Effects of dams in river networks on fish assemblages in non‐impoundment sections of rivers in Michigan and Wisconsin,USA

Regional assessment of cumulative impacts of dams on riverine fish assemblages provides resource managers essential information for dam operation, potential dam removal, river health assessment and overall ecosystem management. Such an assessment is challenging because characteristics of fish assemblages are not only affected by dams, but also influenced by natural variation and human‐induced modification (in addition to dams) in thermal and flow regimes, physicochemical habitats and biological assemblages. This study evaluated the impacts of dams on river fish assemblages in the non‐impoundment sections of rivers in the states of Michigan and Wisconsin using multiple fish assemblage indicators and multiple approaches to distinguish the influences of dams from those of other natural and human‐induced factors. We found that environmental factors that influence fish assemblages in addition to dams should be incorporated when evaluating regional effects of dams on fish assemblages. Without considering such co‐influential factors, the evaluation is inadequate and potentially misleading. The role of dams alone in determining fish assemblages at a regional spatial scale is relatively small (explained less than 20% of variance) compared with the other environmental factors, such as river size, flow and thermal regimes and land uses jointly. However, our results do demonstrate that downstream and upstream dams can substantially modify fish assemblages in the non‐impoundment sections of rivers. After excluding river size and land‐use influences, our results clearly demonstrate that dams have significant impacts on fish biotic‐integrity and habitat‐and‐social‐preference indicators. The influences of the upstream dams, downstream dams, distance to dams, and dam density differ among the fish indicators, which have different implications for maintaining river biotic integrity, protecting biodiversity and managing fisheries. Copyright © 2010 John Wiley & Sons, Ltd.     

Implications of an invasive fish barrier for the long‐term recovery of native fish assemblages in a previously degraded northeastern Illinois River system

Barriers to fish movement have been used to prevent the spread of invasive fishes but may also limit the movements of native fishes. We evaluated the potential consequences of a proposed barrier on the Illinois River Waterway, meant to inhibit the spread of silver and bighead carps, to the continued recovery of native fishes in the Des Plaines River following water quality improvements. We compared changes in upstream cumulative species richness and community structure from 1983 to 2013 in the DuPage River, an adjacent tributary with an impassable dam, to the area upstream of a newly proposed barrier on the Des Plaines River where fish can currently pass through a navigational lock. Fewer species displayed truncated distributions upstream of the passable lock and dam (n = 18) compared with the impassable dam (n = 23). Due to water quality improvements in the Illinois River as a whole, cumulative species richness downstream of both dams steadily increased over time. Richness also increased upstream of the passable dam but plateaued upstream of the impassable dam. Fifteen to 18 species accounted for differences in community structure between areas downstream and upstream of either dam. Most species (78–100%) were found in greater relative abundance downstream of the impassable dam, and only 53% were found in greater relative abundance downstream of the passable dam. The truncation in species richness and abundance at the impassable dam foreshadows the potential consequences of an indiscriminate barrier on native fishes and the continued recovery of native assemblages.     

Dam reverse flow events influence limnological variables and fish assemblages of a downstream tributary in a Neotropical floodplain

We used a dam‐free tributary (the Baía River) in the upper Paraná River floodplain downstream of a major dam in the Paraná River, Brazil to investigate the effects of dam‐regulated reverse flow on limnological variables and fish. We tested the hypotheses that limnological variables in tributaries change based on flow direction and that fish assemblages respond to this variation. Sampling sites were determined considering flow direction (normal or reverse) and position (near or far from the river mouth). Limnological variables showed higher values for transparency, oxygen, pH, and electrical conductivity at sites near the mouth of the Baía River during reverse flow. Species richness and evenness differed significantly in relation to position, with higher values closer to the Paraná River. The average standard length of fish species was higher near the mouth of the Baía River and during the reverse flow period. No significant differences in species abundance were found. Reverse flow into the Baía River brought nutrient‐poor water from the dammed Paraná River, thereby altering the limnological variables. This flow condition impelled the entry of species with higher average standard lengths. However, increased species richness and low evenness were due to the increase in species dispersal rates under all flow conditions. Our results emphasize that the effects of dams can extend several kilometres into the floodplain, provide basic knowledge on the effects of major dams on downstream pristine tributaries, and highlight the need for further studies to understand the wider influences.     

