest contiguous river–floodplain swamp in North America and functions as a distributary for the Mississippi River to the Gulf of Mexico. We assessed the impact of the annual flood pulse on fisheries production at a basin‐wide scale. We modelled flood duration (days/year Butte LaRose gauge height >3.6 m) and magnitude (mean daily gauge height) against long term (1987–2009) fishery‐independent and fishery‐dependent data on largemouth bass (Micropterus salmoides), crappie (Pomoxis spp.), blue catfish (Ictalurus furcatus), buffalofish (Ictiobus spp.), gizzard shad (Dorosoma cepedianum) and crayfish (Procambarus spp.). When the Atchafalaya River at Butte LaRose is at flood stage (≥ 3.6 m gauge height) for approximately 121–157 days/year, then annual relative abundances of largemouth bass, crappie, blue catfish and buffalofish are optimized during the fall (October–December). In contrast to the tenets of the flood pulse concept, gizzard shad abundance was optimized during low flow years, when flood duration is ≤10 days/year. Annual proportion of age‐1 largemouth bass was associated positively with flow magnitude during the previous year, whereas proportions of age‐2 largemouth bass and crappie were associated positively with flood magnitude 2 years prior to the fish sample. Results for commercial production metrics were ambiguous. Crayfish catches were associated positively with flood magnitude and duration, whereas gizzard shad landings peaked at a relatively small flood magnitude level and were not related to flood duration. Blue catfish and buffalofish annual landings were not influenced by the annual flood pulse. The annual flood pulse can be managed to optimize the availability of recreational (largemouth bass and crappie) and some commercial fisheries resources (blue catfish and crayfish). We estimate that a minimum discharge of 8807 m³ s^?1 is required at the river's source for approximately 4–5 months during the winter–spring months to optimize fisheries production. Copyright © 2011 John Wiley & Sons, Ltd.     

MODELLING THE EFFECTS OF RIVER FLOW ON POPULATION DYNAMICS OF PIPING PLOVERS (CHARADRIUS MELODUS) AND LEAST TERNS (STERNULA ANTILLARUM) NESTING ON THE MISSOURI RIVER

Reservoir management on the Missouri River has changed the flow regime that once created dynamic emergent sandbar habitat (ESH) for the interior least tern (Sternula antillarum) and piping plover (Charadrius melodus). High flows that create large amounts of ESH are now rare, but the remaining interannual variability in river stage has strong effects on the amount of ESH available for nesting shorebirds. The scarcity of habitat has led the United States Army Corps of Engineers to develop an adaptive management plan for the restoration of ESH to support nesting terns and plovers. We describe the stochastic simulation models of ESH, plover populations and tern populations used in the adaptive management process, and examine the effects of river flow on projected outcomes of habitat restoration. The population models are most sensitive to uncertainty in adult survival rates. Model validation against historical amounts of ESH and population sizes suggests the model is a reasonable predictor of future dynamics. Flow variability contributes as much uncertainty as parameter estimation error to plover model projections but negligible uncertainty to the tern model. Autocorrelation in flow between years has stronger effects on population outcomes than the intensity of habitat restoration effort does. We compared population outcomes after a habitat‐creating flow with population outcomes following habitat restoration and found that large pulses of habitat creation produced similar or better outcomes in the short term than low but consistent habitat restoration. However, bird populations fared better in the long term with low levels of restoration when habitat‐forming flows were rare. Copyright © 2013 John Wiley & Sons, Ltd.     

TRACING GROUNDWATER DISCHARGE IN THE FLOODPLAIN OF THE PARANA RIVER,ARGENTINA: IMPLICATIONS FOR ITS BIOLOGICAL COMMUNITIES

