title: Long-Term Changes in Sediment and Nutrient Delivery from Conowingo Dam to Chesapeake Bay: Effects of Reservoir Sedimentation.
abstract: Reduction of suspended sediment (SS), total phosphorus (TP), and total nitrogen is an important focus for Chesapeake Bay watershed management. The Susquehanna River, the bay's largest tributary, has drawn attention because SS loads from behind Conowingo Dam (near the river's mouth) have been rising dramatically. To better understand these changes, we evaluated histories of concentration and loading (1986-2013) using data from sites above and below Conowingo Reservoir. First, observed concentration-discharge relationships show that SS and TP concentrations at the reservoir inlet have declined under most discharges in recent decades, but without corresponding declines at the outlet, implying recently diminished reservoir trapping. Second, best estimates of mass balance suggest decreasing net deposition of SS and TP in recent decades over a wide range of discharges, with cumulative mass generally dominated by the 75∼99.5th percentile of daily Conowingo discharges. Finally, stationary models that better accommodate effects of riverflow variability also support the conclusion of diminished trapping of SS and TP under a range of discharges that includes those well below the literature-reported scour threshold. Overall, these findings suggest that decreased net deposition of SS and TP has occurred at subscour levels of discharge, which has significant implications for the Chesapeake Bay ecosystem.
authors.affiliation: Johns Hopkins University , Department of Geography and Environmental Engineering, 3400 North Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Qian
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: U.S. Geological Survey, 432 National Center, Reston, Virginia 20192, United States.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Hirsch
authors.affiliation: Johns Hopkins University , Department of Geography and Environmental Engineering, 3400 North Charles Street, Baltimore, Maryland 21218, United States.|Chesapeake Research Consortium, 645 Contees Wharf Road, Edgewater, Maryland 21037, United States.
authors.forename: William P
authors.initials: WP
authors.lastname: Ball
mesh_terms: D061006:Bays / Q000737:chemistry*; D019015:Geologic Sediments / Q000737:chemistry*; D008396:Maryland; D009584:Nitrogen / Q000032:analysis; D010414:Pennsylvania; D010758:Phosphorus / Q000032:analysis; D045483:Rivers / Q000737:chemistry*; D060753:Water Quality
chemical_list: D010758:Phosphorus; D009584:Nitrogen