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The lower part of the River Verdon is profusely regulated by hydroelectric impoundments and the exploitation of canals by the water supply company. This study is an attempt at determining the effects of mean daily flow on the suspended algae in this river. The sampling sectors were localized downstream to the last hydroelectric dam. Two patterns of hydrological regime were analyzed in this hydrosystem: the river manifests a stable and low residual flow and the canal is characterized by a stronger and annually variable flow. In the canal, the algal community has a lower species richness, cell density and diversity than in the river. The abundance of drift algae decreases significantly with the flow. The composition of the algal community also varies in accordance with the flow. The response of drift algae to hydrological regime shows strong seasonal variations; the hydrological disturbance was heavily linked to the freshwater needs. Copyright © 2001 John Wiley & Sons, Ltd.

Many Baltic salmon rivers have lost their natural juvenile production due to human activities blocking or reducing access to their spawning grounds, e.g. damming, power generation, partial hinders, etc. One such hindrance is a hydroelectric complex located in the lower reaches of River Umealven in northern Sweden. Water from the forbay created by the dam Norrfors is directed to the Stornorrfors power-station. At times, 100% of the river water is directed to the power-station. Water from the power-station then flows via a tunnel and outlet channel to the river. From the point of the tunnel's discharge into the river, the old river bed acts as a bypass channel directing migrating adult fish to a fish ladder located at the base of the dam. In this study, the effect that an additional turbine, that was installed at the power-station in 1986, had on fish passage run-time was examined. Changes in run-time were compared for two periods 1974–1985 and 1986–1995. In 1997, 55 wild and 25 hatchery salmon were captured in the Umealven estuary, radio tagged with uniquely coded tags, and tracked upstream. Both manual and automatic loggers were used to locate fish daily. The main findings show that only 26% of the wild salmon and none of the hatchery salmon found the fish ladder. It is suggested that the salmon followed the main water discharge from the power-station outlet and are thus directed away from the entrance to the bypass channel leading to the fish ladder. Salmon respond by moving upstream or downstream depending on the current flow regimes. The bypass channel consists of partial hinders that may explain why it takes on average 52 days for the salmon to migrate 32 km from the estuary to the fish ladder. Adding a fourth turbine at the power-station did not appear to have changed the timing of the migration or the seasonal distribution of the migrating wild salmon through the fish ladder. There was no significant effect of the fourth turbine on the duration of the middle 50% of the run in any weight-class for wild salmon. Daily turbine discharges or ambient water temperature and daily counts of wild salmon at the fish ladder were not related. Copyright © 2001 John Wiley & Sons, Ltd.

The Snowy Mountains Scheme in southeast Australia provides a good example of the extensive changes that can occur in streams from which water is transferred. The Scheme, now 50 years old, diverts large amounts of water from the Snowy River inland to generate hydro-electricity and to supply water for irrigation. In recent years, growing criticism of the degraded condition of the Snowy River has prompted a number of investigations into the feasibility of providing increased flows in the river and of rehabilitation of the stream environment. A recent Snowy Water Inquiry recommended the partial restoration of streamflow in the Snowy River to 15% of natural levels. However, the Snowy Water Inquiry noted the considerable trade-offs involved in terms of hydro-electricity generation and irrigation, at an overall estimated cost of Australian dollars (A$) 194 million (approximately US$125 million). All monetary values are at 1998 levels (A$ converted to US$ at 0.63). The preferred option identified satisfied none of the interested parties and the matter remains unresolved. Given the complexity of the issue, the Snowy Water Inquiry must be seen as only an initial step in a lengthy process of balancing socioeconomic costs and benefits, and ecological, legal and political considerations, before rehabilitation of the Snowy River can be achieved. Copyright © 2000 John Wiley & Sons, Ltd.

