Tribhuvan University, 12/828 Thahity Tole, Kwabahal, Kathmandu
Nepal has more than 6 000 rivers and streams, most of which originate in the high Himalayas and midhills and are fed by glaciers and springs forming perennial rivers. Those of lowlands are fed by rain and intermittent streams and dry up during the low flow period. The rivers support a large variety of aquatic flora, insects and 185 fish species. The fish are classified as long distance migrants, mid and short distance migrants, or resident fish. Rivers of Nepal are renowned for hydropower potential. The theoretical hydropower potential is estimated to be 83 200 MW, with 42 133 MW as economically feasible. Two methods, i.e. runoff river type and high dam reservoir type, have been adopted for hydropower development in Nepal. Weir type is adopted in the steep northern part of Nepal where low and average flow of water show less variation. High dams with a reservoir are built in the lower hills of Nepal where valleys are wide with small gradient. In the reservoir, some resident and game fish may decline, whereas Neolissocheilus hexagonolepis, resident non-game fish species and hardy fish such as murrels and catfish may find reservoir conditions favorable. Long distant migrants such as Tor sp, Bagarius, Pseudeutropius, Clupisoma etc. and mid-distance migrants as N. hexagonolepis and Labeo sp cannot pass through the physical barrier of a dam. Fish passes are an important mitigation measure at lower dams while fish lifts may be used at higher dams to assist fish in their upstream migration. Minimum downstream discharges recommended in EIA studies of small to large rivers of Nepal vary from 0.5 to 15 m3/s to maintain an aquatic environment suitable for riverine fish.