Key Terms & Concepts — UPSC Mains
Run-of-River Hydropower
"A type of hydroelectric project that generates power using the natural flow of a river with minimal or no water storage — contrasted with storage-based dams that impound large reservoirs."
A run-of-river (RoR) hydroelectric project diverts a portion of a river's natural flow through a turbine to generate electricity, using only the flow that is present at any given time rather than storing water in a large reservoir. Key structural features of RoR projects: - Diversion weir or barrage: A low structure across the river diverts a portion of flow into a headrace channel or tunnel - Forebay: A small settling basin before the penstock - Penstock: A pipe or tunnel carrying water under pressure to the turbines - Powerhouse: Contains turbines and generators (often underground in Himalayan RoR projects) - Tailrace: Returns water to the river after power generation RoR vs. Storage Dam comparison: - Storage: RoR has minimal reservoir (small forebay only); storage dams impound millions of cubic metres - Environmental impact: RoR causes lower submergence, less displacement, lower ecological disruption; storage dams cause large-scale displacement and river ecology changes - Power output: RoR output varies with river flow (lower in dry season); storage dams can regulate output year-round - Cost: RoR projects generally cost less and build faster - Examples: Kalai-II (1,200 MW, Lohit River — RoR); Subansiri Lower (2,000 MW — RoR); Bhakra-Nangal (storage); Tehri (storage) Himalayan RoR projects: Most new Himalayan hydropower projects (in Arunachal Pradesh, Uttarakhand, HP) are RoR because: 1. Himalayan rivers have steep gradients — high head for power generation without needing large storage 2. Environmental clearance is easier for RoR (less displacement) 3. Geological risks of large reservoirs in active seismic zones Policy significance: After 2019, large hydro (>25 MW) was reclassified as renewable energy, making RoR projects eligible for Renewable Purchase Obligation (RPO) targets. This significantly boosted the economic case for new RoR projects.
GS3 Environment + Economy topic covering hydropower technology, environmental trade-offs, and India's renewable energy policy. GS1 Geography: Indian river systems, Himalayan hydrology. Important for Arunachal Pradesh hydro potential questions (50,000 MW theoretical potential) and CCEA approvals.
- 1 Definition: Uses natural river flow without large reservoir storage
- 2 Components: diversion weir → headrace → forebay → penstock → powerhouse → tailrace
- 3 Advantage over storage dam: lower submergence, less displacement, lower ecological impact
- 4 Disadvantage: output varies seasonally with river flow (not dispatchable year-round)
- 5 Himalayan RoR: steep gradient compensates for lack of storage; most new HP projects in AP are RoR
- 6 Classification change: large hydro (>25 MW) reclassified as Renewable Energy in 2019
- 7 Kalai-II (1,200 MW, Lohit River, Arunachal Pradesh) — RoR, approved CCEA April 2026
- 8 Subansiri Lower (2,000 MW, Arunachal Pradesh) — India's largest planned RoR project
The Kalai-II Hydroelectric Project on the Lohit River in Anjaw District, Arunachal Pradesh (approved by CCEA, April 2026, cost ₹14,105.83 crore, capacity 1,200 MW) is a run-of-river project — it will divert a portion of the Lohit River's flow through 7 turbines (6×190 MW + 1×60 MW) without creating a large storage reservoir, minimising submergence and displacement in India's border-sensitive easternmost district.