Large interbasin water transfers for hydro-electric power generation have substantially changed the hydrology of the Snowy River below the Snowy Mountains Hydro-Electric Scheme in southeastern Australia, since 1967. Annual, monthly, mean daily and peak instantaneous flows have been significantly reduced and flood variability greatly increased along 352 km of channel. The first 70 km of river has responded by substantial contraction through the reworking of tributary mouth bars into downstream pools, bars and benches. The tributary mouth bars formed where actively eroding gullies and incised channels debouched into the main stream. These recent deposits have been invaded and stabilized by a number of exotic and native tree and shrub species but particularly willows. Lichens have also colonized the formerly inundated bedrock and boulder surfaces since 1967 in the first 20 km of channel and are now found at elevations up to 2.15 m closer to the river bed. Exotic plants are also invading the riparian corridor downstream of the contracted sections due to reduced disturbance caused by flood suppression. The interaction of reduced flows with high sand and gravel loads has reduced the amount of available habitat. Alternating, bank-attached side bars and their associated pool–riffle sequence have been replaced by transverse and longitudinal bars with a flat featureless bed on the most downstream reach due to a significant reduction in the duration of formative flows between 23.15 and 138.9 m3/s. The rehabilitation of the Snowy River involves the adoption of an environmental flow strategy that includes channel maintenance flows, habitat maintenance flows, minimum acceptable flows, habitat maximizing flows and flow seasonality. This strategy will increase the total amount, duration and variability of releases. A Water Inquiry has been set up as part of the corporatization of the Scheme and will address the issue of environmental flows. Copyright © 1999 John Wiley & Sons, Ltd.

Predation by northern squawfish (Ptychocheilus oregonensis) has been documented to be significant on emigrating juvenile salmonids near juvenile bypass outfalls at hydroelectric dams on the Columbia River. Criteria for siting juvenile fish bypass outfalls to reduce predation were developed using locational data from radio-tagged northern squawfish in The Dalles Dam trailrace, Columbia River. Radio transmitters were surgically implanted in 164 northern squawfish in 1993 and 1994, and their movements and distribution were monitored. Position estimates of northern squawfish were compared with data from a physical hydraulic model of the dam to estimate water velocities where northern squawfish were located. Eighty-two percent of northern squawfish position estimates were in water velocities ≤110 cm/s in 1993 and ≤90 cm/s in 1994. Fish locations were usually associated with water depths ≤10 m (84% in 1993 and 82% in 1994); 90% were within 110 m of the shore or dam structure in 1993, and 86% were within 80 m in 1994. In a related study at John Day Dam, Columbia River, where the juvenile bypass outfall is located 40 m from shore, water depth is 10 m and water velocities typically exceed 75 cm/s, only 13 of 1443 (0.9%) contacts on radio-tagged northern squawfish were located within 200 m of the bypass outfall. We recommend that new or modified juvenile bypass outfalls on the Columbia River be located in water velocities of ≥100 cm/s, ≥75 m from the shore or dam structure, and in water ≥10 m deep.

The stability of habitat conditions determined by the hydrological regime of a river can play a major part in fish spawning success. Application of the classical IFIM approach (Instream Flow Incremental Methodology) does not usually take into account this dynamic aspect of the habitat. We developed a numerical method to simulate the rate of variation in spawning habitat conditions using two-dimensional hydrodynamic modelling coupled with a fish model based on habitat suitability indices (HSI). The approach assumes that, under natural conditions, fish spawning success is maximized under the most common rates of habitat variation. This method was applied during the feasibility study phase of a Hydro-Quebec hydroelectric project on Ashuapmushuan River (St-Jean Lake region, Quebec, Canada). The aim was to evaluate the environmental impact of different exploitation regimes on the rearing and spawning habitats of the landlocked salmon (Ouananiche) of this river. Only the spawning aspect is addressed here. After the hydrodynamic model was well established on a representative reference reach of the river, two spawning areas were identified within this reach and delimited for computational purposes. The per cent usable areas (PUA) were estimated on these two zones for 36 different flow discharges representing a wide rate of the usual hydrological conditions in summer and autumn. In addition, a time series of daily rate of variation of normalized PUA (dPUA′) were obtained from a 35 year habitat time series. Finally, some relevant statistics of dPUA′ (mean and exceedance probabilities of 50, 20, 10, 5 and 2%) were obtained for the natural flow regime. They were then compared with a hypothetical peak exploitation flow pattern currently in use on a neighbouring river. According to our model, this regime would induce a 10–20-fold increase in the rate of variation compared with the most extreme natural conditions (dPUA exceeded 2% of the time). This regime was considered to obtain a contrasting image of different options that were never seriously envisaged by Hydro-Quebec for this river.

