Environmental Flow Releases from Hydroelectric Projects

Environmental flows (EF) are an ecologically acceptable flow regime designed to maintain a river in an agreed or predetermined state. Difficulties in the actual estimation of EF values arise primarily due to the inherent lack of both the understanding of and quantitative data on relationships between river flows and multiple components of river ecology. The major criteria for determining EF should include the maintenance of both spatial and temporal patterns of river flow, i.e., the flow variability, which affect the structural and functional diversity of rivers, and which in turn influence the species diversity of the river. All components of the hydrological regime have certain ecological significance. High flows of different magnitude are important for channel maintenance, bird breeding, wetland flooding and maintenance of riparian vegetation. Moderate flows are critical for cycling of organic matter from river banks and for fish migration, while low flows of different magnitudes are important for algae control, water quality maintenance and the use of the river by local people. Therefore, the elements of flow variability have to be maintained in a modified EF-regime. The present paper describes different methodologies for environmental flow assessment and methodology adopted for the assessment of environmental flow releases from the hydroelectric projects in Siang basin, Arunachal Pradesh.