Serial discontinuities in a Rocky mountain river. II. Distribution and abundance of trichoptera

River regulation in the headwaters and middle reaches of the Gunnison River, Colorado, significantly altered distributions and abundances of Trichoptera fauna. Twenty-five species were collected from mainstream samples, with the greatest species richness occurring at an unregulated, rhithron segment above the central reach dams. At sites immediately below the three hypolimnial-release dams and a reregulation dam, species richness was reduced 35–90 per cent and abundance > 95 per cent. Net-spinning caddisflies were the dominant trichopterans at unregulated sites; Arctopsyche grandis in the upper reaches (218 organisms, 586 mg dry mass m^?2) and Hydropsyche cockerelli, H. occidentalis and Cheumatopsyche pettiti in the lower river (9041 total organisms, 6621 mg m^?2), downstream from the last dam. The observed distributional pattern of low trichopteran densities in dam tailwaters and high hydropsychid densities at sites 60–80 km below the central reach dams is a classic expression of continuum resets and adjustments in response to stream regulation as predicted by the Serial Discontinuity Concept.     

Regulation leads to increases in riparian vegetation,but not direct allochthonous inputs,along the Colorado River in Grand Canyon,Arizona

Dams and associated river regulation have led to the expansion of riparian vegetation, especially nonnative species, along downstream ecosystems. Nonnative saltcedar is one of the dominant riparian plants along virtually every major river system in the arid western United States, but allochthonous inputs have never been quantified along a segment of a large river that is dominated by saltcedar. We developed a novel method for estimating direct allochthonous inputs along the 387 km‐long reach of the Colorado River downstream of Glen Canyon Dam that utilized a GIS vegetation map developed from aerial photographs, empirical and literature‐derived litter production data for the dominant vegetation types, and virtual shorelines of annual peak discharge (566 m³ s^?1 stage elevation). Using this method, we estimate that direct allochthonous inputs from riparian vegetation for the entire reach studied total 186 metric tons year^?1, which represents mean inputs of 470 gAFDM m^?1 year^?1 of shoreline or 5.17 gAFDM m^?2 year^?1 of river surface. These values are comparable to allochthonous inputs for other large rivers and systems that also have sparse riparian vegetation. Nonnative saltcedar represents a significant component of annual allochthonous inputs (36% of total direct inputs) in the Colorado River. We also estimated direct allochthonous inputs for 46.8 km of the Colorado River prior to closure of Glen Canyon Dam using a vegetation map that was developed from historical photographs. Regulation has led to significant increases in riparian vegetation (270–319% increase in cover, depending on stage elevation), but annual allochthonous inputs appear unaffected by regulation because of the lower flood peaks on the post‐dam river. Published in 2010 by John Wiley & Sons, Ltd.     

Flood Effects Provide Evidence of an Alternate Stable State from Dam Management on the Upper Missouri River

We examine how historic flooding in 2011 affected the geomorphic adjustments created by dam regulation along the approximately 120 km free flowing reach of the Upper Missouri River bounded upstream by the Garrison Dam (1953) and downstream by Lake Oahe Reservoir (1959) near the City of Bismarck, ND, USA. The largest flood since dam regulation occurred in 2011. Flood releases from the Garrison Dam began in May 2011 and lasted until October, peaking with a flow of more than 4200 m³ s^?1. Channel cross‐section data and aerial imagery before and after the flood were compared with historic rates of channel change to assess the relative impact of the flood on the river morphology. Results indicate that the 2011 flood maintained trends in island area with the loss of islands in the reach just below the dam and an increase in island area downstream. Channel capacity changes varied along the Garrison Segment as a result of the flood. The thalweg, which has been stable since the mid‐1970s, did not migrate. And channel morphology, as defined by a newly developed shoaling metric, which quantifies the degree of channel braiding, indicates significant longitudinal variability in response to the flood. These results show that the 2011 flood exacerbates some geomorphic trends caused by the dam while reversing others. We conclude that the presence of dams has created an alternate geomorphic and related ecological stable state, which does not revert towards pre‐dam conditions in response to the flood of record. This suggests that management of sediment transport dynamics as well as flow modification is necessary to restore the Garrison Segment of the Upper Missouri River towards pre‐dam conditions and help create or maintain habitat for endangered species. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.     