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 migration pattern of spawners of brown trout (Salmo trutta) in relation to water flow was analysed by radio-tracking in the regulated River Gudbrandsdalslagen, Norway. During the upstream spawning migration in the high flow period, trout (2–12 kg) were caught, tagged and released 15 km downstream of a fish ladder at the Hunderfossen waterfall. The released fish displayed a systematic and directional upstream movement to the outlet of the tunnel from the hydroelectricity plant. In autumn and winter there is a minimum water discharge of 20–2 m3/s on p.s. in the river between the dam and the outlet of the tunnel, which has a discharge of 200–300 m3/s. When the water flowing over the Hunderfossen dam decreased to 20 m3/s, the ascent of brown trout up the river stopped and fish periodically entered the power plant tunnel. To determine the flow necessary to attract fish into the spawning reach above the tunnel outlet, two experiments were undertaken using 12 and 17 radio-tagged trout. In the first experiment, 60 m3/s of water released for 24 h resulted in the migration of 50% of the trout up the river. The second experiment, releasing 60 m3/s for 24 h, followed by 30 m3/s for 24 h two days later, resulted in the migration of 60% of the trout. Only one fish ascended the river at a flow of 30 m3/s. It is recommended that a repeated release of water at 60 m3/s is made in periods of minimum water discharge to save the spawning migration. The results demonstrate the advantage of using radio-tracking in experiments dealing with fish migration in relation to water-flow management.

Dam removal has received increasing attention over the last several years as a viable alternative to rehabilitation of unsafe dams and as an alternative for consideration as many hydroelectric dams come up for relicensing in the USA. The environmental impacts of dams on river ecosystems have been studied far more extensively than have the impacts of removing dams. Comprehensive environmental research is currently being conducted, assessing both the positive and negative impacts of the possible removal of Rodman Dam on the Ocklawaha River, Florida. The 2073 m long and 6·7 m high dam was constructed in 1968, impounding 3642 ha on this 125 km long river. The impacts of dam removal are compared with those expected by retaining and actively managing the reservoir for fish and wildlife. The research approach described here for addressing alternatives is recommended as a holistic procedure in which to make an environmentally based decision regarding dam removal.

The macroinvertebrate communities of the French Upper Rhǒne have been studied since 1975. Three hydroelectric development schemes were erected in this part of the river between 1981 and 1984. The original component of these schemes is the construction of a diversion canal parallel to the old bed, which is closed by a dam and only receives a very reduced compensation flow. The changes of the macrofauna were studied until 1988. The 10 year study of the Chautagne scheme, the oldest scheme, pointed out important alterations in the benthic macro-invertebrate fauna after the completion of this hydroelectric scheme. In the by-passed section, hydrological patterns governed the presence or absence of species, whereas abundance was greatly influenced by substratum stability, food quality and quantity, development of aquatic macrophytes and periphyton. The low hydrological fluctuations in the reservoirs of run of the river schemes favoured limnophilic species, which were previously accidental. The long-term survey of the three schemes (14 years) showed that the period (i.e. before or after the regulation) had a greater influence on the changes in the structure and functioning of the Upper Rhǒne macroinvertebrate communities than the location (upstream or downstream) or the type of environment (i.e. reservoir, by-passed section) considered. No recovery of unregulated and/or upstream fauna appeared. The serial discontinuity concept was not supported because of the absence of abiotic (temperature) discontinuities and of large tributaries. However, the seasonal rhythm was not affected by the regulation.

A study was carried out from 23 October to 19 November 1992 to determine the ability of fish to ascend the fish ladder at the ITAIPU dam, to identify the species attracted and to evaluate possible fish selection. The results show that some species of fish can ascend the ladder during the migratory period; the dam is non-selective with respect to species, but selective with respect to fish size as a result of the presence of reduced openings. The ladder was ascended by both scaly species from surface waters (agile movers) and by deep water species which were characterized by slow movements. An average of 2892 fish ascended the ladder each day. The weight and length of fish recorded ranged from 336.0 to 3676.0 g and from 30.2 to 71.3 cm, respectively. The largest group of migrating fish was the curimba (Prochilodus scrofa). The high ratio of 72% of fish in the gonadal development stage, classified as ‘maturing’, indicates that the species caught in the ladder were migrating for reproduction purposes.

