In physical geography, a floodplain is an area of relatively level land bordering a stream, lake or river. It becomes inundated to various degrees during times of flooding. It is often defined as contained the floodway, which normally is inundated during annual flooding (or less often, say, ten-year flooding), and the floodway fringe (which may be inundated during a "100-year flood" or even "500-year flood").
Floodplains may be extremely broad, as in the case of the Platte River flowing across the Great Plains, where the boundary between river and floodplain is not even clear, or quite narrow, as in the case of entrenched rivers such as the Snake River in the Snake River Canyon or Colorado River in the Grand Canyon.
Historically, many towns, homes and other buildings have been built on floodplains where they are highly susceptible to flooding, for several reasons:
- This is where water is most available
- Floodplain land is usually the most fertile for farming
- Rivers represent cheap sources of transportation, and are often where railroads are located
- The flatter land is easier to develop than hill land
Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream or river. These are accumulations of sand, gravel, loam, silt, and/or clay, and are often important aquifers, the water being drawn from them being pre-filtered compared to the water in the river or stream.