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). An estimated 6% of Michigan's land is flood-prone, including about 200,000 buildings.

Note: Supporting data for some older Flood Insurance Studies (FIS) is maintained at the FEMA Engineering Library at Working with Flood Risk: Engineers, Surveyors or Architects.