Influences of a River Dam on Delivery and Fate of Sediments and Particulate Nutrients to the Adjacent Estuary: Case Study of Conowingo Dam and Chesapeake Bay

Abstract

Dams impact the magnitude and nature of material transport through rivers to coastal waters, initially trapping much material in upstream reservoirs. As reservoirs fill, trapping decreases and bottom sediments can be scoured by high flows, increasing downstream delivery. This is the case for the Conowingo Dam, which historically has trapped much of the sediment and particulate nutrients carried by the Susquehanna River otherwise bound for Chesapeake Bay but has now reached dynamic equilibrium. While previous studies primarily focus on either delivery of river inputs or their fate in the Bay, this study synthesizes insights from field observations and modeling along the Reservoir-Bay continuum to evaluate potential impacts of infilling on Bay biogeochemistry. Results show most Susquehanna sediment and particulate nutrient loading occurs during high-flow events that occur only ~ 10% of the time. While loading during these events has increased since the late 1970s, consistent with a decreasing scour threshold for Reservoir sediments, loading during low-flow periods has declined. Loads entering the estuary are largely retained within the upper Bay but can be transported farther downstream during events. Reservoir sediments are highly refractory, and inputs of reservoir-like organic matter do not enhance modeled sediment-nutrient release in upper Bay sediments. These findings and an emerging literature highlight the Bay’s resilience to large sediment loads during events (e.g., Tropical Storm Lee in 2011), likely aided by ongoing restoration efforts and/or consistently low-moderate recent inflows (2012–2017). Thus, while events can have major short-term impacts, the long-term impact to Bay biogeochemistry is less severe.