Floodplain

"Land area adjacent to a river or stream that can be covered by water when a river or stream over ows its banks. The extent of the  ood plain depends on topography and the magnitude of a given  ood event." (Virginia Polytechnic Institute and State University, n.y.) 

Sources

Virginia Polytechnic Institute and State University. "A Glossary  of Water-Related Terms", by Brian Benham. Publication 442-760