A reservoir is an artificial lake built to store water for irrigation, drinking, hydropower, or flood control. Most are formed by damming a river valley, though some are off-stream basins filled by canals or pumps. The stored water is released when demand rises or rainfall falls.

For agriculture, reservoirs smooth out the mismatch between when water arrives and when crops need it. Snowmelt in spring can be captured and released through the summer. A reservoir also allows a region to survive a dry year, at least for a while. But the buffer is finite, and repeated dry years can drain storage to levels that force cuts.

Uses and trade-offs

• Irrigation supply for dry-season cropping
• Flood control downstream
• Hydropower generation
• Recreation and fisheries

Dams and reservoirs have costs. They displace communities, block fish migration, trap sediment that would otherwise replenish downstream deltas, and lose water to evaporation. Managing a reservoir is a balancing act between competing demands, and the trade-offs rarely satisfy everyone.