What does a giant dam change about a river: Three Gorges, fish and sediment?

A big dam changes a river in three main ways: it makes the flow predictable, it traps the sediment, and it blocks the fish. The Three Gorges Dam on China's Yangtze River shows all three. It is a hydroelectric gravity dam, and the world's largest power station by installed capacity (22,500 MW). It also improves the Yangtze's shipping capacity and provides flood control, helping to protect millions of people on the Yangtze Plain.

The flood control has a history. The Yangtze flood of 1954 killed 33,167 people and forced almost 18.9 million to move. That is the kind of flood the dam's flood control is meant to hold back. But the price was also paid in people: estimates of the number displaced by the dam's construction range from 1.13 million to around 1.4 million.

Then there is the mud. Above the dam the flow is slower, so much of the river's sediment settles there and less travels downstream. Some hydrologists expect downstream riverbanks to become more vulnerable to flooding. In general, sediment flow is reduced after a dam is built because the reservoir traps it, and that can cause river deltas to shrink.

Fish pay too. Dams can impede fish migration by blocking access to spawning habitat upstream, and fish ladders are included in many dams to help. Even so, migratory fish are especially vulnerable: in the Yangtze alone, dams have pushed five species to the verge of extinction. Hydropower is renewable and generally much cleaner than fossil fuels, so a dam is a real trade-off.