Conowingo Dam Impacts

Dundee Creek — The Conowingo Dam.
*Image courtesy of Jane Thomas - IAN, UMCES (ian.umces.edu/media-library).*

Map of Chesapeake Bay watershed — The Susquehanna River watershed - the Conowingo Dam shown as red star. Image courtesy of The Chesapeake Bay Foundation (cbf.org/about-the- bay/susquehanna-river/index.html).

With a watershed covering nearly 27,500 square miles across three states, the Susquehanna River is Chesapeake Bay’s largest tributary and contributes more than half of the Bay’s fresh water. Agricultural runoff and streambank erosion in the watershed contribute nutrients and sediments to the river and Bay. This process can deliver significant pollution to the Bay, particularly during flood events, which decreases the oxygen available for aquatic life.

The Conowingo Hydroelectric Dam, constructed in 1929 to provide power to Philadelphia, is located at the bottom of the Susquehanna River watershed, about ten miles from where the river enters the upper Chesapeake Bay. The 14-mile-long, 9,000-acre reservoir behind the dam historically trapped sediments and nutrients flowing down the Susquehanna River watershed.

Chesapeake Bay food web — The Conowingo Reservoir.
*Image courtesy of Dylan Taillie - IAN, UMCES (ian.umces.edu/media-library)*

Over time, Conowingo Reservoir has filled with sediments and nutrients and lost the capacity to effectively prevent this pollution from entering the Bay. More nitrogen, phosphorus, and sediment are now entering the Chesapeake Bay during high flow events. The increased pollutant load (estimated at 6 million pounds total nitrogen and 260,000 pounds total phosphorus annually) must be addressed to meet the Bay’s water quality standards.

Maryland is implementing three strategies to address pollution loads and negative ecosystem impacts from the Conowingo Dam, including:

Conowingo Watershed Implementation Plan

In July 2021, a multijurisdictional Conowingo Watershed Implementation Plan (CWIP) was developed by the Chesapeake Bay Program (CBP) Partnership to offset the increased pollution from Conowingo infill. Maryland, Pennsylvania, and New York are working together to implement effective nitrogen-reducing best management practices in the Susquehanna River Basin.

Maryland’s Pay for Success Program

Maryland allocated $25 million for an innovative Pay for Success Program that purchases the lowest-cost nutrient reductions throughout the Susquehanna River watershed. The Susquehanna River Basin Commission (SRBC) administers the program and oversees contracts to pay for pollution reduction practices once they have been verified. Seven projects were selected in round one of the program with per-pound nitrogen costs ranging from $6 to $150. Completed projects are estimated to reduce total nitrogen by approximately 165,600 lbs per year. Round two projects are currently under review, and Maryland is expected to achieve its CWIP target once all projects are implemented.

Click here for more information about the CWIP and Maryland’s Pay for Success Program.

Dredging Program within Conowingo Reservoir

Conowingo Sediment Characterization and Innovative Reuse Pilot

In 2019, MDE funded a $3.3 million Conowingo Sediment Characterization and Innovative Reuse and Beneficial Use pilot project to provide Maryland with better information on the quality of sediments behind the dam, dredging costs, dredged material reuse options, scalability, and feasibility for addressing Conowingo’s pollution impacts. Report findings indicate that Conowingo sediments are suitable for a range of potential reuses, including cement manufacturing, bioretention and topsoils, amended agricultural soils, and stabilized fill. The report also concluded that the costs of an environmental dredging program could be offset by the potential revenue generated from selling water pollution reduction credits and implementing different combinations of marketable reuses, though the degree of cost offset depends on a variety of factors. The last phase of the project included a demonstration project where a mix of 90% Conowingo dredged material/10% leaf mulch was used on a sod farm as topsoil to support proper drainage, level the fields, and enhance performance of harvesting equipment.

Conowingo Reservoir Modeling Study

The Maryland Department of the Environment has a Conowingo Reservoir Modeling Contract and Scope cost-share agreement with the Army Corps of Engineers (USACE) Baltimore District (NAB) to develop the scientific framework necessary to evaluate the nutrient reduction effectiveness of Conowingo reservoir dredging. The study will leverage the additional sediment characterization work done and lessons learned through Maryland’s innovative and beneficial reuse pilot, as well as collect additional sediments and bioavailability data to help calibrate the model. The Conowingo Reservoir Modeling Study has three main objectives:

Develop a three-dimensional water quality modeling system. The modeling system will be capable of simulating hydrodynamics, biogeochemical, and sediment transport processes within the reservoir.
Set up the modeling system to simulate both current and future dredging scenarios, specifically the evaluation of sediment and associated nutrient reductions from the different scenarios.
Simulate future hydrologic-climate scenarios.

Information gleaned from this project will help various Chesapeake Bay partnerships better understand and institutionalize the resiliency and response of Conowingo Reservoir to different dredging scenarios and hydrologic conditions. From there, the model can determine scour and sediment resuspension as well as associated nutrient/contaminant increases both within the reservoir and downstream. Below is a timeline of the study's key milestones.

Reviewing Water Quality Certification with Constellation Energy Generation

In 2018, the Maryland Department of the Environment issued the Clean Water Act, Section 401 Water Quality Certification (WQC) 17-WQC-02 to Constellation Energy Generation, LLC (formerly Exelon). In October 2019, MDE and Constellation entered into a water quality settlement agreement which contained agreed-upon proposed Federal Energy Regulatory Commission (FERC) license articles pertaining to water quality. The settlement agreement with Constellation Energy is intended to reduce pollution loads, increase resiliency, and help restore ecosystems and aquatic life in the Susquehanna River Basin watershed. Constellation Energy will invest in environmental and operational enhancements as part of its 50-year license. These projects help restore the river’s ecosystem and support clean water and recreation for the region by:

Improving downstream river flow to support natural hydrology;
Restoring habitats with pollution-filtering mussels and oysters, and funding other nutrient reduction projects;
Enhancing fish passage to support the return of American shad, river herring, and American eels;
Building climate resilience through shoreline restoration, underwater vegetation recovery, and floodplain management; and
Cleaning up trash and debris that collects behind the dam and flows downstream during high water.

In response to timely filed requests for administrative review of the certification, known as Reconsideration, MDE is reviewing its 2018 WQC decision and associated conditions consistent with Maryland’s authority under the Clean Water Act and state law. MDE will make a decision on the Reconsideration at the conclusion of the process.

By collaborating across jurisdictions, financing an innovative Pay for Success Program, coordinating a Conowingo Dredging Program, and working with Constellation Energy, Maryland is investing in the future health of the Bay. MDE will continue to lead Conowingo efforts within the Bay Program Partnership and strive for continuity across administrations in funding and prioritization.

Contact Information

Please direct questions or comments to mde.chesapeakewip@maryland.gov.

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