The reservoir behind Conowingo Dam has long captured sediment flowing downstream, but recent studies have drawn attention to its changing effectiveness as a “pollution gate.”
The Conowingo Hydroelectric Generating Station, or Conowingo Dam, is one of three dams on the lower Susquehanna River. While the reservoir behind the dam has long captured sediment and attached nutrients flowing downstream, recent studies have shown that the dam is less able to capture pollutants. Because the reservoir behind the dam is essentially full, it is only trapping sediment in the short term. During large storms and severe floods, sediment and attached nutrients can “scour” into the Susquehanna River, moving over the dam and into the Chesapeake Bay. While it is possible to manage nutrients and sediment behind the dam, researchers have found that reducing upstream nutrient and sediment pollution would be more beneficial to the Bay.
What is the Conowingo Dam?
The Conowingo Hydroelectric Generating Station, or Conowingo Dam, lies on the Susquehanna River. The river flows downstream from New York and Pennsylvania to Maryland, where the Conowingo Dam is located. The hydroelectric power plant is owned and operated by Exelon Corporation and overseen by the Federal Energy Regulatory Commission. It began operations in 1928, after two years of construction. Conowingo Dam sits downstream from two other dams on the lower Susquehanna River: Safe Harbor and Holtwood dams.
How does the Conowingo Dam impact water quality?
Since its construction, the reservoir behind the Conowingo Dam has captured sediment—and the nutrients that are often attached—flowing down the Susquehanna River, reducing the amount of sand, silt, nitrogen and phosphorus pollution entering the Chesapeake Bay. The reservoir has trapped an average of 3.5 million pounds of phosphorus and four billion pounds of sediment every year since the dam opened, which is approximately a third of the phosphorus and half of the sediment that flows along the Susquehanna River and into the Chesapeake Bay annually.
While the reservoir has long served as an effective way to control pollution, recent studies show that the reservoir behind the Conowingo dam no longer has the long-term ability to store sediment and associated nutrients. In 2015, a U.S. Geological Survey report found that the reservoir behind the dam had reached 92% capacity and had lost its ability to trap sediment and attached nutrients over the long term.
In 2016, the Lower Susquehanna River Watershed Assessment (LSRWA) team released the results of its evaluation of sediment management options at the Conowingo Dam. It found that, since the reservoir behind the dam is essentially full, it is trapping smaller amounts of incoming sediment and, during large storms, sending more silt and attached nutrients over the dam and into the Bay more often. The report concluded that management and mitigation of nutrients and sediment upstream of the reservoir would be more beneficial to Bay health than attempting to manage sediment at the dam through dredging, bypassing or operational changes.
When nutrients enter rivers and streams from urban, suburban and agricultural runoff, they can attach to particles of sediment. While sediment doesn’t take long to settle to the bottom of waterways, nutrients are released back up into the water column in dissolved form.
Nutrient pollution has a lingering effect on water quality. It fuels the growth of harmful algae blooms, which create dead zones that suffocate marine life.
Sediment enters rivers and streams when land, stream banks and shorelines erode. In excess amounts, sediment can cloud water, block light from reaching underwater grasses and smother shellfish. During strong storms and severe floods, particles of sediment can be pushed downstream, or scoured from behind the Conowingo Dam. They then flow into the Bay, decimating underwater grass beds and marine life.
During a major storm event, 20% to 30% of the sediment that flows into the Bay from the Susquehanna River is from material stored behind Conowingo Dam. While the effects of sediment essentially cease once it settles to the bottom, nutrients attached to sediment continue to harm the Bay.
What is being done to address the Conowingo Dam reservoir?
As it has become clear that the Conowingo Dam reservoir can no longer capture pollution flowing downstream, government agencies, nonprofits and businesses have conducted studies to determine the best way to manage pollution from behind the dam. The LSRWA team considered several possible solutions, including large-scale dredging and changing dam operations. However, the team concluded that these efforts on their own, in addition to being less cost effective, do not provide sufficient benefits to offset the adverse impact of the dam in the long-term.
Instead, the team concluded that reducing upstream nutrient and sediment loads would offer the best long-term solution to the Bay’s poor health.
In 2018, the Chesapeake Bay Program partnership formed steering committee that would develop a collaborative strategy to address the increased pollution loads that have resulted from the Conowingo Dam reaching full capacity.
The plan, known as the Conowingo Watershed Implementation Plan (WIP), will detail specific steps each of the seven Bay watershed jurisdictions—Delaware, the District of Columbia, Maryland, New York, Pennsylvania, Virginia and West Virginia—will take in order to offset nutrient and sediment pollution from the dam and restore the health of the Chesapeake Bay ecosystem.
The Conowingo WIP offers a sustainable, long-term solution to the Bay’s water quality issues by addressing the problem at its source. The WIP will outline specific actions the seven jurisdictions must take to directly reduce nutrient and sediment pollution loads.
What you can do
Here are some ways you can reduce the amount of sediment and nutrients flowing into the Bay:
- Install a green roof, rain garden or rain barrel to capture and absorb rainfall.
- Redirect home downspouts onto porous surfaces like grass, gravel or pavers in place of asphalt or concrete.
- If you live on the water, install living shorelines to stabilize waterfront property without damaging underwater grass beds and other nearshore habitats.
- If you have a lawn to take care of, use fertilizers properly: do not use more than needed, and do not apply to dormant lawns or frozen ground.