Conowingo Dam has long trapped sediment runoff originating from farms and other sources upstream. But its capacity is reaching equilibrium, meaning it is trapping less sediment and sending more of it to the Bay. (Photo by Will Parson/Chesapeake Bay Program)

In 1928, a ceremony along the banks of the lower Susquehanna River near Conowingo, Maryland, opened what was then the United States’ second largest hydro-electric project (the first being Niagara Falls). At that time, the Conowingo Dam was celebrated as an engineering feat. These days, it is more well-known for its nearly filled reservoir of nutrient and sediment pollution which is threatening the Chesapeake Bay.

Every day, 25 billion gallons of fresh water from the Susquehanna River flow through the Conowingo Dam on its way to the Chesapeake Bay. The Susquehanna River alone provides 50 percent of the Bay’s fresh water supply. The reservoir behind the Dam has trapped an average of three-and-a-half million pounds of phosphorus and four billion pounds of sediment every year since the dam opened, which is approximately a third of the phosphorus and half of the sediment that flows along the Susquehanna River and into the Chesapeake Bay annually.

The reservoir catches the nutrients and sediment from agricultural, suburban, stormwater and urban runoff that flows downstream, but the problem is that these materials are rapidly loading up the reservoir and within in a few years it should be completely full.

When the Chesapeake Bay Total Maximum Daily Load (Bay TMDL) was put into place in 2010, it was based on the assumption that the reservoir would continue trapping nutrients and sediment as it had always done and the climate would remain steady (meaning hurricanes and other extreme weather events weren’t taken into account). But new models, monitoring data and research show just how much conditions have changed.

In 2015, only five years later, a report issued by the United States Geological Survey estimated that the dam had reached 92 percent of its capacity. Another report by the Lower Susquehanna River Watershed Assessment team confirmed that since the reservoir is essentially full, it is trapping smaller amounts of incoming sediment and sending more and more of it, along with the nutrients that attach to it, over the dam and downstream into the Bay. While the sediment is detrimental enough by reducing water clarity and dissolved oxygen in the Bay, it also carries chemical contaminants such as pesticides and polychlorinated biphenyls (PCBs) in addition to nutrients. All of these harm water quality and the living resources in the Bay, even though the exact amount of toxic pollutants flowing from the reservoir are not well defined at this time.

Hurricanes and other extreme weather events have a strong impact on the dam. Imagine how the situation looked during Hurricane Agnes in 1972 and Tropical Storm Lee in 2011. The gates of the dam were opened to relieve pressure from accumulating water, churning up large amounts of built-up nutrient and sediment pollution and sending them downstream. This decimated underwater grass beds, negatively impacting marine life and fueling the growth of harmful algae blooms, leading to an increased dead zone.

It is clear that the nutrient reductions called for in the Susquehanna River watershed by the Bay TMDL is no longer enough to meet the goals to improve water quality in the deep waters of the upper Bay. The Conowingo Dam is currently owned by Exelon Corporation and physically lies in Maryland, but the Susquehanna River flows downstream from New York and Pennsylvania.

Recently, the Chesapeake Bay Program decided that the best way to address the increased pollution from the Conowingo Dam would be to create a separate Watershed Implementation Plan (WIP) for the dam. WIPs detail the steps that each watershed jurisdiction (Delaware, the District of Columbia, Maryland, New York, Pennsylvania, Virginia and West Virginia) will take to meet their pollution-reduction targets.

A separate steering committee will be tasked with writing this new WIP for the Conowingo Dam and developing a financing strategy. The committee will have representatives from each jurisdiction and the assistance of a third party. The Environmental Protection Agency will provide oversight in the development and implementation of the Conowingo WIP, evaluate and track the progress being made to reduce the additional pollution from the dam and provide technical and contractual support.

This approach will give all the jurisdictions in the Chesapeake Bay watershed the chance to participate in this massive undertaking and contribute in putting conservation practices in place to help offset the nutrient and sediment pollution loads from the Conowingo Dam.

Learn more about the Conowingo Dam.



Paul Bailes

I am Concerned about what is above the Dam . I lived In Havre de Grace in 1972 (June) when TS Agnes dumped 12-14 inches of rain up north in PA and NY . This was Bad !! The Muck left over after the Storm was Horrible! It stayed on the Western Flats for Years and killed lots of Grasses and changed the Echo system for many years. I See a Huge Storm coming in the future , I pray for the health of the Bay. I hope your ideals work but, I think its to late! 92 per cent full ! Wow , this is not good , Dredging is the Answer!!!! It will take to long for your other ideals . But , We know how money seems to put a stop


The report predicted that if the situation at the Conowingo Dam was not addressed, the Chesapeake Bay would not reach the 2025 pollution reduction goals set by the EPA, even if the rest of the plan was implemented fully. 


The dam doesnt create any pollution. If it didnt exist the pollution would still be coming down fact, it may actually be worse as the dam have held back 90 years worth of sediment and nutrients, it would all be in the bay. Having Excelon fit the bill for to clean up the pollution that has been sent from PA and NY simply because they have deep pockets is an asinine concept.

bill ferris

get the dredge boats out on the water and dredge the sediment. Bill Execlon and git er done bill

Michael T Green

Bottom line up front (BLUF)
You must influence the agricultural and outdoor recreation lobbies in PA if you want legislation passed to reduce nutrients coming down the Susquehanna.
I have lived in Pennsylvania and currently live by the Susquehanna flats. When I lived in PA, the only exposure I had to Chesapeake Bay issues were community grants to plant trees along Susquehanna tributaries and the occasional flier about the Chesapeake bay included with my water bill. I knew nothing about the bay, so I didn't care about the bay.
If you want people in PA to care about the bay, you have to relate it to something THEY care about. Two big things in Central PA are Agriculture and hunting / fishing. Most farmers in central PA also hunt and fish so if you can relate their hunting and fishing to the bay you will have a better chance to influence their lobbies at the state capital.
When was the last time an eye catching display booth about the Chesapeake bay was set up at a PA farm show or Outdoor rec show held in Harrisburg? Harrisburg, that large town on the Susquehanna with the state building 4 blocks from the river?


I can tell you that in Southern Maryland as of Friday 9/14/2018

We have MASSIVE AMOUNTS OF DEAD ROCKFISH AND RED DRUM. I have pictures if DNR wants them please come to Calvert Ave and walk the beach.

Its depressing to see a dead 40" rockfish along with about 50 or so 16 to 33inch rockfish all over my beach.

Why is Pennsylvania getting a pass????!!!!! I have to pay bay fee's every year. Why do they get to destroy the bay and walk away!!!!

Where is Hogan!!!!


That's like expecting the owner of a fence to clean up the trash piled up against it. The fence isn't the problem, the trash is. The plan is about how to get people to quit tossing trash out the window. And pay for the time the fence owner spends cleaning up other people's trash.

bob gallagher

This sounds like a plan to let Exelon, the entity that owns and makes millions of dollars in profits from the dam, off the hook and put the burden on taxpayers.

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