1.1 Nepal rivers
Most of the rivers originating in the high Himalayas are nourished by snow and glaciers and flow to the south to provide water to millions. They also threaten the livelihood of people of the Trans-Himalayan region through floods, erosion, landslides, water-borne epidemics, etc. Nepal lies in the central part of the 2 400 km long Himalayan region, and has features of both the east and west Himalayas. This position offers a relative advantage in understanding the problems and prospects of Himalayan rivers. Physiographically, Nepal is divisible into three regions: Himalayan (highlands), Mahabharat (midlands or midhills), and Churia and Terai (lowlands). There are three basic types of rivers: i) perennial glacier- and snow-fed coldwater rivers carrying significant flows in dry season; ii) perennial ground water (e.g. springs) and rain-fed rivers carrying some flow in dry season; and iii) seasonal rain fed rivers with no flow in dry season. Monsoon rainfall is an important source of water for all the river systems of Nepal: 72 percent of rainfall flows as surface run-off from basins. Late monsoon, comprising 12 percent of the total rainfall, comes as snow. The remaining rainfall percolates as ground water to cause delayed run-off. Perennial rivers are supplemented with water from melting of snow during the dry season before monsoon, and by ground water from September to March. In Nepal, 10.7 percent of river basin areas are located in permanent snow regions (5 000 m and higher), 17 percent in seasonal snow regions (between 3 000 m and 5 000 m), and the remaining 70 percent are in no-snow regions (Sharma, 1997).
Nepal has more than 6 000 rivers and streams belonging to four main drainage basins: Sapta Koshi, Gandaki, Karnali and Mahakali. The total area of all the rivers and streams is estimated to be about 395 000 hectares (FDD, 1998). Different rivers of Nepal support a variety of aquatic flora and fauna. Fishes of Nepal are unique depicting the characteristic of three major regions: i) very cold water fishes of the high mountain Himalayan region, ii) cold water fishes of the midland region, and iii) warm water fishes of lowlands. These fishes also show diverse biological and structural adaptations to fluctuating torrents of the rivers flowing through different topographies. So far 185 fish species have been identified for Nepal (Shrestha J., 1997), including a significant number of cold water species. There is a high demand for cold water fish as they fetch a good price for their excellent taste. Some of them have recreational/sport importance. Indigenous fish species of the Himalayan region make a significant contribution to the livelihood of the rural population and are also a valuable genetic resource.
Water resources of Nepal can be profitably utilized for power generation, irrigation, domestic uses, aquaculture and recreational fisheries. Of all the above, hydropower development has become the most attractive as a reliable, cheap and safe energy source. The theoretical hydropower potential of Nepal is 83 200 MW of which 44 552 MW is regarded technically feasible and 42 133 MW economically feasible. Snow-fed Himalayan streams and rivers are small and highly turbulent down to the base of the Himalayas where they get bigger and become less turbulent. In the midhills they are supplemented by springs. The larger perennial rivers are ideal for hydropower development. Run-off river schemes and high dam reservoirs are the major types adopted for power generation in Nepal. In the steep northern parts where average flows of water are lower, the former type is applied. This type generates a lifelong small benefit and one fourth of the Nepal's economically feasible hydropower potential is of this type. In the lower hills or lowlands of Nepal, where rivers flow through flat valleys and wide plains where precipitation and ground water are the major sources of water, river discharges decrease to low or no volume in the dry season. Hydropower development in such areas requires water to be stored in a reservoir established by blocking the river with a high dam to provide for year-round use. It is estimated that under Nepal conditions a high dam storage reservoir can generate electricity for about 50 years. Weak geology, frequent earthquakes, high sediment load and fragile environment are the challenges for a high dam reservoir together with the problems of submergence, stratification, fast sedimentation and erosion of downstream areas.
Cold water fish of Nepal are facing problems due to an increasing number of hydropower projects. Once abundant indigenous fish stocks have been declining due to overfishing, harmful fishing practices (electrofishing, dynamiting, use of chemicals), pollution and developmental works. Developmental works such as river damming have a major impact on river ecology, aquatic flora and fauna, including fish. Cold water fish of Nepal are affected by the increasing number of hydropower dams in the country. In view of this, His Majesty's Government (HMG) of Nepal has made Environment Impact Assessment (EIA) compulsory under the EIA National Guidelines (HMG 1993) for all the hydroelectric projects above 5 MW. In the eyes of HMG of Nepal and international agencies including IUCN, the conservation status of none of the fish species seems yet to be alarming in Nepal. However, an EIA study for the Ilam (1994) has reported Neolissocheilus hexagonolepis and Tor tor endangered and threatened, respectively. The EIA study for the River Gandaki (1996) had listed three fish species endangered, five species threatened and seven species restricted. In a study of fish status in Nepal, Shrestha J. (1997) has listed 34 species threatened and 61 species insufficiently known.