The ubiquitous dams around the world are built to guard against extreme flooding, meet steadily increasing water demands and provide hydroelectric power. They also alter river ecosystems — such as by changing temperatures downstream — and can substantially change nearby fish populations.
In China, the Xinanjiang and Danjiangkou hydroelectric dams caused the peak summer temperature to decrease 7.2 to 10.8˚F (4 to 6˚C) in the downstream reaches of nearby rivers. Fish spawning was delayed by three to eight weeks, causing the local extinction of many of the warm-water fish.
Hydropower dams generally operate by drawing water from the deeper layers of a reservoir into a turbine for energy. This brings colder waters downstream and causes a cooling effect in the summer; the effect reverses in the winter. Some also draw water from the surface or have shallower reservoirs, which could create warmer downstream temperatures.
Worldwide, at least 3,700 medium and large hydropower dams are planned in the coming decades or under construction, heavily concentrated in South America, Africa and South and East Asia. Hundreds of millions of people in large river basins in these areas rely heavily on the river for their livelihoods, Holtgrieve said. For example, Cambodians receive about 80 percent of their animal protein from primarily wild-caught freshwater fish from the Mekong River.
“The Congo, Amazon and Mekong basins are going to have a large number of dams, and that is inevitable,” said Shahryar Ahmad, the lead author of the study.