Following turbine installation at the Kielder dam, the River North Tyne downstream is now characterized by large daily fluctuations in flow. A survey of trichopteran populations (Hydropsychidae, Rhyacophilidae and Polycentropodidae) was carried out to assess the impact of hydroelectric power generation (Phase III), and to compare the results with similar pre-impoundment (Phase I) and post-impoundment (Phase II) surveys. Benthic samples were collected quarterly upstream from the reservoir and at four stations downstream. Although the same three species (Hydropsyche siltalai, Hydropsyche pellucidula and Rhyacophila dorsalis) continue to dominate, there were some notable differences found between sampling sites. Species diversity was drastically reduced at the station nearest the dam, where 99% of all larvae collected were R. dorsalis. At the next station 5 km downstream, where previously H. siltalai had been dominant, R. dorsalis now constitutes 65% of collections. At the two stations farthest away from the dam the former prevalence of H. siltalai was re-established. It is suggested that the unstable, high flows at the reservoir outlet are likely to favour the predatory R. dorsalis rather than the filter feeding hydropsychids whose nets may be susceptible to damage. Upstream from Kielder Water, R. dorsalis was prominent in benthic samples, whereas numbers of H. siltalai were 10 times lower than in Phase II and 60 times lower than in Phase I. The extent to which such upstream changes may be related to altered patterns of community structure below the dam is not known, but these observations may warrant a more cautious approach when considering upstream areas as unaffected ‘reference sites’. Instar analyses suggested that winter-warm, summer-cool discharges were continuing to modify larval growth rates, as noted in Phase II. The results of this study are discussed in relation to present trends in water resource development in Britain, especially with regard to future needs in monitoring and research.

Resource utilization and growth of brown trout were studied in four deep (mean depths 16.2–37.5 m) Norwegian hydroelectric reservoirs by benthic and pelagic gillnet sampling. In all the reservoirs supplementary stockings are carried out. The brown trout were spatially segregated according to size as the habitat use of small individuals (< 180–220 mm) was completely restricted to benthic habitats, whereas larger individuals mainly utilized the upper strata of pelagic waters. It is argued that the pelagic habitat is the more rewarding, and that small-sized brown trout are forced into the less favourable benthic habitat through social interactions with larger specimens. This is supported by an increase in growth of brown trout from their third to fifth year of life, which seems to be related to the shift from benthic to pelagic behaviour. It is also argued that the conditions for small-sized brown trout may be a bottleneck in the capacity to produce brown trout in hydroelectric reservoirs with limited benthic feeding conditions, despite ample access to food in pelagic habitats. When evaluating the possibility of increasing the yield of brown trout through supplementary stockings, it is therefore important to consider food and growth conditions for all age and size groups of brown trout. In reservoirs with poor benthic feeding conditions it may be necessary to stock with brown trout of sizes that are large enough to utilize pelagic habitat, to avoid the limiting benthic living stage.

The effects of intermittent power generation on the fish fauna and benthic invertebrates of several Austrian rivers have been investigated quantitatively. In contrast to the more or less local adverse effects of impoundments or stream channelization, artificial flow fluctuations generally disturb a long section of a given river. Within all the river sections investigated, a breakdown of the benthic invertebrate biomass of between 75 and 95% was observed within the first few kilometres of river length. A reduction of between 40 and 60% of biomass compared with undisturbed areas could be detected within the following 20–40 km. The reduction of the fish fauna is within the same order of magnitude and correlates well with the amplitude of the flow fluctuations. Several reasons for the breakdown are summarized and proposals for the minimization of these detrimental effects of artificial short-term fluctuations are given.

Since its completion in 1973 the Danjiangkou Dam has markedly changed downstream flows, water levels, temperatures, sediment loads and other water quality characteristics in downstream reaches of the Hanjiang River. There have been changes in the growth, spawning behaviour and overwintering condition of local fish populations, in the composition and abundance of food organisms and in the composition of the commercial fish catch. Despite the changed environment and the absence of a fish pass, fish populations are still able to grow and spawn under the new regime. Where conditions are like those of the Hanjiang River, dams may not necessarily have calamitous consequences for fishery production.

