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.
A team of scientists has found that reducing pollution in the Susquehanna River watershed—which includes portions of New York, Pennsylvania and Maryland—could ease the environmental effects of an “essentially full” reservoir behind the Conowingo Dam, whose pollution-trapping capacity has diminished in recent years.
The reservoir behind the Conowingo Dam—as well as those behind the Holtwood and Safe Harbor dams—has for decades trapped particles of sediment flowing down the Susquehanna River, as well as the nutrients that are often attached. But according to research from the Lower Susquehanna River Watershed Assessment (LSRWA) team, this reservoir is full. 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.
While researchers explored strategies for managing sediment at the dam, the team found that reducing pollution loads upstream of the dam would pose a more effective solution to the “full reservoir” problem. Indeed, dredging, bypassing or other operational changes would come with high costs and low or short-lived benefits. But adhering to the Chesapeake Bay’s “pollution diet”—and taking additional steps to reduce pollution where possible—would offer management flexibility and environmental benefits.
The Chesapeake Bay Total Maximum Daily Load (TMDL) was established in 2010 to reduce nutrient and sediment loads across the watershed. Lowering these pollutants is integral to restoring the health of the Bay: excess sediment can cloud the water and harm underwater grasses, fish and shellfish, and nutrients can fuel the growth of harmful algae blooms. While the LSRWA team did find that the effects of the sediment that “scour” from the Conowingo reservoir cease once it settles to the bottom of the river, the effects of nutrient pollution linger. Green infrastructure, forest buffers and sound farm and lawn management can help businesses, landowners and individuals contribute to a restored Chesapeake.
Sediment reservoirs near the mouth of the Susquehanna River are filling up faster than researchers expected, posing a new obstacle for improving water quality in the Chesapeake Bay.
As the holding areas behind the lower Susquehanna's three dams reach capacity, their ability to trap upriver sediment and the phosphorous that is often attached wanes, and the sediment that is held grows more and more likely to flow out of the reservoirs and into the river.
According to a report released by the U.S. Geological Survey (USGS), strong storms, severe flooding and faster-moving water have turned the one-time pollutant blockers into less effective gates.
The Susquehanna delivered more phosphorous and sediment into the Bay last year than it has in more than three decades of monitoring. The past 15 years have seen a 55 percent increase in phosphorous entering the Bay from the river and a 97 percent increase in sediment. And while nitrogen flow has dropped, it shows a jump during large storms--like Tropical Storm Lee in 2011 or Hurricane Ivan in 2004--and the flooding that follows.
Excess nutrients and sediment can harm fish, shellfish and underwater grasses. Nitrogen and phosphorous fuel the growth of algae blooms that rob water of oxygen and, with suspended sediment, cloud the water and block the sunlight that plants need to grow.
A previous USGS report cited improvements in nutrient and sediment trends as a sign of improving Bay health. The USGS has seen significant reductions in nutrient and sediment concentrations upstream of the reservoirs, which reflect the positive impacts of conservation efforts in the Susquehanna watershed. But the filling reservoirs behind the Safe Harbor and Holtwood dams in Pennsylvania and the Conowingo Dam in Maryland overshadow the pollution reduction progress that is being made.
The Lower Susquehanna River Watershed Assessment team, composed of federal, state and regional partners and administered by the U.S. Army Corps of Engineers, is exploring ways to expand the reservoirs' capacity.