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.     

Mitigation of cold‐water thermal pollution downstream of a large dam with the use of a novel thermal curtain

Hypolimnial releases from dams during periods of thermal stratification modify the downstream riverine thermal regime by decreasing water temperature and reducing natural diel thermal variability. This cold‐water thermal pollution in rivers can persist for hundreds of kilometres downstream of dams and impact important ecological processes such as fish spawning. To mitigate this problem, a first‐of‐its‐kind thermal curtain was fitted to the large bottom release Burrendong Dam on the Macquarie River, Australia. The thermal curtain acts by directing warmer, near‐surface epilimnial water to the low‐level hypolimnial offtake. This study aimed to test the efficacy of the thermal curtain by measuring temperatures before and after the curtains installation, quantifying the magnitude and extent of cold‐water thermal pollution along the Macquarie River downstream of Burrendong Dam. Epilimnial releases with use of the curtain increased diel temperature ranges and the mean monthly water temperature below the dam. Epilimnial releases with use of the curtain increased diel temperature ranges from 0.9°C to 2.5°C and reduced the difference between the mean monthly water temperature of an upstream control and a downstream site by up to 3.5°C. A comparison of the monthly temperature means along the river, indicated that thermal recovery, whereby temperatures returned to within the natural range of upstream temperatures occurred 45 km downstream of the dam during summer when the thermal curtain was deployed, compared with approximately 200 km prior to deployment of the curtain. Our study suggests that the use of thermal curtains can reduce cold‐water thermal pollution and improve ecological outcomes for river ecosystems downstream of dams.     

Influence of the Abaeté River on the reproductive success of the neotropical migratory teleost Prochilodus argenteus in the São Francisco River,downstream from the Três Marias Dam,southeastern Brazil

The curimatã‐pacu Prochilodus argenteus is an important characiform from the São Francisco River basin that performs long‐distance migrations for spawning upstream during the rainy season, when the temperature and photoperiod are elevated. Despite the interruption of the migratory routes by the Três Marias Dam and accentuated decline in fishing, the curimatã‐pacu still sustains the fisheries at the Três Marias region in recent decades. The objective of this study was to evaluate the reproductive activity of P. argenteus in two sections of the São Francisco River, downstream from the Três Marias Dam, during the rainy season. In the first 34 km of the river, immediately below the dam, most of the females were in gonadal resting. At 34–54 km downstream from the dam, following the confluence with a medium‐sized tributary, the Abaeté River, there was a high frequency of males and females in reproductive activity. Follicular atresia was more frequent in the upper section of the river while postovulatory follicles occurred predominantly in the lower section. Fulton's condition factor and gonadosomatic index indicated that the females were in a better physiological and reproductive condition below the confluence with the Abaeté River. In contrast to the females, the males were less affected by damming, and testicular maturation was largely achieved in two river sections. Thus, although the section of the São Francisco River immediately below the Três Marias Dam was found to be unfavourable for the reproduction of the migratory fishes due principally to the hypolimnetic water from the reservoir, reproductive success of P. argenteus was achieved below the Abaeté River. In this section, the species encountered appropriate conditions for maturation and spawning, i.e. warm temperatures above 24°C, high water flow and dissolved oxygen, and low water transparency. These results indicate the importance of a non‐regulated tributary to minimize the ecological impact of a dam on the downstream native fish communities. Copyright © 2005 John Wiley & Sons, Ltd.     

A simplified water temperature model for the Colorado River below Glen Canyon Dam

Glen Canyon Dam, located on the Colorado River in northern Arizona, has affected the physical, biological and cultural resources of the river downstream in Grand Canyon. One of the impacts to the downstream physical environment that has important implications for the aquatic ecosystem is the transformation of the thermal regime from highly variable seasonally to relatively constant year‐round, owing to hypolimnetic releases from the upstream reservoir, Lake Powell. Because of the perceived impacts on the downstream aquatic ecosystem and native fish communities, the Glen Canyon Dam Adaptive Management Program has considered modifications to flow releases and release temperatures designed to increase downstream temperatures. Here, we present a new model of monthly average water temperatures below Glen Canyon Dam designed for first‐order, relatively simple evaluation of various alternative dam operations. The model is based on a simplified heat‐exchange equation, and model parameters are estimated empirically. The model predicts monthly average temperatures at locations up to 421 km downstream from the dam with average absolute errors less than 0.5°C for the dataset considered. The modelling approach used here may also prove useful for other systems, particularly below large dams where release temperatures are substantially out of equilibrium with meteorological conditions. We also present some examples of how the model can be used to evaluate scenarios for the operation of Glen Canyon Dam. Published in 2008 by John Wiley & Sons, Ltd.     