Channelization and the construction of 12 dams on the Lower Rhǒne during the last 30 years increased the morphological and biological effects of 19th century embanking along the river. These trends were demonstrated on the main channel, i.e. the central axis of the alluvial floodplain, by the synchronic study of the benthic macrofauna of six lowhead hydroelectric impoundments. The variability of the most significant taxa was investigated by an ANOVA, and the data-samples matrix was processed with a Principal Components Analysis in order to analyse the communities organization and their significance. The benthic macroinvertebrate communities of the channel include some lentic potamic species topographically related with this kind of river (metapotamon), as well as eurytopic and pollution-tolerant species. Thus, the Lower Rhǒne has become a slow potamic river, in the same way as other large mid-European rivers. Flow velocity appears to be the most important structural factor in a large regulated river. The homogeneity of the macroinvertebrate communities and the absence of longitudinal typology show that, in a very intensely regulated river, the River Continuum Concept is too theoretical, and the disturbing effects of dams on longitudinal changes and species response to regulation must be taken into account.

The Tatsumi canal, Japan, constructed by Hyoshiro Itaya in 1632, is of immeasurable cultural value. It has become evident that the canal will be destroyed by construction of a new dam planned by Ishikawa Prefecture. The Tasumi canal is described and an alternative measure that fulfills the purposes of the dam construction and preservation of the Tatsumi canal is proposed. The alternative measure consists of widening the channel of the Sai River for flood control, and modification of the existing Heraso checkdam so as to fulfil the requirements for hydroelectric power generation and water storage. The proposed alternative also allows the preservation of the natural environment around the dam site and maintenance of water-quality in the Tatsumi canal.

This special issue of Regulated Rivers contains most of the contributions presented at an international workshop on Remedial Strategies in Regulated Rivers, held in Lycksele, Sweden, 25-28 September 1995. The workshop was held under the auspices of the Swedish Environmental Protection Agency as part of the final phase of the Swedish research programme, Environmental Improvements in Regulated Rivers, which began in 1991. The research programme has been financed by Vattenfall, a power company responsible for half of Sweden’s hydropower production, and the Swedish National Board for Industrial and Technical Development. The Swedish Environmental Protection Agency has been responsible for the administration of the programme and the scientific evaluation of the projects. The Norwegian Water Resources and Energy Administration and the Swedish World Wide Fund for Nature provided additional funding for the workshop, while Vattenfall organized an excursion to a regulated river, Umealven. We wish to thank all the scientists who kindly agreed to participate in this workshop and thus enabled us to benefit from their knowledge and experience in the field of remedial measures. About half of the electricity in Sweden is produced by hydropower; the other half by nuclear power. Most of the largest hydroelectric power stations in Sweden were built prior to 1970. At that time environmental considerations were not given high priority. However, from the 1960s onwards, environmental awareness has grown and in 1987 a number of rivers were protected from hydropower development. In 1993 further river sections were protected and the four large, free-flowing rivers, Tornealven, Kalixalven, Pitealven and Vindelalven were designated as ‘National Rivers’. However, the decision to phase out nuclear power by 20 10 has increased the pressures for more hydropower. There are now opportunities for improving environmental conditions in existing schemes and this has led to co-operation between environmental authorities, the hydropower companies such as Vattenfall and the National Board for Industrial and Technical Development in order to find appropriate solutions. The present research programme and workshop are evidence of this growing co-operation in river management. Hydropower development, as with most forms of energy production, has an impact on the environment. The workshop has, however, clearly shown that there are also strong international efforts to ameliorate these effects through the employment of remedial measures. Nevertheless, the range of solutions is still rather limited and their long-term sustainability has only been demonstrated in a few cases. Further work on remedial measures is obviously needed, both basic ecological understanding of individual measures and an appraisal of their practical, long-term function in a catchment context. This calls for increasing co-operation between those responsible for regulation, government agencies, engineers and environmental scientists.

A Fortran-based simulation model was written to predict water movement into and out of Flathead Lake. Since tributaries, the lake proper and downstream flows were either freeflowing or regulated by hydroelectric dams, it was necessary for the model to adjust simulated, regulated flows to account for predicted natural flows. Empirical input to the model consisted of daily discharge data or statistical summaries of data sets for an 80+ year period of record for gauging sites on major tributaries, the lake and its outlet river. The model produces graphical output of empirical observations or simulates lake levels and riverine flows on the basis of any selected period of the record. The model was successfully used to evaluate the feasibility of various water management scenarios to enhance or protect fish and wildlife and also permit various hydropower production quotas.

