REGIONAL MEASUREMENT AND MODELING OF FLOODPLAIN WETLAND NUTRIENT AND SEDIMENT FLUXES: THE CHESAPEAKE WATERSHED

Wetlands play a key role in watershed nutrient and sediment dynamics and stream water quality improvement. The USGS Chesapeake Floodplain Network is measuring deposition and erosion of sediment and associated nutrients throughout the Chesapeake non-tidal watershed, with the goal of identifying predictors of reach-scale material fluxes. Forty-three research sites across the Valley and Ridge (V&R), Piedmont (PD), and Coastal Plain (CP) physiographic provinces (PP) were sampled to measure bank erosion and floodplain deposition over long time scales using dendrogeomorphic methods, sediment characteristics, and reach geomorphology. Sites were located on reaches of streams and rivers of varying watershed size and land use. Long-term floodplain accretion and bank erosion rates (cm/y) did not significantly differ among PP. Reach-scale fluxes of sediment (kg/m/y) and its component mineral sediment fluxes from floodplain deposition, bank erosion, and the reach net balance (floodplain – bank) did not differ among PP despite increasing floodplain width from V&R to PD to CP. River reaches in the CP were significantly greater net balance sinks of organic sediment, C, N, and P than those in PD or V&R. Variability in fluxes among reaches could be explained by a combination of GIS derived reach-scale geomorphology and watershed-scale characteristics, with relatively high accuracy (bank and floodplain sediment flux: R²=0.98 and 0.92, respectively). These models are being used to predict bank and floodplain sediment, N, P, and C fluxes for every stream reach in the 167,000 km² Chesapeake watershed. These predictions of wetland effects on downstream water quality, along with estimates of upland erosion and river loads, will then be integrated to generate watershed material budgets throughout the Chesapeake. Our findings suggest a promising approach to combine dendrogeomorphology with GIS analyses to measure and model the effects of floodplain wetlands on downstream water quality.