The impact of cold water releases on the critical period of post‐spawning survival and its implications for Murray cod (Maccullochella peelii peelii): a case study of the Mitta Mitta River,southeastern Australia

The effects of cold water releases, as a by‐product of storing irrigation water in large dams, has been a source of great concern for its impact on native freshwater fish for some time. The Mitta Mitta River, northeast Victoria, is impacted by altered thermal regimes downstream of the fourth largest dam in Australia, Dartmouth dam, with some daily temperatures 10–12°C below normal. Murray cod (Maccullochella peelii peelii) were endemic to the Mitta Mitta River; however, resident Murray cod have not been found in this river since 1992. The response of eggs and hatched larvae from Murray cod to different temperature gradients of water were measured and the post‐spawning survival recorded. As a case study, post‐spawning survival was then inferred from flow data for each year of operation of Dartmouth Dam, recorded since first operation in 1978, and included in a stochastic population model to explore the impact of the altered (historical) thermal regime on population viability. Experimental results revealed no egg and larval survival below 13°C and predicted historical temperature regimes point to more than 15 years of low temperatures in the Mitta Mitta River. Population modelling indicates that the impact of cold water releases on post‐spawning survival is a significant threatening process to the viability of a Murray cod population. Additionally, we consider changes to the thermal regime to explore how the thermal impact of large dams may be minimized on downstream fish populations through incrementally increasing the temperature of the releases. The modelled Murray cod population responds to minor increases in the thermal regime; however, threats are not completely removed until an increase of at least 5–6°C. Copyright © 2005 John Wiley & Sons, Ltd.     

Physical and Biological Responses to an Alternative Removal Strategy of a Moderate‐sized Dam in Washington,USA

Dam removal is an increasingly practised river restoration technique, and ecological responses vary with watershed, dam and reservoir properties, and removal strategies. Moderate‐sized dams, like Hemlock Dam (7.9 m tall and 56 m wide), are large enough that removal effects could be significant, but small enough that mitigation may be possible through a modified dam removal strategy. The removal of Hemlock Dam in Washington State, USA, was designed to limit channel erosion and improve fish passage and habitat by excavating stored fine sediment and reconstructing a channel in the former 6‐ha reservoir. Prior to dam removal, summer daily water temperatures downstream from the dam increased and remained warm long into the night. Afterwards, a more natural diel temperature regime was restored, although daily maximum temperatures remained high. A short‐lived turbidity pulse occurred soon after re‐watering of the channel, but was otherwise similar to background levels. Substrate shifted from sand to gravel–cobble in the former reservoir and from boulder to gravel–cobble downstream of the dam. Initially, macroinvertebrate assemblage richness and abundance was low in the project area, but within 2 years, post‐removal reaches upstream and downstream of the dam had diverse and abundant communities. The excavation of stored sediment and channel restoration as part of the dam removal strategy restored river continuity and improved benthic habitat while minimizing downstream sedimentation. This study provides a comparison of ecological effects with other dam removal strategies and can inform expectations of response time and magnitude. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Thermal regime suitability: Assessment of upstream range restoration potential for Colorado pikeminnow,a warmwater endangered fish