A multitude of Norwegian rivers are managed for generating hydroelectric power (HEP). The hydrology of these hydrorivers changes in various ways which reflect the implemented HEP schemes. Increased winter flows and a concomitant lack of ice-cover are features found downstream of power installations on many Norwegian hydrorivers. Our study objectives were (1) to assess the changes in aquatic macrophyte abundance subsequent to HEP development; and (2) to evaluate the environmental conditions under which nuisance growths of macrophytes are likely to occur. Towards these ends, Norwegian literature data were compiled and assessed. Some case studies are outlined showing the variety of situations under which prolific growths of macrophytes might result. Unfortunately, available data on macrophyte-associated problems proved deficient in many respects. This occurred chiefly because most macrophyte studies are conducted either before, or after, an HEP scheme is completed. Evidently, no concerted efforts are made for assessing the likelihood of macrophyte-dominated nuisance growth in the planning stage of a Norwegian HEP scheme.

Construction of the La Grande Complex in Eastern Canada called for complete or partial diversion of six rivers, the mean annual discharge of which varied from 60 to 1600 m3 s−1, and the transfer of water from two adjoining watersheds (1586m3 s−1) into La Grande Riviere. Three of these rivers with reduced flow, Eastmain (from 700 to 95m3 s−1), Opinaca (from 260 to 35m3 s−1), and Caniapiscau (from 1708 to 960m3 s−1) and the waterways with increased flows, Boyd-Sakami (from 1 to 811m3 s−1) and La Grande Riviere (from 1760 to 3400m3 s−1) were intensively monitored from 1978 to 1988, while the others were surveyed sporadically. Very few modifications were observed, compared to previous conditions, in banks stability, water quality, and aquatic fauna of the affected rivers: those which appeared can be explained by the water level fluctuations, the water quality of the tributaries of the residual drainage basin, and/or by the direct influence of the reservoirs. In James Bay, the areal extent of the Eastmain River and La Grand Riviere plumes varied with discharge in an exponential relation, mostly under ice cover; resources from the coastal environment had not yet undergone any major changes. Because of the very small population dwelling in the area, less than 3000 inhabitants, modifications in the hydrological regime of these rivers did cause relatively few impacts, the main being greater banks instability in specific locations, reduction of the thickness and ice cover on La Grande Riviere, enhancement of navigation constraints on Eastmain River, and higher mercury levels in fish related to operation of the reservoirs.

Biological surveillance techniques have been widely used to assess river water quality. A biological survey using the E.B.I. method has been carried out in the Viola Torrent, a mountain stream subject to water transfers from hydroelectric power plants and receiving the aqueous effluents from some villages. The investigation shows that the oligotrophy of the torrent brings about a simplification of the macrobenthic community and leads to an inaccurate assessment of the quality of the water. It has therefore been necessary to adopt a critical application of the mapping method used.

The Cernadilla reservoir, located on the River Tera (northwest Spain, Duero Basin) is an oligotrophic reservoir with a capacity of more than 250 × 106m3 and a depth of 70 m. The regulated flow pattern is characterized by higher summer flows, higher day flows than night ones, and lower weekend flows. The effects of this artificial flow regime on the stream ecosystem were evaluated by comparing the aquatic communities of an upstream station with those of three downstream stations, located at 1, 8, and 24 km from the dam. Physiocochemical characteristics of the water, fish and macrobenthic communities, and macrophytes were studied at each station. Summer water temperatures were significantly lower downstream of the reservoir. Macrophyte biomass, macrobenthic diversity, and richness were reduced below the dam. The trophic structure was changed by an increase of shredders and grazers. Fishery total biomass greatly fluctuated seasonally, but was higher above the reservoir than below it. However, in spring the trout populations were higher upstream because large numbers of spawning cyprinids from the reservoir invaded upstream reaches of the River Tera and displayed resident trout populations.

The ecological affinity of fish species to different habitat types in the course of their life cycles is investigated in the free-flowing sections of the Austrian Danube. The results point to the great significance of the degree of hydrological integration between the river and its adjoining waters. In the breeding and fry stage, rheophilic species are bound to the river itself, but the preferred zones show great variations according to both fish age and species. A highly structured shoreline is important in providing a gradient of water current and a spectrum of food sources. Regulated embankments are characterized by an extremely reduced fauna with low population densities of eurytopic species. These results give evidence that the conservation of the characteristic species association is incompatible with the planned hydroelectric power dams in the remaining, free flowing zones of the Danube. The importance of the fish fauna as an indicator group for the condition of large river systems is discussed from the conservation point of view.