Impacts of a reservoir on fish assemblages of small tributaries of the Corumbá river,Brazil

Fish community surveys were conducted in five tributaries of the Corumbá River before and after damming. Electrofishing samples were collected monthly in the pre‐impoundment period (March 1996 to August 1996) and 15 in the post‐impoundment period (September 1996 to February 1999). A self‐organizing map (SOM, an Artificial Neural Network algorithm) was used to represent the patterns of fish assemblages. Samples collected in both the pre‐ and post‐impoundment periods were randomly dispersed on the SOM, and, therefore, a clear and significant pattern of separation between samples collected during these two time periods was not found. Mean and maximum water depth, which is correlated with ground water level, did not significantly separate the pre‐ and post‐impoundment samples. However, we found significant differences between the two periods for water temperature, pH, conductivity, DO and current velocity, but abundances of fish species (summarized in the clusters identified by the SOM) did not differ significantly. Instead, the validity of the clusters distinguished by the SOM was confirmed by significant differences in some biotic variables: species richness, equitability and log transformed total abundance. Indicator species values identified the most preferred cluster (and respective complex of environmental factors) for a given species. Only one cluster did not contain any significant species indicator values, but it was dominated by samples from the Furnas Stream, which was the only effluent that could be entered by fish from the main river channel after the damming owing to its location below the dam, which has no fish ladder. Copyright © 2008 John Wiley & Sons, Ltd.     

Low‐Head Dam Impacts on Habitat and the Functional Composition of Fish Communities

The natural flow regime of many rivers in the USA has been impacted by anthropogenic structures. This loss of connectivity plays a role in shaping river ecosystems by altering physical habitat characteristics and shaping fish assemblages. Although the impacts of large dams on river systems are well documented, studies on the effects of low‐head dams using a functional guild approach have been fewer. We assessed river habitat quality and fish community structure at 12 sites on two rivers; the study sites included two sites below each dam, two sites in the pool above each dam and two sites upstream of the pool extent. Fish communities were sampled from 2012 to 2015 using a multi‐gear approach in spring and fall seasons. We aggregated fishes into habitat and reproductive guilds in order to ascertain dams' effects on groups of fishes that respond similarly to environmental variation. We found that habitat quality was significantly poorer in the artificial pools created above the dams than all other sampling sites. Fast riffle specialist taxa were most abundant in high‐quality riffle habitats farthest from the dams, while fast generalists and pelagophils were largely restricted to areas below the downstream‐most impoundment. Overall, these dams play a substantial role in shaping habitat, which impacts fish community composition on a functional level. Utilizing this functional approach enables us to mechanistically link the effects of impoundments to the structure of fish communities and form generalizations that can be applied to other systems. Copyright © 2017 John Wiley & Sons, Ltd.     

Peaking hydropower and fish assemblages: an example from the Tallapoosa River,AL

Dams alter many aspects of riverine environments and can have broad effects on aquatic organisms and habitats both upstream and downstream. While dams and associated reservoirs can provide many services to people (hydropower, recreation, flood control, and navigation), they can also negatively affect riverine ecosystems. In particular, hydropeaking dams affect downstream fish habitats by increasing variability in discharge and temperature. To assess the effects of Harris Dam on the Tallapoosa River, AL, operating under an adaptive management plan implemented in 2005, we sampled fish for community analyses from four sites on the river: three in the regulated reach downstream of the dam, and one unregulated site upstream. Fish were collected every other month using boat/barge electrofishing. We used Shannon's H, nonmetric multidimensional scaling (NMDS), a multiresponse permutation procedure (MRPP), and indicator species analysis to quantify patterns in fish assemblage structure and determine how assemblages varied among sites. NMDS and MRPP indicated significant fish assemblage differences among sites, with the tailrace fish assemblage being distinct from the other downstream sites and sites becoming more similar to the upstream, unregulated site (relative to fish assemblages) with distance downstream of the tailrace. The tailrace fish assemblage included higher proportions of rheophilic species that may be better suited to variable and/or high flows. Altered fish assemblages demonstrated continued effects of Harris Dam on the downstream aquatic systems, particularly close to the dam. These effects may indicate that further mitigation should be considered depending on conservation and management goals.     