Dams have reduced distribution of the endangered Colorado pikeminnow Ptychocheilus lucius in the upper Colorado River basin: low‐head diversion dams blocked upstream passage and large dams inundated free‐flowing segments and cooled downstream reaches with deep‐water releases. To date, range restoration efforts in the Colorado and Gunnison Rivers have focused on building fish ladders around diversion dams to allow recolonization of upstream reaches. Upstream thermal suitability for this warmwater cyprinid was assessed using temperature data and existing distributional information from river reaches where Colorado pikeminnow movements were unrestricted. Among‐site thermal regime comparisons were made using mean annual thermal units (ATU), derived from mean daily temperatures during 1986–2005 and the relation between temperature and Colorado pikeminnow growth. Upstream distributional limits in the Yampa and Gunnison Rivers occurred where in‐channel thermal regimes fell below a long‐term mean of 47–50 ATU, suggesting that two Colorado River fish ladders will make available an estimated 17 km of thermally suitable habitat. A Gunnison River fish ladder successfully re‐established access to 54 km of suitable habitat, but 32 km of critical habitat upstream remains unsuitable. Suitability there could be achieved by raising temperatures only 1–2°C from late May to mid‐October with installation of a temperature control device on an upstream dam. Maximum, main‐channel, summer temperatures did not limit Colorado pikeminnow distribution in downstream reaches of the upper Colorado River. Published in 2010 by John Wiley & Sons, Ltd.     

The effects of flow regulation and climatic variability on obstructed drainage and reverse flow contribution in a Northern river–lake–Delta complex,Mackenzie basin headwaters

A distinctive hydrological feature of the Lake Athabasca–Peace–Athabasca Delta (LA‐PAD) complex is that flow in channels that drain the system reverses direction when stage on the Peace River exceeds that for the central lakes. This river's hydrology has experienced natural and human induced changes since 1968. This study investigates the importance of spring break‐up and open‐water induced outflow obstruction and reverse flow contributions to annual lake level maxima under natural (1960–1967), regulated (1976–2004) and naturalized (1976–1996) flow regimes. Obstructed and reverse flow events during spring break‐up were common prior to and following flow regulation, suggesting that natural climatic variability in source areas below the W.A.C. Bennett Dam exerted a strong influence on their occurrence. Antecedent hydrological conditions, such as fall freeze‐up lake level, break‐up magnitude, peak spring flow and initial open‐water lake level were significantly associated with annual lake level maxima. During the summer period, lake level was linked to sustained high flows on the Peace River. The river obstructed outflow and contributed reverse flow to the LA‐PAD in each year prior to 1968. Following regulation, however, more than half the years did not experience any open‐water obstruction and/or reversal, and those that did were characterized by smaller events. The average estimated duration of obstruction was more than two weeks shorter and reverse flow volume was reduced by ～90% under a regulated regime compared to a simulated naturalized flow regime. This implied a lowered potential for lateral lake expansion into the delta floodplain in some years. The regulated hydrology could produce large stormflow and high lake levels, but only under extreme climatic events in areas below the dam and/or human‐induced alterations to normal reservoir operation. Copyright © 2009 Crown in the right of Canada and John Wiley & Sons, Ltd.     

丹江口水库大坝加高对汉江干流水温影响评价

为探究丹江口水库大坝加高对汉江干流水温的影响,采用汉江干流白河站、黄家港站和襄阳站实测水温资料,基于改进的河流水温变化评价指标,分析了大坝加高前后汉江水温变化趋势,量化评估了水库下游河道水温的变化程度。结果表明：丹江口水库大坝加高显著改变了汉江中游水温情势,坝下河段水温极值变幅指标呈下降趋势,均化程度逐年加深,黄家港站最高、最低水温的出现日期均向后推迟,存在“滞冷”和“滞温”现象;水温情势的改变程度与距离密切相关,距离大坝较近的黄家港站水温情势变化明显,水温集中期相较于大坝加高前滞后约23 d;距大坝较远的襄阳站水温情势变化较小。     

