Sediment in Conowingo reservoir exceeds 90 percent of storage capacity

Sediment building up behind Conowingo Dam has almost reached the reservoir’s capacity for storage, according to a report released by the U.S. Geological Survey (USGS). The reservoir is considered at its limit for holding sediment when it is half full—at present, it is 92 percent of the way toward this maximum.

Since its construction in 1929, the Conowingo reservoir, along with the reservoirs behind the Holtwood and Safe Harbor dams, has trapped sediment and nutrients as they flow down the Susquehanna River—which provides nearly half of the fresh water that flows into the Bay. According to the report, the ability of these reservoirs to trap pollutants has been steadily declining.

“Storage capacity in Conowingo reservoir continues to decrease, and ultimately that means more nutrients and sediment will flow into the Bay,” said Mike Langland, author of the study, in a release. “Understanding the sediments and nutrients flowing into the Bay from the Susquehanna River is critical to monitoring and managing the health of the Bay.”

Excess sediment can cloud the water and harm underwater grasses, fish and shellfish, while nutrients can fuel the growth of harmful algae blooms and the creation of low-oxygen “dead zones,” which suffocate underwater life. Reducing the amount of pollutants in local waterways is integral to Bay restoration efforts, including the Chesapeake Bay Total Maximum Daily Load (TMDL), or “pollution diet,” which Bay Program partners recommitted to achieving as part of the Chesapeake Bay Watershed Agreement. In anticipation of a decline in Conowingo reservoir’s ability to trap sediment, the TMDL includes a mechanism for addressing any increases in nutrient and sediment pollution caused by a full reservoir.

The report from USGS reiterates the findings of a study by the Lower Susquehanna River Watershed Assessment (LSRWA) team, released in November 2014, which found that the once-effective “pollution gate” is trapping smaller amounts of sediment and nutrients and, during large storms, sending more of these pollutants into the Susquehanna River more often. The team found that reducing pollution loads upstream of the dam would pose a more effective solution that dredging, bypassing or other operational changes, which would come with high costs and low or short-lived benefits.

The USGS report, Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, Pennsylvania and Maryland 1900–2012, is available online.