A decades-long effort to clean up the Chesapeake Bay, the nation’s largest estuary, is showing signs of success. But scientists now say progress could be hindered by a hydroelectric dam, located on the Susquehanna River in northern Maryland.

The Conowingo Dam has been holding back pollution for nearly a century, but recent research shows it has filled up with sediment faster than expected.

“It’s now at a point where it’s essentially, effectively full,” says Bill Ball, director of the Chesapeake Research Consortium. “The capacity’s been reached.”

In 2010, the Environmental Protection Agency imposed strict new pollution limits on state and local governments in the bay watershed to sharply curb nitrogen, phosphorous and sediment by 2025. It was believed the Conowingo Dam would continue trapping sediment until then.

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