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Dams alter aquatic ecology and river hydrology upstream and downstream, affecting water quality, quantity and breeding grounds (Helland-Hansen et al., 1995). They create novel and artificial types of aquatic environment for the life span of the dam.

3.1 Water quality and physical changes

Upper reaches of the reservoir may not be affected very much as the original riverine conditions are still retained in most Nepali reservoirs. Downstream of the dam the flow rate in the river will depend on the amount of thecompensation flow. Water volume is considerably reduced during the dry season. As a result the downstream may change to pools alternating with dry stretches for about nine months from November to June. Due to decreased water discharges, water temperature will rise in daytime and decline sharply at night. Rooted plants will grow in the riverbed due to the decrease in water volume.

3.2 Impact on fish

Fish such as snow trout, catfish and loaches may be pulled into the intakes and get killed. Even riverine fish adapted to fast current may be lost. Fish food organisms will be highly affected by reduced flow rates and new species will invade areas with a slow current. Golden and copper mahaseer are known to be affected directly by the changes in their habitat, which leads to stunted growth, diseases and parasite infestation and increased mortality.

3.3 Effects of impoundment

Installation of a weir on the Tamur River and Mewa Khola has changed 200 m and 100 m long river stretches, respectively, into two small reservoirs. Dudh Koshi is now turned into a reservoir of 687.4 mm3 storage, of which 442.1 mm3 is live storage. The riverine habitat of 53.1 km will be converted into a 1989 ha reservoir by the West Seti hydropower dam. Similarly, the Karnali dam will create a 7.0 km long reservoir of an average width of 90 m. Another reservoir will be created upstream of a dam at Likhu Khola, flooding a 700 m long river stretch and having an 18 m daily water level fluctuation.

Water in the reservoirs may become thermally stratified. This will depend on a number of factors, especially on the water retention time and the depth of the reservoir. As the water uptake for turbines is usually in the hypolimnion, cold water will be discharged downstream during the operation of the power station. The hypolimnion may become deoxygenated, and discharge of such water downstream may negatively impact the aquatic fauna in the river below the dam. The reservoir itself may undergo eutrophication as happened for example in the Indrasarovar reservoir (Pradhan and Swar, 1988). Lacustrine conditions of the reservoir will differ from those of the river and this will also have an impact on the original fish fauna. Some fish species may disappear, other will adjust by changing from stenophagous to euryphagous. Omnivorous or planktivorous fish may adapt to the reservoir conditions. But Garra lamta, Glyptosternum, Coraglanis, Puntius sp, Glyptothorax pectinopterus, G. cavia, Psilorhynchus and Pseudecheneis do not like lacustrine conditions. Anguilla bengalensis and Schizothorax richardsonii have been also known to decline abruptly in reservoirs. Snow trout, once abundant in the Kulekhani River, became rare after the river was dammed. The same happened to mahaseer after the impoundment of Phewa and Begnas lakes in Pokhara Valley. Neolissocheilus hexagonolepis, on the other hand, is a common fish in reservoir conditions (Swar, 1992). Carp minnow (Barilius bendelensis), stone loach (Nemacheilus beavani) and stone roller (Garra gotyla) find reservoir conditions favorable, and reservoir conditions are ideal for rapid colonization by hardy fish such as murrels and catfish (Heteropneustes fossilis). Shoreline erosion and rapid and intensive drawdown are other problems faced by fish as they affect spawning and egg incubation.

3.4 Effect of dams on fish migration

A dam will fragment and isolate upstream resident fish such as stone carp and catfish from downstream. The resident species may congregate in the tailwater release site. Fish from upstream will occasionally sweep downstream during the monsoon, stay in the tailwater or swim further downstream. A dam will obstruct the route of the long and mid-distance migratory fish. Upstream migrants will arrive at the dam site during the flow phase. Long distant migrants such as Tor sp., Bagarius, Pseudeutropius, Clupisoma and Anguilla, and mid-distance migrants N. hexagonolepis and Labeo species are most affected by a dam. These species will abandon the original pool and colonize deep pool regions downstream or upstream. Populations of snow trouts are less affected, as they make a small-scale migration to tributaries to breed in clear and cool water during the monsoon and return to the main stream during the low flow period (Shrestha, 1995).

3.5 Socioeconomic impact of dams

A large number of fisher communities are dependent upon fishing for livelihood. These fisher communities are classified as full time, part time and occasional fishermen depending upon the intensity of fishing (Table 7). Full-time fishers devote 200-230 days/year to fishing, part-time 60-65 days/year and occasional about 25-30days/year (EIA-Tamur, 1998). They belong to different ethnic groups: Bote, Majhi, Sarki, Badi, Raji, Gurung, Magar, Chhetri, Kami, Damai, Tamang, Brahmin, Sahi, Gharti, Thapa, Malla, Danwar, Bhujel and some others. The fish catches have been reported to decline due to the high fishing pressure, use of chemicals, dynamiting, electro-fishing and the use of small-meshed nets.

Table 7

Fisher communities recorded in EIA studies

Type of fisher



Upper Karnali*



Sun Koshi**





























* Source - EIA Reports, **Source - Yearly Progress Report (2055/56)

Reduction of riverine fisheries will influence the livelihood of many fishers. Professional full-time fishermen have to seek alternatives to fishing. Some may respond by migrating to new places. There will be intense competition for the permanent jobs created by the project. Social interactions between the local communities and power station employees will have both positive and negative impacts on the traditional culture and values of local communities. Public safety concern is a problem and it may include accidents from sudden peak releases downstream in the dry season. Flooding of land to create a new reservoir also leads to the need to relocate fisher families as well as a permanent loss of arable land and hence of agricultural production,. On the other hand, the relocated fisher families may be provided with electricity and perhaps receive some other benefits arising from the presence of the dam and the reservoir, such as development of local tourism, catering services, and the use of reservoir for fisheries.

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