The Susquehanna River provides nearly half of the Bay’s freshwater, 41% of its nitrogen, 25% of its phosphorus and 27% of its sediment load. These pollutants come from wastewater treatment plants, polluted agricultural, urban, and suburban runoff and other sources of pollution throughout the Susquehanna River watershed. Since its construction in 1928, Conowingo dam has been trapping sediment, nitrogen, and phosphorus pollution in the reservoir behind the structure.

By trapping suspended sediment, the Conowingo has helped reduce contributions of sediment and phosphorus to the Chesapeake, but the sediment storage capacity of Conowingo Reservoir has been gradually declining. Today, scientists estimate the reservoir is almost completely filled. During medium and large storms, the raging river flow can scoop up or “scour” sediment from the reservoir behind the dam, sending additional pollution into the river downstream and into the Chesapeake Bay.

The Chesapeake Bay suffers from too much pollution, but not all of it comes from the Susquehanna River. Local pollution sources drive local water quality. The Chesapeake Bay receives drainage from six states (New York, Pennsylvania, Maryland, Delaware, Virginia and West Virginia) and Washington, D.C. Each of these jurisdictions is bound by the federal Chesapeake Bay Cleanup Effort – known as the Chesapeake Bay TMDL (Total Maximum Daily Load). This clean water plan is authorized by the federal Clean Water Act and requires these jurisdictions to reduce sediment, nitrogen and phosphorus pollution locally.

Some interests are trying to use the Conowingo Dam relicensing as an excuse to distract from, or even abandon, local programs that are reducing pollution. We can’t let this happen – local pollution is driving our local water quality. To make meaningful progress in the Bay cleanup effort, we must address the impacts of the Conowingo Dam AND continue our local programs to reduce pollution, which are working. While sediment behind the dam has an impact on upper Chesapeake Bay water quality, the health of Maryland’s rivers, creeks and streams is driven by local pollution sources.

While Conowingo is only one of many sources of pollution into the Chesapeake Bay, Exelon must still prove to the state of Maryland that its dam will comply with state water quality standards. To the extent that the sediment and nutrient pollution trapped behind Conowingo Dam harms downstream water quality, habitat and living resources, Exelon is responsible for addressing its share of the problem.

In November 2014, an interagency group led by the Army Corps of Engineers released a public draft of the Lower Susquehanna River Watershed Assessment, which was intended to help us better understand the water quality impacts of the Susquehanna River’s sediment and nutrients, and Conowingo’s contribution to water quality problems. Maryland agencies have requested additional information from Exelon to help them measure the impact of the dam on water quality and natural resources in the Bay. Additional information and studies may be necessary to help the State of Maryland and others determine how best to reliably protect crabs, oysters, underwater grasses and the entire Bay ecosystem. In addition, Maryland’s Water Quality Certification for Conowingo will be in effect for as many as 50 years. New science describing Conowingo’s impacts or new technology to enable us to better address those impacts may become available during that time. Maryland should make sure that its certification allows it to retain the ability to update Exelon’s license in the future in order to further improve water quality.