Rivers, Deltas and Drainage Basins

A drainage basin, or watershed, is all the land that drains into one river system, and it is one of the most useful units for understanding how water connects places. Rivers erode valleys in their upper courses and deposit sediment downstream, building floodplains and deltas where they meet the sea. Those deposits create some of the most fertile land on Earth, which explains why early civilizations grew along the Nile, Tigris–Euphrates, Indus and Yellow rivers.

Rivers also link upstream and downstream communities: a dam or a pollution spill in one country affects people hundreds of kilometres away. Large dams such as Egypt's Aswan High Dam provide electricity and flood control, but they also trap sediment, so deltas downstream can shrink and farmland loses its natural fertilizer. Managing a river therefore involves trade-offs among farmers, cities, ecosystems and neighbouring states.