MACo Blog Series: Bay TMDL & WIPs (Part 2) – The Conowingo Dam

This MACo post is the second in a series of blog articles that will examine the issues raised by the Chesapeake Bay Total Maximum Daily Load (TMDL) and the Watershed Implementation Plans (WIPs).  Based on a court consent decree, the Bay TMDL is a United States Environmental Protection Agency (EPA) mandate to the Bay watershed states to reduce nitrogen, phosphorus, and sediment runoff into the Bay and its tributaries.  In Maryland, the State has placed significant implementation responsibility at the county level.


The Conowingo Dam spans the Lower Susquehanna River between Cecil and Harford Counties and is privately owned by the energy generation company Exelon.  It’s approximately 14-mile reservoir is a source of drinking water for the Baltimore region in Maryland and the Chester region in Pennsylvania.  The Dam is also subject to licensing and regulatory oversight by the Federal Energy Regulatory Commission (FERC).

Profile on the Conowingo Dam (Exelon)

Conowingo Dam Background Information (Wikipedia)


The Conowingo Dam is important with respect to the Bay TMDL due to the significant amounts of phosphorus and sediment that is intercepted and trapped in the reservoir.  The Dam reservoir also traps nitrogen, although to a much smaller extent.  However, the reservoir has slowly filled in over the last several decades and there is evidence that during major storm events where the Dam’s floodgates must be fully opened, the Dam releases large amounts of sediment and phosphorus into the main stem of the Bay.  A 2012 United States Geologic Survey report summarizes the concern by highlighting the impact that Tropical Storm Lee (2011) had on the Conowingo’s nutrient and sediment release into the Bay:

Trends in flow-normalized fluxes at the Susquehanna River at Conowingo, Maryland, streamgage during 1996–2011 indicate a 3.2-percent decrease in total nitrogen, but a 55-percent increase in total phosphorus and a 97-percent increase in suspended sediment. These large increases in the flux of phosphorus and sediment from the Susquehanna River to the Chesapeake Bay have occurred despite reductions in the fluxes of these constituents from the Susquehanna River watershed upstream from the reservoirs. Although the Tropical Storm Lee flood event contributed about 1.8 percent of the total streamflow from the Susquehanna River to the Chesapeake Bay over the past decade (water years 2002–11), it contributed about 5 percent of the nitrogen, 22 percent of the phosphorus, and 39 percent of the suspended sediment during the same period. These results highlight the importance of brief highflow events in releasing nitrogen, phosphorus, and sediment derived from the Susquehanna River watershed and stored in the Conowingo Reservoir to the Chesapeake Bay.

The report concludes that the Conowingo reservoir is losing its ability to capture nutrients and sediment during major storm events and that the impact of this reduced capacity must be factored into future Bay restoration efforts:

The three dams at the downstream end of the Susquehanna River are important in mitigating the downstream transport of nitrogen, phosphorus, and suspended sediment from the Susquehanna River watershed to the Chesapeake Bay. The reservoirs are known to be more than 80 percent filled with sediment. A consequence of that filling is that they are no longer a major sink for the nitrogen, phosphorus, and sediment coming from the watershed, but rather are approaching steady state, with an approximately equal balance between the fluxes of these materials that enter the reservoir and those that leave the reservoir and enter the Chesapeake Bay.  …

Therefore, efforts to reduce nutrient and sediment inputs to the Chesapeake Bay will need to include consideration of changes in the trapping of sediment entering, and scouring of sediment in, the reservoirs along with the management actions implemented upstream in the watershed. Continued analysis of water-quality and discharge data that help to improve understanding of the future trajectory of these changes (with and without engineered modifications of the reservoirs) will be crucial to planning for the achievement of restoration goals for the Chesapeake Bay.

While the Dam does contribute pollution to the Bay and should be considered as part of the overall Bay restoration efforts, it is only one piece of the TMDL puzzle and is not a “magic bullet” that will alleviate State and local TMDL efforts.  Addressing the Dam will not address locally generated sources of pollution nor will it address the pollution generated in streams and tributaries outside of the Susquehanna River and the main stem of the Bay.


FERC is the federal agency responsible for licensing the construction and continued operations of hydroelectric dams, including the Conowingo.  Exelon provided notice on September 13 that it was seeking a renewal of its major license for the Dam (FERC Docket P-405, Subdocket 106).  As FERC does have some oversight on the environmental impacts of hydroelectric projects, certain stakeholders have proposed raising the Dam’s impact on the Bay TMDL as part of the licensing renewal process.  Those wishing to receive notices about specific FERC dockets, including the Conowingo, can do so through an eSubscription process.  FERC has requested additional information from Exelon about its initial renewal application and Exelon has 60 days (until early January) to respond.

Any party may submit public comments that will be considered by FERC as part of the hearing process for license renewal.  Additionally, a party has the right to become an intervenor, which allows party to become participants in the hearing proceeding, request a rehearing of a FERC decision, and have standing to challenge FERC decisions in the United States Circuit Courts of Appeal.  A party wishing to become an intervenor must follow certain steps.


Recently, the law firm of Funk & Bolton has proposed to represent counties in addressing the Conowingo Dam issue and possibly challenge other aspects of the Bay TMDL.  Many counties have been approached by the law firm and some have agreed to retain the firm’s services.

Funk & Bolton Conowingo Proposal

Funk & Bolton’s efforts have generated strong reactions from the Office of Governor Martin O’Malley, the Midshore Riverkeeper Conservancy, and the Chesapeake Bay Foundation (CBF).

CBF Letter to Governor O’Malley

CBF Conowingo Dam Fact Sheet

Additionally, Dorchester County submitted concerns over the Dam to MDE in September.  MDE provided a response in November, stating that while the Conowingo Dam should not halt WIP implementation it does pose a concern:

In short, although sediment behind Conowingo Dam is a valid concern that demands our attention and resources, it should not delay action on Bay restoration by Marylanders.


MACo is among many stakeholders reviewing the Conowingo issue and will respond based on the policy implications for county governments as they struggle to meet their TMDL targets.  It is important to neither underestimate or overestimate the impact of the Conowingo Dam on the Bay TMDL.  It is also important to note that the Conowingo Dam is not a “new issue” and stakeholders, including the State and EPA, have been aware of the Dam’s reservoir concerns for many years.

Addressing the problem, and fairly incorporating any connected actions will take a concerted effort — likely involving many key stakeholders, including Exelon, the State, the environmental community, counties and possibly the federal government.  MACo supports raising the awareness of this issue and encourages an ongoing discussion on reaching a solution to this challenge.  As MDE noted in its response letter to Dorchester County, the Conowingo issue “demands our attention and resources.”

For further information about MACo’s efforts regarding the Conowingo Dam, please contact Les Knapp at 410.269.0043 or

Bay TMDL & WIPs (Part 1) – Watershed 101: Q&A About Watershed Implementation Issues

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