Dams block fish migrations and keep sediment from washing downstream. When initially installed, dams usually cause flooding on the side of the dam where the water flows. Dams create large bodies of stagnant water, which have a few dangers, such as encouraging mosquito breeding.
Dams, especially large ones, have the potential to cause a lot of problems for the surrounding area, especially the area behind the dam where the water flows toward the blockage. There are disadvantages for both the nature and any people living in the area.
There are many negative effects on nature. Since dams block up flowing bodies of water, such as rivers, any animals that depend on the flow to reproduce or as part of their life cycle are put in danger. Migratory fish that mate in a different location than they live the rest of their lives, for example, are unable to mate and may decline in population. The buildup of water is also dangerous for plant life that grows on the natural shoreline of the water. The plant life is submerged and dies. In addition, the beneficial sediment that normally is washed down the river is blocked, which decreases the fertility of the soil downriver from the dam.
Many humans are displaced due to dams. Humans who live in an area that is to be flooded due to a new dam have to relocate and lose their homes to the rising water.