MULTISTAGE ANALYSIS OF HYDROLOGIC ALTERATIONS IN THE YELLOW RIVER,CHINA

Natural river flow regimes provide an array of ecological and social functions by sustaining the health of riverine ecosystems. To identify the hydrologic alterations in the lower Yellow River basin caused by natural factors and human activities, we developed multistage hydrologic analysis to investigate the temporal variability of the river's flow regimes. We used a cumulative departure curve and Mann–Whitney–Pettitt nonparametric tests to determine possible change points based on hydrologic data from 1950 to 2006. We then used the range of variability approach to characterize and to quantify the temporal variability of the hydrologic regimes that were associated with perturbations such as dam operation, flow diversions or intensive conversion of land use within the watershed. In the case study, three stages in hydrologic alterations of the flow regime were found: a stage without human impacts, a stage with excessive human impacts and a reservoir‐regulation stage. Our results indicated that (i) after 1997, dam operation efficiently achieved flood control using sediment regulation activities; (ii) although effective in flood control, the Xiaolangdi Reservoir could not handle situations with extremely low flow, such as during droughts; and (iii) under the arid climate of the Yellow River basin, water consumption by agriculture was the main cause of water shortages. The current study shows that multistage hydrologic analysis can greatly assist regional water resources management and the restoration of riparian eco‐environmental systems affected by dam construction under a changing environment. Copyright © 2012 John Wiley & Sons, Ltd.     

山口岩水库水温计算及其对下游河道水温影响分析

对山口岩水库水温分布、水库泄水温度状况及坝下游河道水温沿程变化作了预测,结合灌区工程布置及灌溉农田的基本情况,对水库兴建后水温对下游农田灌溉的影响进行分析评价,结果表明:库区水温结构属典型的分层型;在坝址至半山水汇入口6km长的河段水温回升较慢,年平均水温温升率为0 414℃/km,对应的左岸直灌区和锡陂灌区沿程水温在水稻生育期的月平均值低于水稻生长所需的最低水温,需采取调温措施;在坝址下游10km断面,由于受袁河支流半山水汇入影响,各月平均水温与建库前天然水温相差不超过0 3℃,能满足农田灌溉的要求。最后从工程设计、水库调度运行、灌溉方式等方面提出了对策措施。     

Effects of hydropeaking operations on the growth of Alabama bass Micropterus henshalli and redeye bass Micropterus coosae in the Tallapoosa River,Alabama, USA

Anthropogenic factors such as dam construction and hydropower generation can dramatically alter the flow regime of rivers and may impact growth of aquatic organisms. Using incremental growth techniques, annual growth of Alabama bass Micropterus henshalli and redeye bass M. coosae in the Tallapoosa River, Alabama, USA, was evaluated in response to variation in flow regime. Fish were collected from the Tallapoosa River above Harris Dam (unregulated site) and at two sites downstream of the dam (regulated sites), as well as Hillabee Creek (unregulated tributary). Flow variables were calculated for each growth year, and the best model that described growth for each species at each location was determined using Akaike's Information Criterion. Additionally, growth increments of each species at ages 1, 2, and 3 were compared between years characterized by low and high flow variability. Age was the best explanatory variable that described growth in all models, although flow variables were included in more than half the models. In all cases, annual and seasonal flow variables had low predictive power and explained <2% of the variation in growth. Growth was higher for age‐1 fish in years with less flow variation but was similar among years for age‐2 and age‐3 fish. Overall, this study provided little evidence that annual growth of either species was heavily influenced by flow in this regulated river.     

下岸水库对永安溪水文情势的影响分析

为了定量评估永安溪下岸水库对下游水文情势的影响,利用永安溪柏枝岙水文站1980—2020年的径流序列,采用不均匀系数、完全调节系数和集中度分析水库运行前后永安溪径流年内分配变化,并利用RVA法评估水文整体改变度。结果表明：下岸水库的蓄丰补枯作用明显,径流年内分配趋于均匀化;在取得防洪效益的同时,水库的运行也对永安溪水文情势产生了中度改变,改变度达42.82%,对流域内水生生态系统的健康形成潜在影响。     

Understanding the Thermal Regime of Rivers Influenced by Small and Medium Size Dams in Eastern Canada

