National Wetland Inventory: Wetland Definition

Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. For purposes of this classification wetlands must have one or more of the following three attributes: 1) at least periodically, the land supports predominantly hydrophytes; 2) the substrate is predominantly undrained hydric soil; and 3) the substrate is non-soil and is saturated with water or covered by shallow water at some time during the growing season of each year.
and E. LaRoe. 1979 Classification of wetlands and depwater habitats of the United State. U.S. Fish Wildlife Service 103pp