Conowingo Dam and Chesapeake Bay

Conowingo Dam Eliot Malumuth 695x352

The Issue

Since its construction in 1928, the Conowingo Dam on the Susquehanna River in Maryland has been trapping sediment and phosphorus pollution in the reservoir behind the structure.

In the mid-1990s, researchers estimated that the three upstream Susquehanna dams, including the Conowingo Dam, were trapping about two percent of the nitrogen, 40 percent of the phosphorus, and 70 percent of the sediment that would have entered the Bay from the Susquehanna River. While not designed to do so, 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.

What's Happening in the Reservoir

Today, scientists estimate the reservoir is almost completely filled and has reached a state of "dynamic equilibrium," characterized by alternate periods of pollution loading and transport. In particular, during big storms when the flow through the dam is high, sediments are scoured from the reservoir into the river below, contributing additional pollution downstream and into the Chesapeake Bay. This scouring, however, temporarily restores some of the pollution-trapping capacity of the dam, resulting in a reduction in sediment loads downstream to the Bay. Over the long-term, sediment and associated phosphorus loads into the Conowingo reservoir will equal loads coming out. The loss of this trapping capacity was not considered and accounted for during the initial development of the pollution-reduction targets and Watershed Implementation Plans (WIPs) for the Chesapeake Bay.

A final report of the impacts of the sediments from behind the dam and potential mitigation options and costs was released March 7, 2016, by state officials and the Army Corps of Engineers. Report findings show that the dam's trapped sediment doesn't present as significant a problem as originally feared, in terms of amount of pollution scoured from behind the dam and delivered to the Bay during big storms. It also affirmed that stopping pollution at its source—the larger Susquehanna Watershedis the best bet for restoring the Bay's health.

Sediments—sand, silt, and clay particles—making their way past the dam in storms generally settle to the bottom without threatening the Chesapeake Bay water quality or aquatic life, the report concluded. Underwater grass could be threatened by a storm and sediment surge during the growing season. The study also concluded that dredging material from behind the dam would be prohibitively expensive with little environmental benefit.

The bigger threat to the Bay is nutrients coming through the dam, the report concluded. Nitrogen generally dissolves in water so the dam never trapped or prevented much nitrogen from moving downstream to the Bay. But phosphorus is typically associated with sediment particles and was historically trapped by the dam. With the loss of trapping capacity, more phosphorus pollution is reaching downstream waters and during storm events, sediments and associated nutrients are scoured from behind the dam. The report said the pollution from scouring must be addressed, but the more cost-effective effort will be to stop pollution from entering the Susquehanna in the first place.

A recent study led by the University of Maryland sought to estimate the additional nutrient loads associated with the lost trapping capacity at the Conowingo Dam. The study involved nutrient sampling during storm events and a complex series of hydrological models. Results indicate, on average, the net annual transport of phosphorus to the Chesapeake Bay is about 2 million pounds more than was estimated in 2010, when the WIPs were first developed.

The Chesapeake Bay Program partnership is currently evaluating the impacts of this additional phosphorus load on downstream water quality i.e., dissolved oxygen. In addition, decisions regarding who is responsible for achieving the additional load reductions and by when, will be made by the partnership in the fall of 2017.

Re-Licensing Must Consider Environmental Impact

The Conowingo Dam is owned and operated by Exelon. Exelon's current license for the Conowingo was issued on August 14, 1980 and expired on September 1, 2014. Exelon is asking Federal Energy Regulatory Commission (FERC) for a new license to operate the dam for 46 years. As part of the re-licensing, Exelon must receive a "Section 401 water quality certification" from Maryland. Under this section of the federal Clean Water Act, states must consider potential environmental impacts from the dam and include appropriate conditions in the new FERC permit to minimize these impacts. There are still key pieces of information needed for FERC to issue the new license to Exelon. Hence, their new license will likely not be issued until 2018 or later. In the meantime, FERC will issue a temporary license to continue operation until the Environmental Impact Statement (EIS) is finalized and the State makes a decision on water quality certification.

CBF's Position

As the regional watchdog for the Chesapeake Bay, CBF is seeking a comprehensive solution to this problem, one that addresses both the sediment and associated nutrients that are built-up at the dam in a cost-effective way as well as the sediment and nutrients that enter the Susquehanna upstream. We also continue to advocate for leadership and action to reduce pollution in local waters in Maryland and Virginia, where polluted runoff from farms, lawns, and urban pavement threaten the health of both local rivers and streams and the Bay.

Any solution for the impacts of the Susquehanna on downstream waters will need to be implemented collectively by the state, federal, and private partners, including Exelon.

In considering the impact of the dam's impact on the state’s water quality, CBF believes Maryland must require Exelon to mitigate the harm the dam causes to downstream water quality, including making a financial contribution. A report commissioned by CBF and the Nature Conservancy estimated Exelon could contribute between $27 million to $44 million annually to this mitigation and sustain a healthy profit margin. 

CBF also remains concerned with restoring migratory fish in the Susquehanna. Exelon must also make structural and operational changes to restore safe and effective habitat for migratory fish, mussels and for vegetation. Improving upstream and downstream passage for American shad, hickory shad, American eel, Atlantic sturgeon, shortnose sturgeon, alewife, and blueback herring is essential to the recovery of these fish populations in the Chesapeake Bay watershed. As part of the relicensing of the project, fish passage improvements must be secured, including changes to the existing fish lifts and flow modifications to improve fish migration through the lift and to reduce fish mortality. CBF made these recommendations to Maryland in December, 2017.

What CBF is Doing

Since 1986, CBF has helped reduce pollution coming down the Susquehanna and reaching the dam, but more needs to be done. In the late 1980s, CBF also was involved in the previous relicensing of the dam, and filed comments in support of actions to improve the ability of migratory fish to pass by the dam and access historic upstream habitat.

CBF has filed to intervene in the relicensing of the Dam to ensure the Foundation's voice is heard in that process and to position CBF for a legal challenge if necessary.

CBF submitted comments on the draft Environmental Impact Statement (EIS) issued by FERC on July 30, 2014. The purpose of the EIS is to assess the environmental and economic effects of continuing to operate the facility and make recommendations for how to reduce these negative impacts. These recommended measures then will become part of the new license at Conowingo.

Finally, CBF commissioned an examination of the dam's finances and used that December 2017 report to support a call for Exelon to make a financial contribution to mitigation efforts.

Cutting Pollution to the Susquehanna

Focus can't just be on the dam, however. Pennsylvania and New York must do more to reduce sediment, phosphorus, and nitrogen entering the Susquehanna as required by the Chesapeake Clean Water Blueprint. Pennsylvania is significantly off track in reducing nitrogen and phosphorus pollution from agriculture as well as urban/suburban runoff. Progress in reducing pollution from sewage treatment plants is on track. Overall progress to reduce nitrogen pollution is significantly off track, but efforts to reduce phosphorus and sediment pollution are only slightly off track.

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