SIZE‐FRACTIONATED PHYTOPLANKTON GROWTH AND MICROZOOPLANKTON GRAZING RATES IN THE UPPER ST. LAWRENCE RIVER

Chlorophyll‐a‐specific growth and grazing rates of phytoplankton in three size fractions (0.2 to 2 µm, 2 to 20 µm, and 20 to 153 µm) were determined using dilution assays (n = 38) that were conducted from May to December 2009 in the St. Lawrence River (discharge approx. 7135 m³ s^?1) from its headwaters at Lake Ontario to 180 km downstream. There was no discernible difference in phytoplankton growth or grazing rates as a function of river reach sampled. At water temperatures below 20°C specific rates of growth and grazing were closely matched (0.43 day^?1), suggesting tight coupling of phytoplankton grazing in the water column. However, specific phytoplankton growth rates exceeded specific grazing rates by 0.29 day^?1 when river water temperature exceeded 20°C. Nevertheless, the greater specific growth rate did not manifest in greater phytoplankton biomass with transit downstream suggesting the importance of grazer organisms on benthic surfaces that graze phytoplankton in this reach of a large river. Copyright © 2011 John Wiley & Sons, Ltd.     

Mitigation,compensation, and future protection for fish populations affected by hydropower development in the upper Columbia system,Montana, U.S.A.

Pursuant to the Northwest Power Planning Council's Fish and Wildlife Program for the Columbia River system, we estimated losses in fish populations and developed mitigation, compensation, and protection alternatives for reservoirs and downstream river reaches affected by hydropower development in the Flathead and Kootenai River systems in northwest Montana, U.S.A. The construction of Hungry Horse Dam has resulted in estimated annual losses of 65500 migratory juvenile westslope cutthroat and 1965 adult migratory bull trout from the Flathead Lake and River system. In addition, operations of Hungry Horse and Kerr dams caused annual losses conservatively estimated at 96300 river-spawning and 131000 lakeshore-spawning kokanee adults. Water level fluctuations caused by dam operations at Libby and Hungry Horse reservoirs result in: (1) altered thermal stratification, (2) indirect losses in phytoplankton and zooplankton production, (3) direct washout of phytoplankton and zooplankton through dam penstocks, (4) reductions in standing crop of benthic organisms and of insects on the water surface, and (5) reduced fish growth in the late summer and fall. Mitigative measures include: (1) 99.2 and 113.3 m³ s^?1 minimum flows in the Flathead and Kootenai rivers respectively, to protect salmonid eggs and juveniles, (2) improvement of fish passage to restore migrations between the Flathead and Swan systems, and (3) biological rule curves for operations at Libby and Hungry Horse reservoirs. To compensate for fisheries losses, we recommend enhancement of spawning and rearing habitat, introductions of hatchery juveniles, and spawning channels. We recommend protection from further hydropower development for 100 stream reaches (1386 km) for fish species of special concern, and for outstanding sport fisheries. These and other measures will be considered by various agencies in developing an overall fisheries restoration plan which should be flexible, and employ principles of adaptive management. Effectiveness of the plan may be limited by heavy reliance on hatchery fish. Although mitigation efforts may not restore fish populations to pre-dam levels, substantial benefits should be realized.     

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.     

Channel response to sediment replenishment in a large gravel‐bed river: The case of the Saint‐Sauveur dam in the Buëch River (Southern Alps,France)

The Saint‐Sauveur dam was built in 1992 in the middle section of the Buëch River. Downstream of the dam, a channel incision by several meters was observed. A gravel replenishment operation was planned in order to restore the active channel. An equivalent of two times the mean annual bedload‐transport capacity (43,500 m³) was replenished downstream of the dam in September 2016. The aim of this paper is to quantify morphological change associated with sediment remobilization in order to evaluate the efficiency of the restoration works. The monitoring was based on a combination of (a) change detection using sequential high‐resolution digital elevation models (from airborne LiDAR data), (b) bedload tracing using active ultrahigh‐frequency radio‐frequency identification technology, and (c) complementary field surveys of channel grain‐size distribution and morphology for bedload‐transport computation. Field monitoring allows us to capture a net aggradation along a 2‐km reach after the first post‐replenishment flood. A sediment balance analysis was performed to back‐calculate bedload supply coming from the sluicing operation during the flood. Although the sediment replenishment operation clearly had a positive impact on the morphological conditions of the starved river reach, the effective bedload supply from artificial berms (22,650 m³) was insufficient to initiate substantial channel shifting along the restored reach and a subsequent amplification of the sediment recharge. The combination of high‐resolution topographic resurveys and sediment tracing was successful to evaluate the downstream propagation of sediment replenishment effects.     