Riverine trichopteran (caddisfly) communities were sampled seasonally at eight locations over a two-year period to determine if a multiple-level release hydroelectric dam created a faunal discontinuity along the longitudinal profile of the Clearwater River in northern Idaho. The confluence of a non-regulated river with a regulated reach provided an unusual opportunity to examine the recovery of community parameters in a semi-regulated river. A marked discontinuity in the caddisfly community occurred in the regulated reach immediately below the dam, reflected by greatly reduced diversity and abundance values, and by shifts in functional relationships. These faunal changes are primarily attributable to the modified temperature, discharge, and food resources induced by river regulation. The non-regulated river exerted a major mitigative effect on the trichopteran fauna below its confluence with the regulated reach, despite maintenance of semi-regulated conditions. Trichopteran community parameters approached near normal levels concomitant with partial reductions in flow fluctuations, and partial recovery of temperature and food resources. In addition, the non-regulated river provided an important colonization pathway for riverine species to enter the semi-regulated lower reaches.

An attempt is made to appraise the present and likely future environmental impacts of the Tucurui Dam upon the mid and lower Araguaia-Tocantins Basin (Brazilian Amazonia). Information has been obtained by literature review and from field visits made by the author in 1981, 1983, and 1985. The problems of impoundment-related environmental impact assessment in tropical developing countries are also discussed. Tucurui is the world's fourth largest hydroelectric project and is the first large dam to be completed in an Amazonian rainforest environment. It is also the first of a number of large dams which Brazil plans to construct in the Araguaia-Tocantins Basin and on other tributaries of the Amazon. The appraisal of problems associated with Tucurui Dam should help planners minimize difficulties with future Amazonian hydroprojects. Like many developing country environmental impact assessments, that carried out at Tucurui was begun after the construction site had been selected and construction had begun. Tucurui impact studies have tended to concentrate on the environmental and socioeconomic impacts generated in the vicinity of the dam and reservoir; downstream impacts have received less attention but are already apparent. Major impacts identified include reservoir siltation, disruption of fisheries and agriculture (especially floodland cultivation) downstream, and human disease problems.

Recent studies on the impact of peaking hydropower have indicated that impacts directly associated with peaking releases can be categorized as those which result from the hydraulic surge during start-up of generation and the chronic effects of frequent flow fluctuations to biota occupying the fluctuation zones. The instream flow incremental methodology (IFIM) was not designed for application to dynamic flows. In order to determine if such an application was appropriate, we chose to conduct exacting habitat descriptions at the unique zones associated with peaking hydropower (degradation, aggradation, and recovery) and combine this information with suitability criteria for the banded sculpin (Cottus carolinae) and stage/discharge records to obtain weighted usable habitat estimates under typical peaking releases. The results demonstrate that habitat declines as the peaking surge passes an area and rises during low flow periods. However, the difficulties in interpretation of the results remain. That is, the biological response (lateral or longitudinal movement or entrainment) as well as the refuge value of certain hydraulic conditions remain to be investigated. This suggests that the present management value of IFIM applied to peaking hydropower is in examining minimum habitat values that occur during the generation cycle and a comparison of gains or losses in the minimum (greatest amount of hydraulic refuge available) WUA estimates under various proposed release scenarios.

Spring and summer chinook salmon Oncorhynchus tshawytscha populations from the Snake River have declined precipitously from historical levels. Declines initially were the result of overfishing, but since the late 1950s they have been primarily the result of hydroelectric dams that blocked access to spawning areas, created slack water, and caused mortalities of juvenile outmigrants. Mitigation hatcheries and juvenile collection and bypass systems at dams, transportation of juveniles from upriver dams to a release site on the lower Columbia River, a water budget, allocated spill for fish at main-stem dams have not increased these populations appreciably. Ocean conditions may have been a factor in the low returns from 1976 through 1984, but the factors currently considered as controlling the viability of the run are the low genetic variability, lack of stress tolerance, and incidence of bacterial kidney disease inherent in the stock. With control of disease, reduced stress at main-stem projects, and continued efforts to maintain genetic variability, the chinook salmon populations should increase and their long-term viability should be assured.