WASHINGTON -- When Tropical Storm Lee sent record amounts of water down the Susquehanna River from Pennsylvania into Chesapeake Bay in September, it also sent an estimated 4 million tons of sediment, including large amounts of nitrogen and phosphorus into the region's fragile bay ecosystem. It was the third-highest recorded amount of freshwater from the Susquehanna into the bay since 1967.
As The Washington Post reports, sediment has been building up for decades behind the 83-year-old Conowingo Dam in Maryland, about 10 miles upriver from where the Susquehanna empties into the Chesapeake.
Per the Post:
According to a U.S. Geological Survey estimate, more than 160 million tons of sediment floats behind the Conowingo, built near Darlington, Md., in 1928. About 3 million tons arrive there each year, and about a million tons of that sloshes over the gates, said Mike Langland, a USGS hydrologist.
Environmentalists say the sediment dump during the storm was so high that it could spawn another mammoth, oxygen-depleted "dead zone" like this past summer's.
If the storage capacity is reached in about 20 years, as Langland predicted, at least 3 million tons of sediment would wash into the bay yearly, making matters far worse.
The dam, according to the Baltimore Sun, traps two-thirds of the 3 million tons of sediment that flows down the river each year. Phosphorus and nitrogen from sediment can harm the bay. As the Sun reported in September:
All this is worrying federal and local officials, who are looking for ways to reduce the environmental risk from sediment before similar storms pour more sediment into the bay.
Those two plant nutrients foul the bay by feeding massive algae blooms, which subsequently rob the water of oxygen that fish, crabs and shellfish need to breathe. An even larger outpouring of sediment from the river during Tropical Storm Agnes in 1972 smothered underwater grasses and killed oyster beds.
"Tropical Storm Lee provided a vivid demonstration of the need to take steps to head off what could be a catastrophic event causing immediate and enormous damage to our restoration processes," Maryland Gov. Martin O'Malley said in September, when he initiated a study of the sediment that's building up behind the dam. "The time to address this threat is now."
RELATED VIDEO: Water Pours Through Conowingo Dam's Gates Following Tropical Storm Lee