Hydropower development in the Himalaya: identifying critical river stretches

Two hydroelectric projects are operational in the upper and lower stretches of the river Alaknanda near Lambagarh and Srinagar that are respectively ~ 140 km apart. The health of the river ecosystem in the flowing stretches between and below these projects was bio-assessed, based on widely used diatom indices and van Dam ecologic values computed by OMNIDIA software. Samples were collected from Birahi (S3), the semi-natural stretch ~ 53 km downstream of the powerhouse, Narkota (S5), the impacted stretch ~ 27 km upstream Srinagar dam and Bagwan (S8), the highly regulated stretch ~ 22 km downstream Srinagar. IPS and IBD revealed mild pollution in the upper and lower stretch compared to relatively elevated pollution at S5. However, TDI indicates moderate organic pollution at S3 and S5, whereas the IDS/E indicates low degradation at all stations. The prevalence of tolerant N-autotroph, β-mesosprobe, mesotrophic, and aquatic to aerophilic categories of van Dam environmental variables indicate that a certain level of impact already persists due to organic-cum-nutrient load and perturbed flow regime. The prevalence of eutrophic and moderate O₂ categories shows that the ecosystem is in a critical state at S5, as also observed for species diversity. Based on the dominant categories, the river stretch was classified as "polyoxybiontic-cum-mesotrophic" at S3 and S8 while "moderate O₂-cum-eutrophic" at S5, implying higher organic and nutrient load in the river section at S5.