Dams have many effects on the wildlife in and around the rivers they divide. Because the dam prevents seasonal flooding, some animal species that usually thrive in floodplains are deprived of a habitat. Also, thermal pollution from the dam itself causes the river’s natural temperature variations to stop, and can actually make the water colder. Colder temperatures effect animals (ectoderms) whose body temperatures change to suit the environment’s temperature. Dams can also alter the flow of sediment (increasing it), which causes problems for animals that rely on clear waters to eat. Finally, dams can cause problems for migrations patterns of some fish; the structure prevents them from going upriver.
One of the biggest problems is that dams can damage or destroy migratory fish habitat. Unless a fish ladder is installed a dam blocks fish like salmon from swimming back upstream to the river where they were born to reproduce.
As mentioned by another respondent, dams have a big impact on the plantlife surrounding the riverbed, which in turn eliminates the habitat of many land animals that live alongside rivers. Animals that depend on riparian vegetation that thrives in a marshy floodplain, such as cottonwoods, are forced out when a dam is put in place, as is seen in the Southwest United States. In addition, dams keep debris and other natural detritus from flowing through the river, which deprives it of the nutrients these items deliver as they disintegrate and decay as they float along. This lack of nutrients impacts every level of the food web depending on the river, from fish and phytoplankton all the way up to birds of prey and bears.
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