The potential role of tributaries as seed sources to an impoundment in Northern Sweden: a field experiment with seed mimics

Fragmentation and flow regulation of rivers by large dams are known to obstruct the longitudinal dispersal of waterborne plant propagules between impoundments, and to affect plant community composition. However, even several decades after a dam has been built, impoundments may still have a relatively species‐rich riparian flora. We hypothesized that free‐flowing tributaries act as the major gene pools for such impoundments, thus alleviating the fragmenting effect large dams have on the main channel. The importance of tributaries as seed sources was tested by releasing wooden seed mimics in three different‐sized (0.22–6.93 m³ s^?1) tributaries of an impoundment in the Ume River in Northern Sweden. In each tributary seed mimics were released, during the spring flood peak, from three points approximately 1, 2 and 3 km upstream the outlet in the impoundment. The importance of a tributary as a seed source increased with tributary size. Of the 9000 released seed mimics 1.5% reached the impoundment; 1.2% of the 9000 originated from the largest tributary and 0.3% from the middle‐sized one. The smallest tributary retained all its mimics. Copyright © 2007 John Wiley & Sons, Ltd.     

Geomorphic response to river flow regulation: Case studies and time-scales

River regulation imposes primary changes on flow and sediment transfer, the principal factors governing the alluvial channel regime. In this study, the effect of flow regulation is isolated from sediment delivery. Peace River (Q? = 1080m³s^?1, increasing to 2110m³s^?1 downstream) was regulated in 1967 for hydropower. The gravel-bed reach immediately downstream from the dam has become stable. Gravel accumulates at major tributary junctions, so the river profile is becoming stepped. Further downstream, the river has a sand bed. It can still transport sand, so morphological changes along the channel include both aggradation and channel narrowing by lateral accretation. In the gravel-bed Kemano River (Q? = 150m³s^?1), the addition of water by diversion from another river caused degradation when additional bed material was entrained below the inflow point. However, the effect became evident only after many years, when a competent flood occurred. The short-term response was channel widening. The time-scale for the response depends on the size of the river and the nature and severity of regulation. In both rivers, significant adjustment will require centuries and will intimately involve the riparian forest.     

Impact of an urban centre on the nitrogen cycle processes of epilithic biofilms during a summer low‐water period *

Nitrogen transformations in epilithic biofilms of a large gravel bed river, the Garonne, France, has been studied upstream (one site) and downstream (four sites) of a large urban centre (Toulouse, 740 000 inhabitants). High biomass, up to 49 g AFDM m^?2 (ashes free dry matter) and 300 mg chlorophyll a m^?2 (Chl. a), were recorded at 6 and 12 km downstream from the main wastewater treatment plant outlet. The lowest records upstream and larger downstream (less than 16 g AFDM m^?2 or 120 mg Chl. a m^?2) could be explained by recent water fall (early summer low‐water period). Measurements of nitrogen exchange at the biofilm–overlying water interface were performed in incubation chambers under light and dark conditions. The addition of acetylene at the mid‐incubation time allowed evaluation of both nitrification (variation in NH₄⁺ flux after the ammonium monooxygenase inhibition) and denitrification (N₂O accumulation related to the inhibition of N₂O reduction). Denitrification (D_w) and nitrification rates were maximum at sites close to the city discharges in dark conditions (up to 9.1 and 5.6 mg N m^?2 h^?1, respectively). Unexpected denitrification activities in light conditions (up to 1.4 mg N m^?2 h^?1) at these sites provided evidence for enhanced nitrogen self‐purification downstream. As confirmed by most probable number (MPN) counts, high nitrification rates in biofilm close downstream were related to enhanced (more than almost 3 log) nitrifying bacteria densities (up to 7.6×10⁹ MPN m^?2). Downstream of an urban centre, nitrogen transformations in the biofilm appeared to be influenced by the occurrence of an adapted microflora which is inoculated or stimulated by anthropic pollution. Copyright © 2002 John Wiley & Sons, Ltd.     

Development and habitat selection of chironomid communities

The macrobenthos (especially Chironomidae) along channel margins has been compared for two 5th-order rivers, the River Grabia, which has not changed its morphology for many years, and the River Widawka, which experienced an increase of channel width (by over 5 m) about eight years before the present study due to human impact. Also, the effect of frequent short-term high water levels on the macrobenthos near the bank was investigated in the 7th order section of the Warta River downstream of a dam. The macrobenthos distribution was found to be similar in the ‘impacted’ Warta and Widawka Rivers, with the highest abundance near the bank, decreasing towards the mid-channel. In the marginal zone of the Widawka River, abundant detritivorous invertebrates having low oxygen tolerance (mainly Prodiamesinae and Chironomini) were observed. The percentage of Chironomini in this zone of the Warta River was even higher than that in the Widawka River. A different pattern was observed in the ‘natural’ Grabia River—the lowest species number and density were at the banks: Orthocladiinae dominated especially in the mid-river. A high abundance fluctuation of one of two dominant species in the margin of the Warta River—Glyptotendipes gripekoveni K. is remarkable. The larvae temporarily reached high densities on snags, which were periodically exposed and submerged by water, although over a year their density was lower (399 ind m^?2) than that of those inhabiting the bottom (543 ind m^?2).     

