Effects of Reservoir Filling on Sediment and Nutrient Removal in the Lower Susquehanna River Reservoir: An Input-Output Analysis Based on Long-Term Monitoring

Abstract:

Reduction of suspended sediment (SS), total phosphorus (TP), and total nitrogen is an important focus for Chesapeake Bay watershed management. Susquehanna River, the bay’s largest tributary, has drawn attention because SS load from behind Conowingo Dam (near the river fall-line) has risen dramatically recently. To better understand these changes, we evaluated decadal-scale (1986-2013) history of concentrations and fluxes using data from sites above and below the reservoir. First, observed concentration-discharge relationships show declined SS and TP concentrations at the reservoir inlet under most discharges in recent years, but such changes have not been propagated to emerge at the outlet, implying recently diminished reservoir trapping. Second, best estimates of loadings show declined net depositions of SS and TP in recent decades, which occurred under a range of discharges, with the 75th~99.5th percentile of Conowingo discharge dominating such changes and carrying most sediment/nutrient loadings. Finally, stationary models that better accommodate effects of riverflow variability also show diminished reservoir trapping of SS and TP, which occurred under a range of flows including those well below the literature-documented scour threshold. These findings have significant implications in regard to our understanding and management of this major reservoir and illustrate the value of long-term monitoring programs.