Koshi Basin
- Page status
- Registry record
- Updated
- 2026-05-11
Summary
The Koshi (Sapta Koshi) basin is Nepal's eastern major river system. It drains ~27,818 km² within Nepal through seven major tributaries, including the Arun, Tamor, and Sunkoshi. The Arun is one of Nepal's oldest rivers — older than the Himalayas themselves, predating the mountain uplift.
The basin is the largest glacierized in Nepal (278 glaciers, 51 km³ ice volume) and carries the largest gross hydropower potential of any single basin.
Key Facts
| Fact | Value |
|---|---|
| Area in Nepal | 27,818 km² |
| Average discharge (border) | 1,827 m³/s (~57.6 BCM/yr) |
| Monsoon share | 67–78% |
| Peak-to-dry ratio | 25–50:1 |
| Gross hydropower potential | 27,805 MW (38.3% of national total) |
| Glaciers | 278 (51 km³ ice) |
| Sediment significance | ~120 Mt/yr, nearly 25% of Ganges sediment load |
Hydropower Significance
Largest gross potential of any basin in Nepal per WECS Assessment of Hydropower Potential of Nepal (2019). Contains the Dudhkoshi Storage Hydroelectric Project project (670 MW) and Upper Tamakoshi (456 MW, largest operating plant). The Arun corridor hosts Upper Arun (900 MW, under construction).
Constraints
- Extreme sediment loads — ICIMOD framing puts the basin near 120 million tonnes/year into the Ganges system, or nearly 25% of the total sediment load. See Sediment as Design Constraint.
- Flood hazard — known as the "River of Sorrow" in Bihar due to catastrophic flooding and course shifts. The 2008 Koshi breach displaced 2+ million people.
- GLOF Risk — the 2024 Thame Valley GLOF traveled >80 km down the Dudh Koshi, destroying hydropower facilities.
- High hazard does not eliminate the basin's value, but it demands Buildability filters on every project.
Downstream Significance
Koshi irrigates ~500,000–600,000 ha in Bihar. Supports >40 million people downstream. Contributes disproportionate sediment to the Ganges system. Central to Hydro-Geopolitics and Nepal's Contribution to the Ganges.
Limitations & Controversies
Extreme sediment loads. The basin carries ~120 Mt/yr into the Ganges system — nearly 25% of the total sediment load. This makes every project in the basin a Sediment as Design Constraint case study, with turbine erosion and reservoir siltation as permanent operating risks.
Flood hazard. Known as the "River of Sorrow" in Bihar due to catastrophic flooding and course shifts. The 2008 Koshi breach displaced 2+ million people. A dam or barrage failure in this basin would have cross-border consequences an order of magnitude beyond any single project.
GLOF exposure. The 2024 Thame Valley GLOF traveled >80 km down the Dudh Koshi, destroying hydropower facilities. With 278 glaciers and 51 km³ of ice volume, the basin has the highest GLOF frequency risk in Nepal.
Peak-to-dry ratio of 25–50:1. The most extreme seasonal flow variation of any major basin, making run-of-river projects structurally winter-poor and concentrating value in the monsoon months when prices are lowest.