The Gangetic river dolphin Platanista gangetica, locally known as sihu, inhabits major river systems such as the Ganga and the Brahmaputra in the Indian subcontinent. The population of the Gangetic river dolphin is falling fast (only a few hundred remain) due to habitat loss and poaching. The IUCN lists the Gangetic river dolphin as 'Critically Endangered'.
The Brahmaputra population of the Gangetic dolphin is still relatively safe from population fragmentation and other ecological complexities resulting from water development projects. But the recent trend of sanctioning hydroelectric projects without thorough environmental impact assessment studies threatens the future of the sihu. A combination of many factors is responsible for the establishment and maintenance of a dolphin habitat. In the entire Brahmaputra river system, dolphins have been reported in only a few tributaries even in the flood season. The river Kulsi (a southern tributary) has the only resident population (now recommended as a dolphin sanctuary by the MoEF) and the lower stretches of the Subansiri (a northern tributary) has an important migratory dolphin population (also a proposed dolphin sanctuary). The proposed Lower Subansiri dam, upstream of this area, is a great threat to this dolphin population. The dam will cause major changes in the flow regime, sediment load and water quality. It will degrade the dynamic attributes of downstream waters and eliminate the flow of sediments essential to the formation of islands and bars, required for an ideal dolphin habitat. Downstream discharge will be prevented from reaching the critical magnitude necessary for water to overspill the bank on to adjacent floodplains, thereby resulting in a dramatic decrease in biological production.
The dam will suppress the natural fluctuation in flow, temperature and detritus loading, which generate optimum conditions for a large number of organisms. The number of ecological niches available to support diverse biotic communities will be reduced. This ultimately will result in a serious shortage of food fish for the dolphins. More importantly, the dam will be a permanent barrier, stopping the dolphins' ascent during the rainy season in search of food. The construction of the Lower Subansiri dam will be highly detrimental to the dolphin population downstream and is against the interests of conservation of this rare creature.