A riparian ecosystem is land which is adjacent to a stream or river. It is a characteristic association of substrate, flora, and fauna within the 100-year floodplain of a stream or if a floodplain is absent, a zone hydrologically influenced by a stream or river. They are maintained by high water tables and periodic flooding. Examples are, bosques of the American Southwest, gallery forests of prairie regions, and wetlands or adjacent slopes that border streams of humid eastern states. These zones contain or adjoin riverine wetlands and share functions such as surface and subsurface water storage, sediment retention, nutrient and contaminant removal, and the maintenance of habitats for plants and animals.
Riparian ecosystems are important to regional bio-diversity.
Alteration of these zones has accompanied regulated activities such as gravel mining, bridge crossings, and the creation of new dams and diversions. These zones are also closely associated with the maintenance of the physical, chemical, and biological processes of streams. Although they are important to the goals of the Clean Water Act, they are not fully protected by it. Some are unregulated because they do not satisfy any of the definitions used of wetlands and because they lie outside of the ordinary high-water mark. Nationwide Permit 26 affects isolated wetlands and headwaters by authorizing the filling of relatively small areas if the permitted area is consistent with CWA regulations.