Rivers change in character along the lengths of their courses, changing the types of habitat the river provides. The land surrounding rivers provides diverse habitats, including wetlands, grasslands, and woodlands. These lands also link river ecosystems with upland ecosystems. Rivers are important to people for many reasons including fishing, swimming, boating, and bird watching. When a dam is built, the river's water rises behind the dam, flooding the land behind the dam that is a lower elevation than the crest of the dam.
Impoundment is a general term for the body of water, regardless of size, formed by damming a river.