Dam effects on bedload transport on the upper Santa Ana River,California, and implications for native fish habitat

Dams disrupt the flow of water and sediment and thus have the potential to affect the downstream geomorphic characteristics of a river. Though there are some well‐known and common geomorphic responses to dams, such as bed armouring, the response downstream from any particular dam is dependent on local conditions. Herein, we investigate the response of the upper Santa Ana River in southern California, USA, to the construction of a large dam at the transition from mountains to valley, using calculations of bedload transport capacity on the mainstem below the dam and for major tributaries. Approximate sediment budgets were constructed for downstream reaches to estimate deposition and erosion rates for sand, gravel, and cobble particle sizes. Our results indicate that the classical response of bed armouring and erosion is likely limited to a short reach immediately below the dam. Farther downstream, though transport capacity is reduced by flow regulation by the dam, the channel reaches are likely to remain depositional but with reduced deposition rates. Persistent deposition, as opposed to erosion, is the result of the replenishment of flow and sediment supply by large downstream tributaries. In addition, the calculations indicate that the composition of the bed is unlikely to change substantially in downstream reaches. A Monte Carlo approach was employed to estimate the uncertainty in the sediment budget predictions. The impacts of the dam on the geomorphic character of the river downstream could have implications for native fish that rely on coarse substrate that supports their food base.     

EFFECTS OF FLOW RELEASES ON MACROINVERTEBRATE ASSEMBLAGES IN THE INDIAN AND HUDSON RIVERS IN THE ADIRONDACK MOUNTAINS OF NORTHERN NEW YORK

The effects of flow releases (daily during spring and four times weekly during summer) from a small impoundment on macroinvertebrate assemblages in the lower Indian River and upper Hudson River of northern New York were assessed during the summers of 2005 and 2006. Community indices, feeding guilds, dominant species and Bray–Curtis similarities at three sites on the Indian River, below a regulated impoundment, were compared with those at four control sites on the Cedar River, below a run‐of‐the‐river impoundment of comparable size. The same indices at four less‐likely affected sites on the Hudson River, below the mouth of the Indian River, were compared with those at an upstream control site on the Hudson River. Results show that the function and apparent health of macroinvertebrate communities were generally unaffected by atypical flow regimes and/or altered water quality at study reaches downstream from both dams in the Indian, Cedar and Hudson Rivers. The lentic nature of releases from both impoundments, however, produced significant changes in the structure of assemblages at Indian and Cedar River sites immediately downstream from both dams, moderate effects at two Indian River sites 2.4 and 4.0 km downstream from its dam, little or no effect at three Cedar River sites 7.2–34.2 km downstream from its dam, and no effect at any Hudson River site. Bray–Curtis similarities indicate that assemblages did not differ significantly among sites within similar impact categories. The paucity of scrapers at all Indian River sites, and the predominance of filter‐feeding Simulium gouldingi and Pisidium compressum immediately below Abanakee dam, show that only minor differences in dominant species and trophic structure of macroinvertebrate communities occurred at affected sites in the Indian River compared to the Cedar River. Thus, flow releases had only a small, localized effect on macroinvertebrate communities in the Indian River. Copyright © 2011 John Wiley & Sons, Ltd.     

Fish catch and fishing practices in the Nam Theun 2 Reservoir and watershed (Lao PDR)

A fish catch and fishing practice monitoring survey were conducted from 2008 to 2014 in villages adjacent to the Nam Theun 2 Reservoir and its upstream tributaries. The reservoir fisheries exhibited – the three expected phases following impoundment (i.e. trophic upsurge, trophic depletion and stabilization, respectively). This study focused on assessing fish catches and fishing practices from communities living next to the reservoir, and those living upstream of the reservoir, over these phases, and to identify factors that could influence total fish catch. The fish catch appeared to be higher among communities located next to the reservoir, compared to communities living in upstream areas. Fish catches in the upstream tributaries remained relatively stable after impoundment, with a slight increase in the average catch. The reservoir exhibited a low fish yield, compared to similar reservoirs, which could be linked to its oligotrophic status. The majority of the total catch biomass of the reservoir was comprised primarily of two species (i.e. Oreochromis niloticus; Hampala macrolepidota) since a stabilization of the catches was observed. Reservoir fisheries appeared to be mainly driven by hydrological factors, specifically the influence of the rainy season peak. The results of this study indicated no over‐exploitation of fisheries occurred overall, although the fishery resource is still fragile. Fishing activities are known to occur in protected zones (productive areas with large inundated habitat areas), with higher annual total catch being observed during periods of poor enforcement of these zones. As fisheries have become an important income source for villagers living along the NT2 Reservoir, regulation and adequate management of the reservoir are recommended to maintain the reservoir fisheries as a sustainable activity.     

