MURKY WATERS: The Conowingo Dam and Chesapeake Bay

By H.S. Parker

Susquehanna Rising

The story of the Conowingo Dam begins in the foothills of the Catskills near Cooperstown, New York. There the North Branch of the Susquehanna River spills out of Otsego Lake and starts a meandering journey of 444 miles to Chesapeake Bay. Joined by the river’s West Branch in western Pennsylvania, and augmented by numerous tributaries, the Susquehanna finally empties into the Bay near Havre de Grace, Maryland.

Drawing from a three-state watershed of 27,500 square miles – half in Pennsylvania – the Susquehanna is the largest river east of the Mississippi and Chesapeake Bay’s most voluminous freshwater source. One hundred fifty major rivers and over 100,000 tributaries discharge into the Bay. The Susquehanna accounts for half that – some 25 billion gallons of water every day, enough to fill 38,000 Olympic swimming pools.

 All that moving water means vast potential for power: there are three hydroelectric dams on the lower Susquehanna. The largest of these, the Conowingo, opened in 1928. Today the dam’s 11 turbines can produce 1.6 billion kilowatt-hours of renewable energy a year, enough to supply 165,000 homes and businesses, mostly around Philadelphia.

Moving More than Water

As it scours the countryside, the Susquehanna annually transports downstream some four million tons of sediments. For much of its operating life, the Conowingo Dam has unintentionally acted as a “pollution gate,” trapping sediments, nutrients (especially sediment-bound phosphorus), and debris in a 9,000-acre reservoir behind its 94-foot-high barrier (you can drive across it on Route One). Sediments smother marine life and excess nutrients can trigger algal blooms that can cause dead zones. 

Up to the mid-1990s half the Susquehanna’s entrained sediments and a third of its phosphorus were confined behind the dam. That’s no longer the case; the reservoir is now close to the limit of its storage capacity. Today the dam annually discharges over a million tons of formerly retained sediments, releasing most of these during large storms. That’s about a fifth of all sediments entering the Bay. Debris also tumbles over the dam, including logs and uprooted trees that can damage boats and impede navigation. 

Licensed to Spill?

The Conowingo Dam operates under a license from the Federal Energy Regulatory Commission (FERC). This first requires a Clean Water Act water quality certification from Maryland. The dam’s FERC license expired in 2014. After years of wrangling, Maryland issued a new permit in April 2018 with the condition that then-owner Exelon take primary responsibility for reducing nutrient pollution coming through the dam. Exelon objected. Maryland and Exelon then agreed to revised, softer terms requiring that Exelon invest $200 million for environmental remediation. Maryland recertified the dam and the FERC issued a new 50-year license in 2021. 

Bring in the Lawyers

Several environmental groups, including Waterkeepers Chesapeake, Lower Susquehanna Riverkeeper, ShoreRivers, and the Chesapeake Bay Foundation, challenged the new license. They contended that Maryland’s negotiated settlement with Exelon was illegal and that the original April 2018 provisions should stand. In 2022 the U.S. Court of Appeals in Washington, DC agreed and vacated the FERC license. Then Constellation Energy assumed ownership of the dam. With Constellation now operating under one-year licenses, intense negotiations among Maryland, Constellation, and environmental groups ensued.

In October 2025, Maryland Governor Wes Moore announced a new settlement involving Maryland Department of the Environment (MDE), Constellation, Waterkeepers Chesapeake, and Lower Susquehanna Riverkeeper. The agreement places stringent, enforceable water quality requirements on Constellation and requires the company to invest $340 million for pollution remediation over the next fifty years. This includes $18.7 million for reservoir-dredging studies. 

Glowing press releases followed and all involved parties breathed a sigh of relief. The goodwill lasted about a month. On Nov. 14, 2025, Clean Chesapeake Coalition, an organization representing four Maryland Eastern Shore communities, appealed the settlement. They had two primary objections: that the agreement should have required that Constellation dredge accumulated sediments; and that the settlement process occurred “behind closed doors,” freezing out organizations like Clean Chesapeake.

Murky Waters Ahead

What happens next? The future is murky, but three things are clear: First, further delays in permitting the dam will be costly, both economically and environmentally. Second, any agreement will not satisfy all interested parties. Third, and probably most important, consequential actions to remediate environmental impacts at the Conowingo Dam must ultimately address water quality issues far upstream of the dam.

Stay tuned.

Henry S. Parker is a scientist and writer who previously lived in Annapolis and now resides in Vermont.