The flow of sediment into the bay from the Susquehanna would increase by at least 150 percent if the dam’s reservoir filled.
On top of that, phosphorus levels would increase by 40 percent and nitrogen by 2 percent.
Sediment in the Chesapeake Bay.
Not all sediment is bad. Some is composed of gravelly material that helps support bay grasses that protect young fish from predators.
But a lot of it is granular stuff that blots out light and smothers grasses in the Chesapeake. Nutrients such as nitrogen and phosphorus feed algae blooms that suck out oxygen, suffocating endangered bay oysters and other slow-moving shellfish.
“Anything that moves upstream eventually finds its way to the Conowingo,” said Mark Bryer, director of the Chesapeake Bay program for the Nature Conservancy. “Since we’ve been building dams, sediment has been a problem.”
The study is tasked with finding a way around the problem, getting sediment downriver in the least harmful way. “You don’t want it coming down in one big punch,” Bryer said.
Having a filter like the Conowingo helps, said Bruce Michael, director of resource assessment for Maryland’s Department of Natural Resources. “There’s a benefit to that dam being there” corraling it, Michael said. “We’re all trying to see how we can extend that trapping capacity behind the dam.”
Serena McClain, a director of river restoration for the conservation group American Rivers, had a different view. “Rivers move sediment. That’s their job,” she said. “When you put a dam in the river, that’s going to trap sediment. It’s only going to trap so much sediment before it goes over and is released.”
American Rivers advocates removing some dams, McClain said. The sediment the dams collect is often grit that pours through the orange mesh fencing of home builders or comes from mud loosened by bathing cattle. The Susquehanna is fed by 49,000 miles of rivers, streams, creeks, brooks and runs.
“There’s no obstruction to stop” it from being injected into the Susquehanna’s veins, McClain said.
With the help of the Nature Conservancy and other partners, state and federal officials will try to determine whether conservation measures such as cover crops, stream buffers and other methods can become an obstruction, soaking up rain before it can push sediment into waters, Michael said.
Some of those same measures are similar to what the federal Environmental Protection Agency calls for in its controversial “pollution diet” to reduce harmful sediment, phosphorus and nutrients that pour into the bay from cities and farms.
Environmentalists support stronger conservation measures, but farm lobbies and housing developers have fiercely contested these efforts, saying they’re expensive and extreme.
An American Farmers Association lawsuit against the EPA is making its way through a federal court in Pennsylvania, with support from home builders.
The federal government is putting up more than $1 million for the sediment study, according to the announcement. The remainder is being split by the Maryland departments of the Environment and Natural Resources, the Susquehanna River Basin Commission and the Nature Conservancy.
“We have to somehow defuse the impact of the time bomb that’s been waiting for the next big flow,” Bryer said.
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