R/EH-15

Impact/Outcome:

This section describes how this project has advanced scientific knowledge and made a difference for coastal residents, communities, and environments. Maryland Sea Grant has reported these details to the National Oceanic and Atmospheric Administration (NOAA), one of our funding sponsors.

Summary: Researchers used modeling tools to develop novel insights into trends in the accumulation and loss of sediments and nutrients in the reservoir behind a major hydroelectric dam on the Susquehanna River. The findings can inform management decisions about the dam in ways that support a multi-state effort to reduce nutrients and sediments in Chesapeake Bay to improve water quality.

Relevance: The Conowingo Dam hydroelectric facility on the Susquehanna River has became an important focus of scientific study and public interest in an ongoing discussion about effective ways to improve water quality in Chesapeake Bay. Over decades of the dam’s operation, sediments and associated nutrients have accumulated in the reservoir behind the dam. The potential for these to become resuspended and flow downstream poses significant, negative consequences for water quality in the estuary; the Susquehanna flows into the Chesapeake’s head 10 miles downstream of the dam. The dam’s federal operating license is up for renewal, and some citizens have proposed that the new license require new measures, including dredging of the sediments. Future progress in Chesapeake Bay restoration will depend on accurate predictions of how outputs of sediments and nutrients from the reservoir may change over time.

Response:  Maryland Sea Grant (MDSG) supported principal investigator William Ball of the Johns Hopkins University and colleagues to develop new statistical modeling of rates of sediment accumulation and loss in the Conowingo’s reservoir. This work was a component in a broader, MDSG-funded project by the researchers to use a state-of-the-art riverine loading estimation method, WRTDS (Weighted Regressions on Time, Discharge, and Season), to generate models of nutrient and sediment loadings in the nine major tributaries of the Chesapeake Bay watershed, including the Susquehanna. MDSG funded a graduate fellow and doctoral candidate, Qian Zhang, who made substantial contributions on this project.

Results: The researchers’ modeling offers new information describing historical trends in the Conowingo reservoir’s inputs and outputs of sediments and associated nutrients as the dam has filled and under varying levels of river flow. An important implication for the Chesapeake’s water quality is that the reservoir is trapping less sediment over time as its storage capacity has decreased. The modeling indicated that inputs of suspended sediments and total phosphorus at the reservoir inlet have declined during a recent 30-year period, reflecting the effects of upstream management controls on a variety of sources. However, there was not a corresponding decline in outputs from the reservoir. Although storm-driven scour has been implicated in this diminished trapping, the modeling indicated an increase in output of sediment relative to previous decades that occurred under a range of river flows including ones well below the literature-reported scour threshold. Model results were consistent with monitoring data. The research team presented the findings at scientific conferences; in journal articles, including one in Environmental Science & Technology; and in meetings with agencies studying the Conowingo Dam including the Chesapeake Bay Program Office, the U.S. Geological Survey, and the Maryland Department of Natural Resources. The modeling approaches used in this project are helping to inform – and can be used to calibrate and verify – additional modeling that is underway to quantify temporal changes in the reservoir’s sediment output under differing rates of river flow.  Improvements in modeling informed by this project can improve future decisions about how best to manage the dam and the Susquehanna River Basin in order to help reduce future impacts on water quality in Chesapeake Bay. The results and methods are also applicable to other reservoir systems that may be similarly approaching a state of dynamic equilibrium with respect to sediment storage.