Groundwater discharge can be an important determinant of the functioning of aquatic environments and their associated biological communities. However, the presence and the importance of groundwater have not been considered in the Parana River floodplain owing to the large quantity of surface water. The present study aimed to identify groundwater discharge conditions in a sector of the middle of the floodplain of the Parana River by studying groundwater flow systems. Eight piezometers were installed to record groundwater movement in the vertical plane weekly for 2 years. Water samples were collected in piezometers, domestic wells, the river and other water bodies to study the groundwater flow systems. Rising of the water level during piezometer installation and recording suggested that the study zone represents groundwater discharge conditions. Residence time proxy allowed identification of local flows and intermediate flows. Local rainfall (in Santa Fe) showed an isotopic signature (δD and δ¹⁸O) similar to some local flows detected in the study zone, and this suggested local recharge. The chemical characteristics of an intermediate flow suggest that water would have travelled from a recharge area ~30 km from the study zone. Local rainfall and the intermediate flow have different isotopic signature. Results suggest that the willow forest is associated with the recharge area of a local flow, which plurispecific‐canopy forest is related to a transit area of a semi‐intermediate flow, and that tall grassland and marshy community colonize discharge areas of local and intermediate flows, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

CLASSIFICATION OF NATURAL FLOW REGIMES IN THE EBRO BASIN (SPAIN) BY USING A WIDE RANGE OF HYDROLOGIC PARAMETERS

This paper presents a classification of different natural flow regimes found in Ebro basin, one of the largest in the Mediterranean region. Determination of flow regimes was based on multivariate analyses using long‐term discharge series of unaltered flow data. Mean monthly discharges of the 30 ‘best’ flow series and a total of 52 flow series containing unaltered flow data were selected to represent baseline flow conditions for tributaries throughout the basin. Metrics representing magnitude, duration and frequency components of flow were used to identify hydrologic differences across the basin. A total of six natural flow regimes were identified in the Ebro Basin, using a Ward cluster method. The flow patterns identified and their spatial distribution largely corresponded with climatic zones previously reported for the Ebro Basin, with regime types ranging from pluvio‐oceanic in the western part of the basin to Mediterranean in the eastern region. Geologic characteristics of the catchment and altitude of headwaters were also found to play an important role in defining flow regime type. A 19‐hydrologic variable subset was used to explain main hydrologic differences among groups (such as magnitude and frequency of extreme flow conditions or magnitude and variance of average flow conditions). However, stepwise discriminant analysis was not able to identify consistent subsets of hydrologic variables that adequately identified the six natural flow regime types in this basin. Canonical discriminant analysis was useful to understand class separation and for the interpretation of results. Copyright © 2012 John Wiley & Sons, Ltd.     

MAGNITUDE,FREQUENCY AND DURATION OF INSTREAM FLOWS TO STIMULATE AND FACILITATE CATADROMOUS FISH MIGRATIONS: AUSTRALIAN BASS (MACQUARIA NOVEMACULEATA PERCIFORMES,PERCICHTHYIDAE)

The migratory response and behaviour of catadromous Australian bass with regard to hourly mean river flows and water temperatures was assessed over 15 months. Fish movement was assessed using a 75‐km passive acoustic telemetry array in the regulated Shoalhaven River below Tallowa Dam, NSW, Australia. The majority (62%) of downstream pre‐spawning migrations from freshwater to estuarine habitats were stimulated by a series of flow pulses from April to September, but a proportion of fish (38%) commenced downstream migrations under regulated baseflow conditions after a sustained decrease in water temperature to below 15°C in late autumn. Equal numbers of fish undertook post‐spawning upstream return migrations during flow pulses and during regulated baseflow conditions, with regulated baseflow migrants exhibiting a preference for dusk–dawn passage through freshwater pool–riffle sequences. The median magnitude of flow pulses at the time of commencement of downstream and upstream freshwater migrations by Australian bass was not large, equivalent to natural (in the absence of river regulation) flows equalled or exceeded for 56% and 48% of time, respectively. There was no evidence for increased numbers of migrants with increasing flow pulse magnitude, with individual fish ignoring some flow pulses but responding to subsequent events. In regulated rivers, the release of more frequent flow pulses with peak magnitudes approximating the natural 50th flow duration percentile may be more effective in stimulating greater numbers of Australian bass to undertake pre‐spawning and post‐spawning migrations between freshwater and estuarine habitats than the release of a single, larger event. The propensity of Australian bass to also undertake spawning migrations under regulated baseflow conditions emphasizes the need for provision of baseflow regimes in regulated rivers that can facilitate migrations by large bodied fishes. Copyright © 2011 John Wiley & Sons, Ltd.     