Environmental impacts of hydroelectric projects on fish resources in China

China has embarked on a programme to vastly expand its hydroelectric generating capacity and this is certain to alter its freshwater and anadromous fish communities. To provide some insight into the direction and consequences of the likely changes, four (>250 000 kW) existing facilities were selected for review. The Gezhouba Dam, on the Changjiang River, commissioned in 1981, is a low-head run of the river facility. The Xinanjiang Dam (1959) is a high-head dam and the Fuchunjiang Dam (1968) is a low-head, run of the river dam, both sited on the Quiantang River. The Danjiangkou Dam (1968) is a high-head dam in the Han River, a tributary of the Changjiang River. Impacts on fish were classified as those caused directly by the structures, those resulting from changes in physical and chemical factors in their environment and those induced through biotic changes in their habitat. Migrations of anadromous and semi-migratory fish were blocked by the Gezhouba Dam, although some species adapted to the new environment by reproducing downstream. Below the Xinanjiang and Danjiangkou dams spawning was delayed 20–60 days by lower water temperatures. Reduced water velocities and less variable discharges caused spawning grounds below the dams to be abandoned. Marked changes in the hydrological regime caused the extinction of Macrura reevesii, a highly valued fish, in the Qiantang River. The fish communities in the Qiantang estuary were affected by the regulated discharge. Freshwater species fell from 96 to 85, whereas marine species increased from 15 to 80. Loss of habitat eliminated torrential habitat species from the areas inundated by Xinanjiang and Danjiangkou Reservoirs; lentic fish replaced lotic species and now dominate the reservoir fish communities. The expanded aquatic habitat was beneficial for fishery production. Catches from the two reservoirs continue to increase 20 years after impoundment, but are supported by extensive artificial propagation and stocking. There is no doubt that, when the expansion of China's hydroelectric facility network is complete, the fish communities in its rivers will be markedly changed.     

Influence of navigational lock and dam structures on adjacent fish communities in a major river system

We assessed the effects of a series of navigational lock and dam (L/D) structures on the composition of their adjacent fish communities. The Allegheny and Monongahela Rivers of southwestern Pennsylvania contain 14 lock and dam installations, eight and six respectively, which transform their river corridors into a series of contiguous pools. We selected two targeted fish assemblages, large‐bodied fishes (>250 mm TL) susceptible to gill‐netting and small‐bodied benthic species captured by trawling, for assessment upstream and downstream of each L/D installation. Gill nets were fished for approximately 16 h/net, while trawls were performed across three parallel 2‐min hauls. A total of 56 samples were collected over the spring/summers, 2004–2008. Species richness, abundance and the Jaccard Coefficient of Community Similarity (JCS) were calculated for each targeted fish community. Small‐bodied species, particularly darters, were depauperate upstream L/D while abundant and diverse downstream L/D on the Allegheny River. However, Monongahela River upstream and downstream L/D communities were similar. Jaccard Coefficient of Similarity values were comparable for both targeted fish assemblages on the Monongahela River, but differed markedly among Allegheny sites. While JCS values of large‐bodied fish assemblages of both rivers were strongly correlated with lockage frequency, this pattern was not replicated among small‐bodied assemblages. The near doubling of yearly lockages on the Monongahela River compared with the Allegheny may account for the similarity of its upstream and downstream ichthyofauna. Serial L/D navigational facilities on a large river may alter biotic connectivity patterns through physical isolation of adjacent fish communities. Copyright © 2010 John Wiley & Sons, Ltd.     

