PHYSICAL MODIFICATIONS TO COASTAL PLAIN DRAINAGE NETWORKS AND THEIR IMPLICATIONS TO NUTRIENT AND SEDIMENT DELIVERY

Drainage networks within the Mid-Atlantic Coastal Plain have experienced extensive physical alterations from human activities. Upland channels have been extensively eroded from agriculture and urban runoff. Lowland valleys have been modified by the placement of fill or impoundment for water supply, stormwater management, and recreation. Watershed management efforts over the past two decades have included rehabilitation of upland and lowland tributaries through stabilization and morphological sculpting of channels, floodplains and pools. One of the objectives of the mitigation activities is to reduce loadings of nitrogen (N) and sediment to tidal estuaries. In this study, we collected discharge and water quality data during a wide range of hydrological conditions in stabilized upland channels and constructed valley swamp systems on the Western Shore of the Chesapeake Bay to assess whether they are effective at reducing the export of N and sediment to downstream waters. The results show that the tributary modifications can reduce N loads from surface flows, but at modest rates compared to total inputs of N into the associated watersheds. Reduction of erosion in upland channels through stabilization with placed boulders can be substantial, decreasing the export of sediment to downstream waters. Retention of suspended solids was observed in the constructed valley swamp systems, but the longevity of the storage capacity and impacts on biogeochemical processes need to be examined. The use of highly interventionist approaches to create desired ecosystem functions in Coastal Plain drainage networks is still controversial and should be evaluated in relation to other approaches to address problematic loadings of N and sediment to estuaries.