Although small and medium‐size dams are prevalent in North America, few studies have described their year‐round impacts on the thermal regime of rivers. The objective of this study was to quantify the impacts of two types of dams (run‐of‐river, storage with shallow reservoirs) on the thermal regime of rivers in eastern Canada. Thermal impacts of dams were assessed (i) for the open water period by evaluating their influence on the annual cycle in daily mean water temperature and residual variability and (ii) for the ice‐covered winter period by evaluating their influence on water temperature duration curves. Overall, results showed that the run‐of‐river dam (with limited storage capacity) did not have a significant effect on the thermal regime of the regulated river. At the two rivers regulated by storage dams with shallow reservoirs (mean depth < 6 m), the annual cycle in daily mean water temperature was significantly modified which led to warmer water temperatures in summer and autumn. From August to October, the monthly mean water temperature at rivers regulated by storage dams was 1.4 to 3.9°C warmer than at their respective reference sites. During the open water period, the two storage dams also reduced water temperature variability at a daily timescale while increased variability was observed in regulated rivers during the winter. Storage dams also had a warming effect during the winter and the winter median water temperature ranged between 1.0 and 2.1°C downstream of the two storage dams whereas water temperature remained stable and close to 0°C in unregulated rivers. The biological implications of the altered thermal regimes at rivers regulated by storage dams are discussed, in particular for salmonids. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

The influence of major dams on hydrology through the drainage network of the Sacramento River basin,California

This paper reports basinwide patterns of hydrograph alteration via statistical and graphical analysis from a network of long‐term streamflow gauges located various distances downstream of major dams and confluences in the Sacramento River basin in California, USA. Streamflow data from 10 gauging stations downstream of major dams were divided into hydrologic series corresponding to the periods before and after dam construction. Pre‐ and post‐dam flows were compared with respect to hydrograph characteristics representing frequency, magnitude and shape: annual flood peak, annual flow trough, annual flood volume, time to flood peak, flood drawdown time and interarrival time. The use of such a suite of characteristics within a statistical and graphical framework allows for generalising distinct strategies of flood control operation that can be identified without any a priori knowledge of operations rules. Dam operation is highly dependent on the ratio of reservoir capacity to annual flood volume (impounded runoff index). Dams with high values of this index generally completely cut off flood peaks thus reducing time to peak, drawdown time and annual flood volume. Those with low values conduct early and late flow releases to extend the hydrograph, increasing time to peak, drawdown time and annual flood volume. The analyses reveal minimal flood control benefits from foothill dams in the lower Sacramento River (i.e. dissipation of the down‐valley flood control signal). The lower part of the basin is instead reliant on a weir and bypass system to control lowland flooding. Data from a control gauge (i.e. with no upstream dams) suggest a background signature of global climate change expressed as shortened flood hydrograph falling limbs and lengthened flood interarrival times at low exceedence probabilities. This research has implications for flood control, water resource management, aquatic and riparian ecosystems and for rehabilitation strategies involving flow alteration and/or manipulation of sediment supplies. Copyright © 2006 John Wiley & Sons, Ltd.     

Reconciling Hydropower and Environmental Water Uses in the Leishui River Basin

Today's water systems require integrated water resource management to improve the water supply for conflicting water uses. This research explores alternative policies to improve the water supply for two conflicting uses, hydropower and environmental, using the Leishui River basin and Dongjiang reservoir as a case study. First, the natural flow regime prior to reservoir construction (pre‐1992) was estimated by performing a statistical analysis of 41 years of daily streamflow data (March 1952–February 1993). This natural flow regime was used as a template for proposing environmental flow (e‐flow) requirements. The post‐reservoir flow regime (post‐1992) (March 1993–February 2011) was analysed to estimate the streamflow alteration. Results show that the natural flow regime has been completely transformed; post‐1992 winter normal flows are greater, and summer flows are smaller than pre‐1992 conditions. Also, the occurrence of natural floods has been prevented. Second, a planning model was built of the current operation of the Dongjiang reservoir and used for comparison of four alternative water management policies that considered e‐flow releases from the Dongjiang reservoir. The scenarios that considered combinations of the current operational policy and e‐flow releases performed better in terms of hydropower generation than the current operation. Different volumes of e‐flow requirements were tested, and an annual e‐flow volume of 75% of the pre‐1992 hydrograph was determined to generate the most hydropower while providing for environmental water needs. Trade‐offs are essential to balance these two water management objectives, and compromises have to be made for both water uses to obtain benefits. Copyright © 2013 John Wiley & Sons, Ltd.     