Ascent of neotropical migratory fish in the Itaipu Reservoir fish pass

The Piracema Canal is a complex 10‐km fish pass system that climbs 120 m to connect the Paraná River to the Itaipu Reservoir along the Brazil–Paraguay border. The canal was constructed to allow migratory fishes to reach suitable habitats for reproduction and feeding in tributaries upstream from the reservoir. The Piracema Canal attracted 17 of the 19 long‐distance migratory species that have been recorded in the Paraná River Basin and Paraguay–Paraná Basin. However, the incidence of migratory fish decreased from downstream to upstream, with the pattern of decrease depending on species. Overall, 0.5% of the migratory fish that entered the Piracema Canal and segment 1, eventually were able to reach segment 5 and potentially Itaipu Reservoir. Ascension rate was examined relative to various physical attributes of canal segments; maximum water velocity emerged as the most influential variable affecting fish passage. Water velocity may be manipulated by controlling water discharge, and by re‐engineering critical sections of the canal. Because the Itaipu Reservoir flooded a set of falls that separated two distinct biogeographical regions, facilitating fish movements through the Piracema Canal into the Itaipu Reservoir presents a management dilemma that requires deliberation in the context of the fish assemblages rather than on selected migratory species. Copyright © 2010 John Wiley & Sons, Ltd.     

Patterns of channel adjustment to impoundment of the upper River Spey,Scotland (1942–2000)

Over the last 30 years, understanding of how river channel morphology adjusts to upstream impoundment has been improved considerably with clearwater erosion close to the dam and aggradation downstream of unregulated tributary confluences being identified. In a UK context our understanding of channel response to impoundment has been based principally on analysis of a number of regulated rivers using a time–space substitution approach but the temporal pattern of channel change was not fully observed. This paper details channel change over almost six decades following impoundment of the River Spey, Scotland, primarily via the use of cartographic and aerial photographic evidence. Channel change initially involved the formation of tributary confluence bars followed by bench development and vegetation colonization and development on the benches and gravel bars. Sixty years after impoundment, the channel is still adjusting its form at distance from the dam to be in quasi‐ equilibrium with its regulated flow regime; indeed adjustment was most rapid between 1989 to 1995. This accelerated adjustment is due to the occurrence of a greater number of large and geomorphologically significant flood events in this period. Sediment supply limitation from unregulated tributaries will have stemmed the rate of narrowing and bench development prior to 1989. This spatial and temporal pattern of adjustment supports, for the most part, the model advocated by Petts (Progress in Physical Geography, 1979, Vol. 3, pp. 329–362). The wider implications of the channel change for river conservation and management are addressed. Copyright © 2003 John Wiley & Sons, Ltd.     

An Assessment of Fish Assemblage Structure in a Large River

The Penobscot River drains the largest watershed in Maine and once provided spawning and rearing habitats to 11 species of diadromous fishes. The construction of dams blocked migrations of these fishes and likely changed the structure and function of fish assemblages throughout the river. The proposed removal of two main‐stem dams, improved upstream fish passage at a third dam, and construction of a fish bypass on a dam obstructing a major tributary is anticipated to increase passage of and improve habitat connectivity for both diadromous and resident fishes. We captured 61 837 fish of 35 species in the Penobscot River and major tributaries, through 114 km of boat electrofishing. Patterns of fish assemblage structure did not change considerably during our sampling; relatively few species contributed to seasonal and annual variability within the main‐stem river, including smallmouth bass Micropterus dolomieu, white sucker Catostomus commersonii, pumpkinseed Lepomis gibbosus, and golden shiner Notemigonus crysoleucas. However, distinct fish assemblages were present among river sections bounded by dams. Many diadromous species were restricted to tidal waters downriver of the Veazie Dam; Fundulus species were also abundant within the tidal river section. Smallmouth bass and pumpkinseed were most prevalent within the Veazie Dam impoundment and the free‐flowing river section immediately upriver, suggesting the importance of both types of habitat that supports multiple life stages of these species. Further upriver, brown bullhead Ameiurus nebulosus, yellow perch Perca flavescens, chain pickerel Esox niger, and cyprinid species were more prevalent than within any other river section. Our findings describe baseline spatial patterns of fish assemblages in the Penobscot River in relation to dams with which to compare assessments after dam removal occurs. Copyright © 2014 John Wiley & Sons, Ltd.     