THRESHOLD FLOWS FOR THE BREAKDOWN OF SEASONALLY PERSISTENT THERMAL STRATIFICATION: SHOALHAVEN RIVER BELOW TALLOWA DAM,NEW SOUTH WALES,AUSTRALIA

Reduced mixing of deep pools attributable to river regulation and downstream flow suppression can lead to an increase in the magnitude, frequency and duration of thermal stratification in riverine pools over summer. This study monitored hourly temperature profiles with five thermistor loggers in a 15 m deep natural pool over 12 months from May 2005. Detailed bathymetric and topographic survey data and HEC‐RAS hydraulic modelling of layer Richardson numbers were used to extend thermistor observations of flow‐related stratification breakdown in this single deep pool to a 20 km long pool‐riffle dominated river reach below the dam. Reach‐wide breakdown of persistent thermal stratification in deep pools over spring and summer was likely to be achieved by a flow rate of 3000 ML day^?1. This flow rate approximates the long‐term mean annual natural flow (2860 ML day^?1) and the 16th flow duration percentile (mean daily flows equalled or exceeded for 16% of time), indicating that thermal stratification of the deepest pools in the Shoalhaven River is a common, natural phenomenon not solely attributable to river regulation. Should reasonably consistent hydraulic geometry relationships exist between low salinity rivers in similar climatic, hydrologic and geomorphic settings, then we suggest that the mean annual natural flow is likely to achieve widespread breakdown of thermal stratification across lengthy reaches of similar pool‐riffle sequence rivers elsewhere. Hourly mean wind speeds of up to 65 km h^?1 recorded at an automated weather station 25 km from the study site were found to suppress of the degree of thermal stratification in the study pool but did not achieve deep mixing of persistent seasonal thermoclines. Large, rapid and sustained air temperature decreases associated with the passage of cold fronts across southeastern Australia in summer were found to be more effective than wind and achieved mixing to depths of at least 4.2 m. Copyright © 2011 John Wiley & Sons, Ltd.     

THE MANAGEMENT MODES OF SEASONAL FLOODS AND THEIR IMPACT ON THE RELATIONSHIP BETWEEN CLIMATE AND STREAMFLOW DOWNSTREAM FROM DAMS IN QUEBEC (CANADA)

The goal of the study was to compare the modes of management of seasonal floods for different dams and to constrain their impact on the relationship between climate variables and streamflow downstream from the dams. At the Rawdon dam, downstream from which the Ouareau River is characterized by a natural‐type regulated flow regime, a ‘type A’ flood management mode prevails, in which the same rainfall and/or snowmelt events account for seasonal floods both in the unregulated (natural) stretch of river upstream from the dam and in the river downstream from the dam. As a result, seasonal floods in the natural setting and downstream from the dam are nearly synchronous. In contrast, downstream from the Matawin dam (Matawin River), which produces an inversion‐type regulated flow regime, the prevalent flood management modes are of types B and D, whereby seasonal floods observed upstream and downstream from the dam are not caused by the same rainfall and/or snowmelt events and, as a result, are not synchronous. This difference in seasonal flood management modes affects the interannual variability of the magnitude of seasonal daily maximum flows related to the seasonal floods. Thus, the interannual variability of these flows downstream from the Matawin dam differs significantly from that of flows upstream. No correlation is observed between climate variables and streamflow downstream from the Matawin dam. This absence of correlation disappears gradually at the annual scale, at which streamflow is correlated with rainfall, as is observed upstream from the dam. Copyright © 2013 John Wiley & Sons, Ltd.     

ASSESSING THE IMPACTS OF RIVER REGULATION ON NATIVE BULL TROUT (SALVELINUS CONFLUENTUS) AND WESTSLOPE CUTTHROAT TROUT (ONCORHYNCHUS CLARKII LEWISI) HABITATS IN THE UPPER FLATHEAD RIVER,MONTANA, USA

