Dams do a great many things. They:  •  control flooding  •  provide water for drinking and irrigating crops—even during droughts  •  generate power. Water flowing through hydroelectric stations, built near dams, turns turbines. The result is cheap, clean electricity. The power generated from dams is important to the world's well being. Consider this:  •  About two billion people throughout the world have no electricity at all.  •  Wind and solar power have great potential, yet won't produce anywhere near enough power for decades.  •  Burning coal provides 80 percent of the world's energy—along with huge quantities of greenhouse gases that contribute to global warming. Unlike coal-burning plants, hydroelectric turbines are clean and release no damaging gases. But hydropower can also drain the life out of rivers and damage farmland. In the U.S., about 10 percent of the energy now comes from hydroelectric power.

People who lived in early communities didn't turn on the faucet to get fresh water. They simply made their home by the bank of a river. It's no accident that ancient cities arose near major waterways.

Clean, drinkable water can mean the difference between life and death Source: USAID

To live near a waterway also meant growing food without complete reliance on rainfall, and faster ways to carry goods from one place to another.

But living near water can also be dangerous. Because precipitation is unpredictable and never uniform, most rivers often overrun their banks. These floods carry vital nutrients to nearby land but can bring disaster to homes and farms—and living things in their wake. Floods are on the rise—in the 1990s, floods and droughts accounted for 86 percent of all natural disasters worldwide.

As technology evolved, people found ways to move water instead of themselves. By redirecting water into canals and irrigation ditches, they could magically make crops grow on land that had been dry and barren. Constructing dams allowed them to minimize flooding and control moving water—one of nature's most powerful forces—to run waterwheels. People were also freed from having to live near water.

Our control of water for irrigation makes possible most of the food we eat today Source: USDA NRCS

Roman engineers even designed stone aqueducts that carried water for hundreds of miles. Some are still operating today.

This Roman aqueduct in Southern France has worked for nearly 2000 years Source: UNESCO (B. Liegeois)

Armed with modern technology, humans have become increasingly skilled at pushing water around. Huge concrete dams now span rivers, creating giant, artificial lakes.

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When it was completed in 1931, Nevada's Hoover Dam was the world's largest Source: USDA NRCS

Dams save us from flooding and create great water sources and new farmlands. But they also can harm the environment—and people.

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An abandoned boat in what was the Aral Sea Source: UNEP

For much of the past century, farmers in the former Soviet Union diverted water from rivers that flowed into the Aral Sea, the world's largest inland salt-water sea. They transformed dry, arid land into productive cotton farms. But they diverted so much water that the Aral Sea, which has existed for tens of thousands of years, is now in danger of disappearing. Over-irrigation and poor water management have shrunk the Sea to a third its size. The region is becoming a desert.

If you hold the water back, you also miss out on the silt and rich nutrients it carries. Farmers downstream then grow fewer crops and are forced to rely on chemical fertilizers. Egypt's farmers now use about 13,000 tons of lime-nitrate fertilizer each year because silt from the Nile River, stopped by the Aswan Dam, delivers fewer nourishing compounds.

Damming a river can also disrupt the ecosystems in downstream wetlands and floodplains. Fish, important for food and enterprise, become much less plentiful. In some areas, water evaporates so much in the newly created lakes that humans can't drink it or use it for farming. It's too salty and full of minerals and can also harm wildlife.

Every year, tens of millions of people get sick from bathing or swimming in dirty, slow-moving water. Source: Schistosomiasis Control Initiative

Dams and irrigation canals, furthermore, can create bodies of standing or slow-moving water that serve as the perfect breeding ground for disease vectors. Certain snails carry the parasite that causes schistosomiasis. And mosquitoes cause any number of problems, including malaria and encephalitis. To make matters worse, recent flooding in the Upper Mississippi and Missouri Rivers revealed that a system of levees—river embankments designed to prevent flooding—may actually channel river water into higher surges and leave people more vulnerable.

For decades, people have questioned the wisdom of constructing huge dams in the U.S., pointing to the undesirable effects of changing the natural flow of water. A major political controversy is now brewing about whether to remove several of the more than 400 dams on the Columbia River in the northwest U.S. The dams produce hydroelectric power and irrigate farmland but also affect the spawning of salmon, a critical source of jobs and food.

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