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 flowing downstream, recent studies have drawn attention to its changing effectiveness as a “pollution gate.” 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, moving over the dam and into the Chesapeake Bay. While it is possible to manage 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, is one of three dams on the lower Susquehanna River. The hydroelectric power plant is owned and operated by the Exelon Corporation, overseen by the Federal Energy Regulatory Commission and located downstream of the Safe Harbor and Holtwood dams. It began operations in 1928, after two years of construction.
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 phosphorous entering the Chesapeake Bay. While this reservoir has long served as an effective “pollution gate,” recent studies have drawn attention to its changing effectiveness.
In 2012, the U.S. Geological Survey (USGS) reported that the reservoir behind the Conowingo Dam had lost its ability to trap sediment and attached nutrients over the long term.
In 2014, the Lower Susquehanna River Watershed Assessment (LSRWA) team released the results of its evaluation of sediment management options at the Conowingo Dam. It found:
- The reservoir behind the Conowingo Dam is trapping sediment in the short-term. Because the reservoir 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.
- Nutrients that enter the river upstream and attach to particles of sediment are a bigger threat to water quality than sediment alone.
- 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. Because of this, lowering both nutrient and sediment pollution upstream of the Conowingo Dam would benefit Bay health.
Sediment enters rivers and streams when land, stream banks and shorelines erode. In excess amounts, sediment can cloud the water, blocking light from reaching underwater grasses and smothering shellfish. During strong storms, severe floods and fast-moving water, particles of sediment can be pushed downstream, or scoured from behind the Conowingo Dam. While the LSRWA team found the effects of sediment essentially cease once it settles to the bottom, lowering both sediment and nutrient pollution upstream of the Conowingo Dam would benefit Bay health.
What is being done to address the Conowingo Dam reservoir?
Between 2008 and 2011, just 13 percent of the Susquehanna River’s sediment load came from the reservoir behind the Conowingo Dam. The remaining 87 percent originated from the broader Susquehanna River watershed. Therefore, reducing upstream nutrient and sediment loads through the Chesapeake Bay’s “pollution diet”—or Total Maximum Daily Load (TMDL)—and supporting Watershed Implementation Plans (WIPs) would offer the best long-term solution to the Bay’s poor health, helping jurisdictions attain their water quality goals.
For Chesapeake Bay restoration to be a success, we all must do our part. Our everyday actions can have a big impact on the Bay. By making simple changes in our lives, each one of us can take part in restoring the Bay and its rivers for future generations to enjoy.
To lower nutrient pollution, consider reducing the amount of pollution that can run off your property. Install a green roof, rain garden or rain barrel to capture and absorb rainfall. Use porous surfaces like gravel or pavers in place of asphalt or concrete. 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.
To lower sediment pollution, consider combating erosion on your property. Spread mulch over bare ground, or plant buffers of trees and shrubs to capture runoff and hold soil in place.