Hungry Horse Dam on the South Fork Flathead River, Montana, USA, has modified the natural flow regimen for power generation, flood risk management and flow augmentation for anadromous fish recovery in the Columbia River. Concern over the detrimental effects of dam operations on native resident fishes prompted research to quantify the impacts of alternative flow management strategies on threatened bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarkii lewisi) habitats. Seasonal and life‐stage specific habitat suitability criteria were combined with a two‐dimensional hydrodynamic habitat model to assess discharge effects on usable habitats. Telemetry data used to construct seasonal habitat suitability curves revealed that subadult (fish that emigrated from natal streams to the river system) bull trout move to shallow, low‐velocity shoreline areas at night, which are most sensitive to flow fluctuations. Habitat time series analyses comparing the natural flow regimen (predam, 1929–1952) with five postdam flow management strategies (1953–2008) show that the natural flow conditions optimize the critical bull trout habitats and that the current strategy best resembles the natural flow conditions of all postdam periods. Late summer flow augmentation for anadromous fish recovery, however, produces higher discharges than predam conditions, which reduces the availability of usable habitat during this critical growing season. Our results suggest that past flow management policies that created sporadic streamflow fluctuations were likely detrimental to resident salmonids and that natural flow management strategies will likely improve the chances of protecting key ecosystem processes and help to maintain and restore threatened bull trout and westslope cutthroat trout populations in the upper Columbia River Basin. Copyright © 2011 John Wiley & Sons, Ltd.     

HISTORIC CHANGES (1941–2008) IN SIDE CHANNEL AND BACKWATER HABITATS ON AN UNCHANNELIZED REACH OF THE MISSOURI RIVER

Flow regulation has had pervasive effects on aquatic ecosystems within the world's large rivers. While channelization on the lower Missouri River has led to major changes in the river and its floodplain, including the loss of shallow water habitats, effects of upstream dams on unchannelized reaches on the Missouri have not been formally assessed. We quantified changes in the number and size of off‐channel habitats, specifically backwaters and side channels, on the 95‐km unchannelized reach of the Missouri below Gavins Point Dam (Yankton, South Dakota) using historical (1941, 1983–1985, 2008) aerial imagery. Total and mean areas of side channels declined by 77% and 37% and total and mean length decreased by 79% and 42% from 1941 to 2008. Total area of backwaters increased by 40% from 1941 to 2008, whereas mean area decreased by 36%. Our findings suggest that sharp declines in the area and length of side channels have occurred on this unchannelized remnant reach of the Missouri River, with likely significant impacts on aquatic ecosystem processes. Copyright © 2011 John Wiley & Sons, Ltd.     

POST‐DAMMING FLOW REGIME DEVELOPMENT IN A LARGE LOWLAND RIVER (VOLGA,RUSSIAN FEDERATION): IMPLICATIONS FOR FLOODPLAIN INUNDATION AND FISHERIES

Periodic flooding plays a key role in the ecology of floodplain rivers. Damming of such rivers can disturb flooding patterns and have a negative impact on commercial fish yield. The Volga River, the largest river in Europe, has a regulated flow regime after completion of a cascade of dams. Here, we study effects of damming on long‐term discharge variability and flood pulse characteristics. In addition, we evaluate the effects of the altered flood pulse on floodplain ecosystem functioning and commercial fish yields. Our results indicate that both flood pulse and fish populations of the Volga–Akhtuba floodplain have varied considerably over the past decades. After damming, annual maximum peak discharges have decreased, minimum discharges increased, but average discharges remained similar to pre‐damming conditions. Moreover, because of bed level incision of over 1.5 m, a higher discharge is needed to reach bankfull level and inundate the floodplains. Despite this significantly altered hydrological regime and subsequent morphological changes, current discharge management still provides significant spring flooding. However, commercial fish catches did decrease after damming, both in the main channel and in the floodplain lakes. All catches were dominated by species with a eurytopic flow preference, although catches from the main channel contained more rheophilic species, and floodplain catches contained more limnophilic and phytophilic species. The strong increase of opportunistic gibel carp (Carassius gibelio) around 1985 was apparent in the main channel and the floodplain lakes. Despite the hydrological changes, the decrease in overall catches, and the upsurge of gibel, we found a strong positive effect of flood magnitude in the previous year on commercial fish yield in the floodplain lakes. This suggests that under the current discharge management there still is an increased fish growth and/or survival during high floods and that functioning of the floodplain is at least partly intact. Copyright © 2011 John Wiley & Sons, Ltd.