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.     

Restoration of Hydrochory Following Dam Removal on the Elwha River,Washington

Hydrochory, seed dispersal by water, affects riparian vegetation by contributing to downstream community composition and diversity. However, dams can block hydrochory, reducing downstream species diversity and fragmenting riparian corridors. Dam removal is becoming more prevalent for economic and ecological reasons and is expected to restore hydrochory; however, this has never been documented in rivers. The largest dam removal project to date was the 2011 to 2014 removal of the Glines Canyon and Elwha dams on the Elwha River in Washington. Prior to dam removal, hydrochory was lower below Glines Canyon Dam compared with an upstream reach; our objective was to test the hypothesis that dam removal would restore downstream hydrochory to levels observed in the upstream reach. To test this, we collected seeds in nets above and below the dam during three sample periods (early July, late July and early August), growing out seeds in a greenhouse and comparing seed abundance and species richness above and below dams, before and after dam removal. We found that after dam removal, the average number of hydrochorous seeds and species increased below Glines Canyon Dam to levels similar to or higher than that of the upstream reach; hydrochory levels in the upstream reach did not change. This study is the first to document the restoration of hydrochory in rivers following removal of a large dam. Restoration of hydrochory may ultimately increase downstream vegetation diversity and play a role in the recolonization of reservoir sediments deposited in the riparian zone in the years following dam removal. Copyright © 2016 John Wiley & Sons, Ltd.     

Impact Assessment of Mtera and Kidatu Reservoirs on the Annual Maximum Floods at Stiegler's Gorge of the Rufiji River in Tanzania

Abstract

The impoundment of any river causes changes in the downstream flow regime. The effect of a dam on flow regime depends on both the storage capacity of the reservoir relative to the volume of river flow and the way the dam is operated. The most common attribute of flow regulation is a decrease in the magnitude of the flood peaks and an increase in low flows. This paper reports the findings of a study to assess the cumulative impact of two dams on the Great Ruaha River in Tanzania (the Mtera and Kidatu Reservoir System) on flood flows. The method used was to compare the observed annual maximum flows at downstream locations after the construction of the dams with mathematically modeled estimates of the annual maximum flows at the same locations, assuming that the dams were not built (i.e. generating naturalized flows). Contrary to expectations, the study found that annual maximum peaks were supposed to be less than those actually observed. For instance, in 1989, annual maximum peak flow of 1,400 m³/s was recorded at Kidatu, but the estimated uncontrolled peak flow should have been 800 m³/s, signifying an artificially-induced flood wave. Although, there was a significant impact on the peak flows at Kidatu awing to the dams, the impact was found to be minimal on the flow peaks at Steigler's Gorge.     

水电开发影响下河岸带植被结构和生物量分布

为了解水电开发不同阶段对河岸带植被的影响差异,在澜沧江中下游小湾电站、漫湾库区、漫湾坝下、大朝山库区、大朝山坝下、景洪电站河岸带设置6个样带进行植被结构指标和生物量空间分布特征调查分析。结果表明:澜沧江中下游河岸带植被以乔木为主体,乔木层结构指标以已建库区和坝下样带较高,灌木层和草本层结构指标以在建电站和坝下样带较高;已建库区、坝下和在建电站样带乔木层和群落生物量的最高值分别出现在距离河岸100,50 m和200 m处;乔木层生物量以库区样带最高,灌木层生物量以坝下样带最高,草本层生物量以在建电站样带最高;库区和坝下样带群落生物量大致相当,高于在建电站样带;漫湾库区及其坝下乔木层结构指标以及群落生物量相对变化幅度小于大朝山库区及其坝下样带,小湾样带小于景洪样带。研究区植被结构和生物量的分布格局反映了库区植被恢复程度的差异和电站施工影响强度的差异。     

Computational Fluid Dynamics–Habitat Suitability Index (CFD–HSI) Modelling as an Exploratory Tool for Assessing Passability of Riverine Migratory Challenge Zones for Fish

