Conowingo Dam. Photo copyright Eliot Malumuth Laurel MarylandPhoto © Eliot Malumuth, Laurel Maryland

The Conowingo Dam and Chesapeake Bay

Infographic: Pollution Flow from the Conowingo Dam to the Chesapeake Bay

Thumbnail - Pollution flow from the Conowingo Dam to the Chesapeake Bay

Download our infographic "Pollution Flow from the Conowingo Dam to the Chesapeake Bay"

The Facts About the Conowingo Dam

  • The dam is not the largest source of pollution to the Bay, the Susquehanna River is. The Susquehanna contributes, on average, roughly one third of the sediment, one quarter of the phosphorus, and 46% of the nitrogen flowing into the Bay.
  • The Susquehanna mostly affects the central stem of the Bay. Many of Maryland's local creeks and rivers are on the EPA's "impaired waters list" and are polluted almost entirely by local sources—farms, sewage plants, septic systems, urban and suburban runoff, and other sources which must be addressed locally. These need to be cleaned up if we ever expect to have a lasting impact on Bay cleanup.
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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. Today, researchers estimate the reservoir is almost completely filled and, as a result, has lost much of its capacity to trap. In particular, during big storms when the flow through the dam is high, these sediments are scoured from the reservoir into the river below, contributing additional pollution downstream and into the Chesapeake Bay.

As the regional watchdog for the Chesapeake Bay, CBF is seeking a comprehensive solution to this problem, one that addresses both the sediment build-up at the dam and sediment entering the Susquehanna upstream.

In addition, CBF insists that improvements in "fish passage" at the dam also be part of the relicensing settlement. 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.

CBF is participating in a committee working on a solution to reduce legacy sediment at the dam. The group is expected to finish its work in 2014. CBF also has filed to intervene in the relicensing of the dam by the Federal Regulatory Energy Commission (FERC) to ensure the Foundation's voice is heard in that process, and to position CBF for a legal challenge if necessary. 

Scientists predicted the Conowingo Dam  reservoirs would be full by 2015-2020. Capacity was reached early. With the reservoirs full, when extreme storm events occur:


  • The dam has little impact on nitrogen pollution loads, which are showing a downward trend on the Susquehanna. Nitrogen is water soluble so little is trapped behind the dam, unlike sediment and phosphorus (which is often attached to sediment). Therefore, even if the reservoir completely filled, the amount of nitrogen reaching the Bay would continue to decrease due to implementation of upstream pollution reduction measures.
  • Pennsylvania and its upstream neighbor, New York, are being held to the same requirements to meet the EPA's pollution limits as every other state in the Bay watershed, and are developing their own Watershed Implementation Plans to meet those requirements.
  • Any solution for the dam will need to be implemented collectively by state, federal and private partners, including Exelon, the owner of the dam. The State of Maryland, in partnership with the Army Corps of Engineers, are undertaking a study to determine the options for dealing with the sediment behind the Dam.
  • The EPA explicitly included the dam and its pollution removal capacity in developing the pollution limits.

The choice to use taxpayer money to challenge the Clean Water Blueprint puts the state in a bind. Maryland must demonstrate progress is being made to reduce pollution or face consequences such as reduced federal funding and more stringent permitting—which would affect local communities.

The Dam's Impact on Bay Water Quality

In 2012, the law firm of Funk & Bolton began to urge Maryland counties to delay their pollution-reduction efforts, required by the Chesapeake Clean Water Blueprint, until the Susquehanna River and the Conowingo Dam are cleaned up. The firm inaccurately asserts that pollution sources upstream are the reason for local Maryland water quality problems. As a result, several counties are considering not moving forward with their local water quality efforts.

Science is very clear that local creeks are fouled by local farms, sewage plants, and other sources. The Susquehanna River affects the Bay, but it doesn't flood local Maryland Bay tributaries with pollution.

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. A recent study by CBF and the Choose Clean Water Coalition found Pennsylvania was making progress toward specific pollution-reduction goals, but was still well short of goals. Of eight actions promised by Pennsylvania and evaluated in the study, the state was on track or exceeded its goals for three actions and fell short on five.

In a November 2012 press statement, CBF Senior Scientist Beth McGee said, "Pennsylvania has a plan to cut pollution to the Susquehanna, and is making progress. Local governments here should stay the course, and continue to reduce pollution to local waterways using proven strategies like upgrading sewage plants."

(Graphic) Pollution impact of the Conowingo Dam with the Chesapeake Clean Water Blueprint implemented.

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