Riparian (streamside) zones are critical transition zones in the landscape, situated in between upland and aquatic ecosystems. There is a complex web of interactions between riparian zones and surrounding ecosystems that have an important influence on the movement of water and nutrients across the landscape and on biodiversity. Many nutrients are transported from upland ecosystems, especially those dominated by agricultural and urban land uses, in surface runoff and groundwater flow. Riparian zones have the capacity to intercept these flows and to function as "buffer zones" preventing the movement of nutrients into streams. These flows, as well as floodwaters from streams, help to support unique plant and animal communities in riparian zones. These communities in turn, help to support stream communities. Many studies in agricultural and forested watersheds have shown that riparian zones are particularly effective "sinks" or buffer zones for nitrate, the most common groundwater pollutant in the world and a prime cause of excess nutrient levels (eutrophication) in coastal waters such as the Chesapeake Bay. Much of this sink potential derives from the ability of wet, oxygen-poor (anaerobic) riparian soils to support denitrification, an anaerobic microbial process that converts nitrate into nitrogen gases.
BES research has found that urban land cover change dramatically alters the structure, function and importance of riparian zones in the landscape. Increases in impervious surface associated with urbanization change the way that water moves across the landscape. In urban areas, large volumes of water are delivered directly to streams with great energy via storm drainage infrastructure. This energy degrades urban stream channels, washing away sediments and incising (deepening) stream channels and destroying habitats for stream and riparian organisms. Incision of the stream channel creates a profile like a drainage ditch, lowering the water table in the riparian zone. Connections between the riparian zone and its surrounding environments are cut; groundwater flows deep beneath the biologically active zone of the riparian ecosystem, floodwaters stay within the deepened channels and seldom interact with the riparian zone, and support of stream communities by riparian vegetation is reduced. Riparian soils become drier and less anaerobic leading to decreases in denitrification and increases in nitrification, an aerobic microbial process that produces, rather than consumes nitrate. BES research has shown that urbanization can convert riparian zones from sinks to sources of nitrate in the landscape.
The suite of changes in stream and riparian ecosystem structure caused by urbanization -- the urban stream syndrome -- creates great practical problems for municipalities with damage to real estate and infrastructure, e.g., sanitary sewer lines, in the near stream zone. These changes have led to the emergence of active stream and riparian restoration programs around the world. While the goal of most restoration projects is geomorphic stability, BES research has determined that there can be a nitrogen benefit to these projects as well. Results have shown that restorations that create hydrologic connections between the stream and the riparian zone foster the development of "hotspots" of riparian denitrification. Long-term studies will be critical to determining if these hotspots persist under urban conditions.