Fishway passage and post‐passage mortality of up‐river migrating sockeye salmon in the Seton River,British Columbia

Adult sockeye salmon (Oncorhynchus nerka) were studied to assess the consequences of a dam and vertical‐slot fishway on mortality during their spawning migration in the Seton–Anderson watershed, British Columbia, Canada. Since previous research suggests fishway passage may be difficult, our main hypothesis was that the dam and fishway have post‐passage consequences that affect subsequent behaviour and survival. Eighty‐seven sockeye were caught at the top of the fishway, implanted with an acoustic telemetry transmitter, non‐lethally biopsied to obtain a small blood sample and released either upstream or downstream of the dam. Indices of physiological stress (i.e. plasma cortisol, glucose, lactate and ions) indicated that fish were not stressed or exhausted after capture from the fishway, and were not unduly stressed by transportation to release sites or net‐pen holding. Of 59 fish released downstream of the dam, 14% did not reach the dam tailrace. Overall passage efficiency at the fishway was 80%. Mortality in two lakes upstream of the dam was greater in fish released downstream of the dam (27%) compared to fish released upstream of the dam (7%; p = 0.04) suggesting that dam passage has consequences that reduce subsequent survival. Cumulative mortality of fish released downstream of the dam (n = 55) resulted in only 49% survival to spawning areas, compared to 93% of fish released upstream of the dam (n = 28). Survival was significantly lower for females (40%) than for males (71%; p = 0.03), a finding that has implications for conservation because spawning success of sockeye salmon populations is governed primarily by females. Copyright © 2010 John Wiley & Sons, Ltd.     

Importance of Reservoir Tributaries to Spawning of Migratory Fish in the Upper Paraná River

Regulation of rivers by dams transforms previously lotic reaches above the dam into lentic ones and limits or prevents longitudinal connectivity, which impairs access to suitable habitats for the reproduction of many migratory fish species. Frequently, unregulated tributaries can provide important habitat heterogeneity to a regulated river and may mitigate the influence of impoundments on the mainstem river. We evaluated the importance of tributaries to spawning of migratory fish species over three spawning seasons, by comparing several abiotic conditions and larval fish distributions in four rivers that are tributaries to an impounded reach of the Upper Paraná River, Brazil. Our study confirmed reproduction of at least 8 long‐distance migrators, likely nine, out of a total of 19 occurring in the Upper Paraná River. Total larval densities and percentage species composition differed among tributaries, but the differences were not consistent among spawning seasons and unexpectedly were not strongly related to annual differences in temperature and hydrology. We hypothesize that under present conditions, densities of larvae of migratory species may be better related to efficiency of fish passage facilities than to temperature and hydrology. Our study indicates that adult fish are finding suitable habitat for spawning in tributaries, fish eggs are developing into larvae, and larvae are finding suitable rearing space in lagoons adjacent to the tributaries. Our findings also suggest the need for establishment of protected areas in unregulated and lightly regulated tributaries to preserve essential spawning and nursery habitats. Copyright © 2014 John Wiley & Sons, Ltd.     

Ecological consequences of hydropower development in Central America: impacts of small dams and water diversion on neotropical stream fish assemblages

Small dams for hydropower have caused widespread alteration of Central American rivers, yet much of recent development has gone undocumented by scientists and conservationists. We examined the ecological effects of a small hydropower plant (Doña Julia Hydroelectric Center) on two low‐order streams (the Puerto Viejo River and Quebradon stream) draining a mountainous area of Costa Rica. Operation of the Doña Julia plant has dewatered these streams, reducing discharge to ～10% of average annual flow. This study compared fish assemblage composition and aquatic habitat upstream and downstream of diversion dams on two streams and along a ～4 km dewatered reach of the Puerto Viejo River in an attempt to evaluate current instream flow recommendations for regulated Costa Rican streams. Our results indicated that fish assemblages directly upstream and downstream of the dam on the third order Puerto Viejo River were dissimilar, suggesting that the small dam (< 15 m high) hindered movement of fishes. Along the ～4 km dewatered reach of the Puerto Viejo River, species count increased with downstream distance from the dam. However, estimated species richness and overall fish abundance were not significantly correlated with downstream distance from the dam. Our results suggested that effects of stream dewatering may be most pronounced for a subset of species with more complex reproductive requirements, classified as equilibrium‐type species based on their life‐history. In the absence of changes to current operations, we expect that fish assemblages in the Puerto Viejo River will be increasingly dominated by opportunistic‐type, colonizing fish species. Operations of many other small hydropower plants in Costa Rica and other parts of Central America mirror those of Doña Julia; the methods and results of this study may be applicable to some of those projects. Copyright © 2006 John Wiley & Sons, Ltd.     