Discharge pulses of hydroelectric dams and their effects in the downstream limnological conditions: a case study in a large tropical river (SE Brazil)

To address daily fluctuations in electricity demands, the quantities of water passing through the turbines of hydropower plants can vary significantly (up to fourfold) during a 24‐h cycle. This study evaluates the effects of hourly variations in water discharges on the limnological conditions observed in two below‐dam river stretches. The study reservoirs, Capivara and Taquaruçu, are the 9th and 10th reservoirs in a cascade of dams in the Paranapanema River in south‐east Brazil. The reservoirs exhibit different trophic conditions, water retention times, thermal regimes and spillway positions. Capivara Reservoir is deeper, meso‐eutrophic, with a high water retention time and hypolimnetic discharges (32 m) varying between 500 and 1400 m³ s^?1. In contrast, Taquaruçu Reservoir is relatively shallow, oligo‐mesotrophic, and has a low retention time, with water discharges varying between 500 and 2000 m³ s^?1. Its turbine water intake zone also is more superficial (7 m). For two periods of the year, winter and summer, profiles of limnological measurements were developed in the lacustrine (above‐dam site) zones of the reservoirs, as well as in the downstream river stretches (below‐dam site). In both cases, the sampling was carried out at 4‐h intervals over a complete nictemeral cycle. The results demonstrated that the reservoir operating regime (water discharge variations) promoted significant differences in the conditions of the river below the dams, especially for water velocity, turbidity, and nutrient and suspended solids concentrations. The reservoir physical characteristics, including depth, thermal stratification and outlet structure, are also key factors influencing the limnology and water quality at the below‐dam sampling sites. In the case of Capivara Reservoir, for example, the low dissolved oxygen concentration (<5.0 mg L^?1) in its bottom water layer was transferred to the downstream river stretches during the summer. These study results demonstrated that it is important to continue such investigations as a means of verifying whether or not these high‐amplitude/low‐frequency variations could negatively affect the downstream river biota.     

基于分布式水文模型的水库生态调度方案 修正研究——以双峰寺水库为例

结合下游生态环境用水要求,使用分布式水文模型模拟双峰寺水库调度前后坝址下游的水文循环过程,指出了规划阶段水库调度存在的生态问题,进而修正调度方案并分析采用生态调度修正方案后对武烈河下游水文情势及坝址下游河流生态环境需水的影响。模拟结果表明：在修正后的水库生态调度方案下,双峰寺水库建成运行后,库区段流速从库尾至坝前断面不断减小;水库下泄流量大于坝址处多年平均流量的10%。而且,水库建成后防洪能力提高;75%保证率年份和特枯年份均可保证地下水位。     

Impact of Dam Construction on Water Quality and Water Self-Purification Capacity of the Lancang River, China

Along with the sequent completion of Manwan and Dachaoshan Dam, the river continuum of the middle and lower reaches of the Lancang River was separated into three types of segments: reservoir, below-dam segment and downstream flowing segment. The long-term series of water quality and river flow data over 20 years were analyzed in order to study the impact of dam construction and operation on water quality and water self-purification capacity of these different river segments. From pre-dam period to the first 7 years after Manwan Dam had been accomplished, the water quality of Manwan Reservoir became worse due to the accumulation of pollutants, and then to the next 5 years the water quality became better in virtue of the water self-purification of the reservoir. The cooperative operation of Manwan and Dachaoshan Dam had cumulatively positive impacts on water quality of their below-dam segment but no impacts on that of downstream flowing segment. From pre-dam period to the first 7 years after the closure of Manwan Dam, the water self-purification capacity of Xiaowan–Manwan segment for BOD₅, COD_Mn and NH₃–N decreased. Also, the water self-purification capacity of Manwan–Dachaoshan segment for BOD₅ and COD_Mn decreased but for NH₃–N increased. However, the water self-purification capacity of Jinghong–Ganlanba segment changed contrary to Manwan–Dachaoshan segment. In general, the construction of Manwan Dam negatively affected the water self-purification capacity of reservoir and below-dam segment but impose little impact on that of downstream flowing segment. This study suggested that it is necessary to pay attention to the effect of complicated temporal and spatial characteristics of dam on aquatic ecosystem.