Explanation:

 The river's most famed fauna is the freshwater Ganges river dolphin (Platanista gangetica gangetica), which has been declared India's national aquatic animal.
This dolphin used to exist in large schools near to urban centres in both the Ganges and Brahmaputra rivers, but is now seriously threatened by pollution and dam construction. Their numbers have now dwindled to a quarter of their numbers of fifteen years before, and they have become extinct in the Ganges' main tributaries. A recent survey by the World Wildlife Fund found only 3,000 left in the water catchment of both river systems.