In recent decades, humans have built many dams. These are designed to regulate flow for irrigation, hydropower and water supply. Most major rivers in the world are dammed.
But there are detriments to damming rivers. Many people depend on the natural ebb and flow of unrestricted rivers that swell with water in the rainy season and wane in the dry season. When the natural flow is changed, people and ecosystems are affected: globally, an estimated 472 million people living downstream of dams have suffered adverse effects from changes to the rivers’ flows.
Free flowing rivers bring huge benefits. Fishermen benefit from healthy fish populations that need flooded areas to spawn and feed. In dry regions, livestock keepers often depend on rivers spilling out over the floodplains to maintain grasslands for grazing cattle. Farmers and ecosystems depend on naturally high flows that transport and deposit sediment along the river, leaving important nutrients that enrich soil and boost crops.
Dams that regulate water flows reduce all these benefits. In exchange they provide water for hydropower and domestic supply. It’s a trade-off in which some people gain and some lose.