Eco-geomorphological approach for environmental flows assessment in monsoon-driven highland rivers: A case study of Upper Ganga, India

Abstract Study region Upper Ganga reaches up to Rishikesh town, India. Study focus Environmental Flows (E-Flows) assessment in the upper stretches of the Ganga river has been carried out by integrating ecological and geomorphological parameters with hydraulic analysis to estimate the flow depths and flow volumes necessary for river ecology and channel maintenance. We have used a modified version of Building Block Method (BBM) for computing E-Flows for lean period, for monsoon period and for high floods based on the flow requirements of keystone species for different sites and geomorphic considerations. We define three flow depths, D1, D2 and D3 which correspond to the minimum flow depths required for sustenance of keystone species during lean period, for breeding and spawning of keystone species during monsoon period, and for maintaining lateral connectivity during floods respectively. New hydrological insights for the region Annual hydrographs for E-Flows have been developed and compared with the observed flows for each site under natural flow conditions. Our computation shows that for the wet period, which is taken as the period from mid-May to mid-October, monthly E-Flows vary from ∼23% to ∼40% of the monthly natural flows at different sites. However, dry season E-Flows as percentages of natural flows, taken for the period from mid-October to mid-May, vary over a wider range of 29%–53% for these sites.