Stream Meander Curvature As a Control Of Riparian Water Table Dynamics, Groundwater Flow Path, and Nitrogen Dynamics.

Excessive fertilizer use in agricultural watersheds has been linked to water quality problems throughout the nation. Nitrogen management is therefore often a stated goal of many agricultural best management practices. Stream restoration can enhance longer hydraulic residence times within the floodplain; this can, in turn, increase biogeochemical processing and allow for less nitrate contaminant loads to reach surface water systems. Hydraulic gradient data, groundwater chemistry and greenhouse gas flux data were used to evaluate stream meander curvature and hydrologic pulse as a control of groundwater flow, water table and nitrogen dynamics in three riparian zones in the US Northeast. Data was obtained via piezometer and static chamber transects in riparian areas directly adjacent to an agricultural draining stream with differing channel curvature, (innermeander, straight section and oxbow depression sites) between May-November 2011 and 2012. Heterogeneous meander curvature and changes in hydrologic regime had a strong affect on riparian groundwater and nitrogen dynamics. Spatially, location within the riparian zone influenced nitrogen dynamics. Areas in the middle of the riparian zone that were affected by flow reversals (i.e. stream-riparian exchange within the innermeander site) were areas with low levels of groundwater NO₃^- (median 0.034 mg/L) vis a vis adjacent locations within the riparian area, which was indicative of high rates of nitrogen removal. Areas that had consistently elevated water tables (i.e the oxbow depression) also had minute levels of NO₃. Although differing channel geomorphology affected hydrologic and biogeochemical conditions, temporal changes in hydrologic regime had the most impact on groundwater and nitrogen dynamics. Until only recently, riparian studies have focused primarily on nitrogen removal from the field edge to the stream, while stream re-meandering projects generally have implemented a homogenous sinusoidal channel curvature and considered only in-stream dynamics. As inferred by the study, areas of stream-riparian exchange and elevated water tables within the riparian zone can have enhanced nitrogen removal. Therefore, implementation of more heterogeneous floodplain restoration designs that promote riparian-stream connectivity can improve the efficiency and function of restoration projects at nitrogen mitigation within the floodplain.