We developed two‐dimensional computational fluid hydraulics–habitat suitability index (CFD–HSI) models to identify and qualitatively assess potential zones of shallow water depth and high water velocity that may present passage challenges for five major anadromous fish species in a 2.63‐km reach of the main stem Penobscot River, Maine, as a result of a dam removal downstream of the reach. Suitability parameters were based on distribution of fish lengths and body depths and transformed to cruising, maximum sustained and sprint swimming speeds. Zones of potential depth and velocity challenges were calculated based on the hydraulic models; ability of fish to pass a challenge zone was based on the percent of river channel that the contiguous zone spanned and its maximum along‐current length. Three river flows (low: 99.1 m³ sec^‐1; normal: 344.9 m³ sec^‐1; and high:792.9 m³ sec^‐1) were modelled to simulate existing hydraulic conditions and hydraulic conditions simulating removal of a dam at the downstream boundary of the reach. Potential depth challenge zones were nonexistent for all low‐flow simulations of existing conditions for deeper‐bodied fishes. Increasing flows for existing conditions and removal of the dam under all flow conditions increased the number and size of potential velocity challenge zones, with the effects of zones being more pronounced for smaller species. The two‐dimensional CFD–HSI model has utility in demonstrating gross effects of flow and hydraulic alteration, but may not be as precise a predictive tool as a three‐dimensional model. Passability of the potential challenge zones cannot be precisely quantified for two‐dimensional or three‐dimensional models due to untested assumptions and incomplete data on fish swimming performance and behaviours. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

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.     

Benthic macroinvertebrate communities in nonregulated and regulated waters of the clearwater river,Idaho, U.S.A.

Benthic macroinvertebrates were examined over a two-year period in nonregulated, regulated, and semiregulated reaches of the Clearwater River in northern Idaho. Macroinvertebrate communities in the nonregulated reach above Dworshak Reservoir were taxonomically diverse at all stations. In contrast, the macroinvertebrate community in the regulated reach of the North Fork of the Clearwater River (NFCR) was severely altered with high macroinvertebrate abundance and low taxa richness (2 to 8 taxa). Orthoclad chironomids dominated this reach composing between 68 and 99 per cent of total benthic invertebrate numbers. The mayfly Ephemerella infrequens was the only other macroinvertebrate to be abundant in the regulated reach. The major factors contributing to the simplified macroinvertebrate community are reduced habitat diversity, fluctuating water levels, altered thermal regime, and possibly an altered food supply. The effects of the dam were mitigated in the semiregulated Mainstem of the Clearwater River (MSCR) due to the merger of a nonregulated fork entering 2.5 km downstream from the dam. The macroinvertebrate community in the semiregulated MSCR was more complex than the regulated reach with a community structure resembling that found above the reservoir.     

Effect of Proposed Large Dams on Water Flows and Hydropower Production in the Sekong,Sesan and Srepok Rivers of the Mekong Basin

Water flow patterns in the Mekong are changing because of on‐going rapid hydropower development triggered by economic growth. Of immediate concern are the current and proposed hydropower dams in the transboundary Srepok, Sesan and Srekong (3S) Rivers, which contribute up to 20% of the Mekong's annual flows, have a large potential for energy production and provide critical ecosystem services to the downstream Tonle Sap Lake and Mekong Delta. The objective of this paper is to determine how the operation of the proposed largest individual dams and cascade dam schemes in the 3S Rivers will affect flow regimes and energy production. Daily flows were simulated over 20 years using the Soil and Water Assessment Tool and HEC‐ResSim models for a range of dam development and operations scenarios. The development of all dams in the 3S basin under an operation scheme to maximize individual electricity production results in an average 98% increase in dry season flows at the 3S outlet. Over 55% of dry season flows changes are caused by seven proposed large dams, with the Lower Srepok 3 project causing the highest impact. The seven large dams will generate 33.0 GWh/day with a water volume of 17 679 x 10⁶ m³, compared with the current and definite future dams generating 73.2 GWh/day with a much lower volume of 6616 x 10⁶ m³. When a cascade of dams are operational, downstream dams with small reservoirs will produce more energy. However, the marginal increase in energy production from the development of additional dams in the 3S basin will decline rapidly relative to the required water storage increase,. Strategic decision‐making on the future of each large proposed dam in the 3S basin needs to be considered by local governments after understanding cumulative operation effects and with further consideration to the potential impact on downstream ecosystem productivity and livelihoods. Copyright © 2016 John Wiley & Sons, Ltd.