Photo © Eliot Malumuth, Laurel Maryland
The Conowingo Dam and Chesapeake Bay
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 suspended sediment that would have entered the Bay from the Susquehanna River. By trapping suspended sediment, the Conowingo has helped reduce contributions of sediment and phosphorus to the Chesapeake and helped to restore the Bay. 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 removal. In particular, during big storms (which occur roughly every four to five years) 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 loads into the Conowingo reservoir will equal loads coming out. The loss of this trapping capacity was considered and accounted for when the most recent pollution-reduction targets and plans were developed for 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 percent of the nitrogen flowing into the Bay.
- Scientists estimate that during the most recent large scour event, Tropical Storm Lee in September 2011, the Susquehanna River watershed located above the Conowingo Dam provided 80 percent of the load delivered to the Bay, with the remaining 20 percent scoured from the sediment trapped in the Conowingo Reservoir. Hence, upstream sources deliver more sediment and nutrients and, therefore, more impacts on the Bay ecosystem, than do the scoured sediment and associated nutrients from the reservoir behind the dam.
- 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 clean water.
- The dam has little impact on nitrogen pollution loads, the worst form of pollution for the Bay. Nitrogen is showing a downward trend on the Susquehanna. Nitrogen is water soluble, so little was ever trapped behind the dam, unlike sediment and phosphorus (which is often attached to sediment). Implementation of upstream pollution reduction measures will continue to reduce nitrogen loads from the Susquehanna.
- 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.
- The EPA explicitly included the dam and its pollution removal capacity in developing the Bay pollution limits and will be revisiting the topic as part of the 2017 "midpoint evaluation" for updating the science behind the pollution limits.
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 Watershed—is 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 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.
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. A temporary, one-year license was issued in early September to permit the dam's operation while ongoing studies and the relicensing process are completed. As part of the re-licensing, Federal Energy Regulatory Commission (FERC) must consider potential environmental impacts from the dam and include appropriate conditions in the new 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 2016 or later. In the meantime, FERC will issue a temporary license until the Environmental Impact Statement (EIS) is finalized and the State makes a decision on water quality certification.
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.
CBF also remains concerned with restoring migratory fish in the Susquehanna. 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. With decades of experience in Bay fisheries, CBF has much to contribute to discussions on fish passage issues at the Conowingo, and must have recourse should proposed solutions be inadequate.
For More Information
Download our fact sheet
November 13, 2014—(CBF Press Release) Study Reveals Conowingo Dam Only One of Many Pollution Sources
November 13, 2014—(Cecil Whig) Study: Nutrients, not sediment, are Bay's biggest issue
November 13, 2014—(The Capital) Scientists: Conowingo sludge not the sole solution to bay pollution; Say best cleanup results come from controlling pollutants at their source
November 12, 2014—(Baltimore Sun) Dredging Conowingo little help to bay, study finds
November 12, 2014—(DelmarvaNow.com) Report: Conowingo Dam not major threat to bay
August 27, 2014—Study of Conowingo Provides Groundbreaking Findings on Sediment Impact
Watch video here
August 12, 2014—CBF Letter to Maryland's General Assembly
May 5, 2014—Upriver Pollution, Not Dam, Bay's Major Threat
May 5, 2014—Statements from the Senate Hearing (Committee on Environment and Public Works, Subcommittee on Water and Wildlife) on Finding Cooperative Solutions to Environmental Concerns with the Conowingo Dam to Improve the Health of the Chesapeake Bay
August 20, 2013—CBF Files to Intervene in Conowingo Dam Relicensing
October 10, 2012— Baltimore Law Firm Urges Counties to Delay Implementation of the Clean Water Blueprint
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.
More recently, 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, as part of the FERC re-licensing, Exelon must receive a "Section 401 water quality certification" from Maryland. Under this section of the federal Clean Water Act, states need to evaluate the potential for dams to affect downstream waters and insert conditions in the FERC permit to protect them. CBF will comment on this certification.
What You Can Do
Marylanders: Contact Governor Hogan and let him know this latest study highlights how critical it is to contine with local cleanup efforts and urge him to complete these critical actions to save the Bay.
Join CBF's Action Network to stay informed about this and other issues affecting the Bay.
What Others Are Saying
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 from the dam and the Susquehanna 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 Maryland 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.
Furthermore, the choice to use taxpayer money to challenge the Clean Water Blueprint instead of reducing pollution 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.
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. 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 four actions and fell short on four.
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 and reducing polluted runoff from urban areas."