The Conowingo Dam stores enormous amounts of sediment, phosphorus and other pollutants. When too much sediment reaches the Chesapeake Bay it clouds the bay’s waters, harming the underwater vegetation there and the animals that rely on it. While the dam has been trapping sediment since it began operation in 1928, scientists estimate that its retention capacity is rapidly diminishing. Already, major storms scour sediment from behind the dam, leading to giant influxes of sediment and other pollutants into the Bay during events like Tropical Storm Lee in 2011, which swept a 100-mile-long plume of sediment—visible from space—from the Susquehanna into the Bay. Although Exelon has asked for a 46-year license to continue operating the dam, the company has no plan for addressing this serious problem.

The dam is also a problem for American eels that historically made up as much as 25 percent of the Susquehanna River’s biomass. It effectively cuts off eels’ access to any part of the river basin more than 10 mile upstream of the river’s mouth. As a result, only relatively few eels exist in the Susquehanna River, and, of those, many cannot survive the attempt to get through the dam’s turbines as they try to return to the Sargasso Sea to spawn. The American eel is important for the ecosystem and water quality in the Susquehanna because they are the best host for the larva of Eastern elliptio mussels that filter the river’s water, making it cleaner.