Dam: Ecological Impact

Building a dam changes the ecology of the surrounding area. Among the most affected animals are fish that depend on free-flowing water to live. Some kinds of salmon, trout, and other fish species migrate downstream to spend part of their lives in the open ocean. As adults, they return upstream to lay their eggs in the gravel bottoms of the rivers where they were born. Large dams block the passage of such migratory fish.

Some dams incorporate a fish pass to allow fish a chance to swim around the dam and reach upstream spawning grounds. Fish passes called fish ladders comprise a series of small pools arranged like stair steps. Each pool is slightly higher than the previous one. Fish ladders are based on the idea that a fish swimming upstream cannot leap over a dam that is more than about 5 meters high, but it can leap up a series of pools, each slightly higher than the one below it. Despite fish passes and other efforts to help fish bypass dams, the cumulative effect of multiple dams built along the length of a river can exact a heavy toll on fish populations. In rivers blocked by many dams, salmon populations have dropped by as much as 95 percent, a decline many experts attribute, at least in part, to dams.

Dams also alter the water temperatures and microhabitats downstream. Water released from behind dams usually comes from close to the bottom of the reservoirs, where little sunlight penetrates. This frigid water significantly lowers the temperatures of sun-warmed shallows downstream, rendering them unfit for certain kinds of fish and other wildlife. Natural rivers surge and meander, creating small pools and sandbars that provide a place for young fish, insects, and other river-dwelling organisms to flourish. But dams alter the river flow, eliminating these microhabitats and, in some cases, their inhabitants.

Dams prevent nutrient-laden silt from flowing downstream and into river valleys. Water in a fast-moving river carries tiny particles of soil and organic material. When the water reaches a pool or a flat section of a river course, it slows down. As it slows, the organic matter it carries drops to the river bottom or accumulates along the banks. Following heavy rains or snowmelt, rivers spill over their banks and deposit organic matter on their floodplains, creating rich, fertile soil. Some of the organic matter makes it all the way to river mouths, where it settles into the rich mud of estuaries, ecosystems that nourish up to one-half of the living matter in the world’s oceans. Large dams artificially slow water to a near standstill, causing the organic matter to settle to the bottom of the reservoir. In such cases, downstream regions are deprived of nutrient-laden silt.

Dams can also wreak havoc on human populations. Reservoirs created by dams can inundate entire riverside communities that may be centuries old and filled with rich archaeological treasures. Community inhabitants are forced to seek out new places to live and work. Even those who do not have to leave suffer from forced change. People who depend on rivers for their livelihood may need to change their way of life when dams destroy natural river flows.