Dams are huge man-made structures that act as barriers on a river. Today, the main reason people build dams is to produce electricity. They are also built to channel,cross,span,restrictand control the flow of water in a river. In,For,Against,Throughouthistory, dams have been used to prevent flooding and to irrigate (water) farmland. Dams supply about a sixth of the world's electricity and they significantly reduce the risk of floods and droughts. They also make water easier to access, especially in desertlike areas, where water is in low supply. There are, again,never,also,however, some negative effects of damming rivers. Many people's homes are traded up,worn off,knocked down,moved onto make space for the dam, and flooding can occur in the reservoir, which is the area behind the dam where water collects . This can cause valuable farmland to become submerged under the lakes.