三峡坝下游河道造床流量与水流挟沙力的变化

通过对三峡水库蓄水前后坝下游水沙条件代表站--宜昌水文站的资料进行统计,分析了三峡水库蓄水前后坝下游河道造床流量的变化,并结合水力学计算对水流挟沙能力进行估算,简要分析了坝下游河道水流挟沙能力沿程的变化情况。分析认为三峡水库蓄水后引起的坝下游第一、第二造床流量变化和水流挟沙能力变化反映了坝下游河床以纵向冲刷为主的发展趋势;三峡蓄水前后同流量级条件下沿程水流挟沙能力的变化反映了三峡蓄水以来坝下游河段以中低水冲刷为主、中枯水位下降相对明显及河床沿程变化受节点控制的客观事实。     

三峡坝下游河道造床流量与水流挟沙力的变化简

 通过对三峡水库蓄水前后坝下游水沙条件代表站——宜昌水文站的资料进行统计,分析了三峡水库蓄水前后坝下游河道造床流量的变化,并结合水力学计算对水流挟沙能力进行估算,简要分析了坝下游河道水流挟沙能力沿程的变化情况。分析认为三峡水库蓄水后引起的坝下游第一、第二造床流量变化和水流挟沙能力变化反映了坝下游河床以纵向冲刷为主的发展趋势;三峡蓄水前后同流量级条件下沿程水流挟沙能力的变化反映了三峡蓄水以来坝下游河段以中低水冲刷为主、中枯水位下降相对明显及河床沿程变化受节点控制的客观事实。     

Toledo Bend reservoir and geomorphic response in the lower Sabine River

Downstream geomorphic responses of stream channels to dams are complex, variable, and difficult to predict, apparently because the effects of local geological, hydrological, and operational details confound and complicate efforts to apply models and generalizations to individual streams. This sort of complex geomorphic response characterizes the Sabine River, along the Texas and Louisiana border, downstream of the Toledo Bend dam and reservoir. Toledo Bend controls the flow of water and essentially prevents the flux of sediment from three‐quarters of the drainage basin to the lower Sabine River. Although the channel is scoured immediately downstream of the dam, further downstream there is little evidence of major changes in sediment transport or deposition, sand supply, or channel morphology attributable to the impoundment. Channels are actively shifting, banks are eroding, and sandbars are migrating, but not in any discernibly different way than before the dam was constructed. The Sabine River continues to transport sand downstream, and alluvial floodplains continue to accrete. The relatively small geomorphic response can be attributed to several factors. While dam releases are unnaturally flashy and abrupt on a day‐to‐day basis, the long‐term pattern of releases combined with some downstream smoothing creates a flow regime in the lower basin which mimics the pre‐dam regime, at least at monthly and annual time scales. Sediment production within the lower Sabine basin is sufficient to satisfy the river's sediment transport capacity and maintain pre‐dam alluvial sedimentation regimes. Toledo Bend reservoir has a capacity: annual inflow ratio of 1.2 and impounds 74% of the Sabine drainage basin, yet there has been minimal geomorphic response in the lower river, which may seem counterintuitive. However, the complex linked geomorphic processes of discharge, sediment transport and loads, tributary inputs, and channel erosion include interactions which might increase as well as decrease sediment loads. Furthermore, if a stream is transport‐limited before impoundment, the reduced sediment supply after damming may have limited impact. Copyright © 2003 John Wiley & Sons, Ltd.