A river, stream, lake, or drain may on occasion overflow their banks and inundate adjacent land areas. The land that is inundated by water is defined as a floodplain. In Michigan, and nationally, the term floodplain has come to mean the land area that will be inundated by the overflow of water resulting from a 100-year flood (a flood which has a 1% chance of occurring any given year). It is estimated that about 6% of Michigan's land is flood-prone, including about 200,000 buildings.

The leading cause for disaster declarations by the Governor or the President is flooding. In 1968, Congress instituted the National Flood Insurance Program (NFIP) to reduce the economic and social cost of flood losses. This program makes flood insurance available in communities that agree to manage their floodplains in such a manner that additional flood prone structures are not built.