Riparian areas filter nutrients and improve water quality. In agricultural-use watersheds, nutrient filtering in riparian zones can help control agricultural nonpoint-source pollution. Sediment deposition is a natural process that takes place during periodic flooding. Accelerated upland erosion can increase sediment deposition in riparian and wetland areas because of downslope movement of dislodged soil material. Such deposition can change the soils, drainage, and vegetation associated with riparian or wetland areas. Riparian vegetation also reduces sediment and nutrient transport in a number of ways. Roots, especially those of woody vegetation such as willows, help stabilize streambanks by holding soil intact. Vegetation also increases hydraulic resistance to flow, thereby lowering flow velocities and causing sediment deposition.
Another important role of wetland vegetation is uptake and long-term storage of nutrients. Wetland areas are more productive because of the nutrient and water subsidies provided by periodic flooding (Brinson and others 1980). Nutrient uptake into leaves and other deciduous parts can be an important factor in short-term nutrient storage. However, because deciduous plant parts drop each year, nutrients soon become available for transport. High litter fall and generally wet conditions also result in soils higher in organic matter.
In general, surface runoff slows as it flows through the riparian zone, causing sediment deposition and diminishing the water’s erosive potential.
The loss of wetlands is responsible, in part, for the change from perennial to intermittent flow in some streams. Many alluvial aquifers in the western United States are maintained by infiltration of upland runoff in the stream channel or alluvial deposits. These aquifers provide an important source of water for human use. Water storage in such aquifers was once partially responsible for maintaining base flow in western rivers, which are now dry beds much of the year. Learn more about restoring your increasingly precious natural resources.
