In Bhutan there are four major river systems, and all of them flow out of the glaciers in the north through the central mountains and valleys, and into the lower Siwalik range in the south to join the Brahmaputra River in India. However, the bulk of the water volume (60-70) is attributable to rain-fed (Monsoon) recharge and rest to winter snow and glaciers. Glaciers cover about 10% of the area of Bhutan, providing an important source of renewable water. Thus, hydrological regime is characterized by low flow in the winter and high flow during summer caused by monsoon precipitation and melting of glacier ice and snow. Due to the mountainous topography, rivers flow swiftly through narrow valleys, resulting in high hydropower potential (30,000 MW) but less used for drinking and irrigation purposes.
Although inland freshwater biodiversity of Bhutan is considered as both highly diverse and representative to the regional and global significance, representation of Bhutanese freshwater biodiversity is inadequate due to lack of readily available information on the status and distribution of inland water taxa.
There are few scattered information on fish, water birds and invertebrates but are not exhaustive. Ichthyofaunal diversity survey carried out by College of Natural Resource (CNR) in 2011 provides an annotated checklist of 91 species of fish that are now known to occur in Bhutan. IUCN (2009-2010) reports 7.2% of species of the 1,073 freshwater taxa as threatened with extinction and 5.4% as near threatened in the Eastern Himalayan region (Nepal, Bangladesh, Bhutan, and parts of Myanmar, China).
Habitat loss and degradation, mainly due to sedimentation, pollution (from urban, agricultural, and industrial sources), forest clearance, and the development of hydropower dams were identified as the major causes of species decline.
Goal: By 2020, ecological integrity of major river basins maintained to ensure long term sustainable use of water resources, biodiversity conservation and resilience to climate change.