New research shows that while stores of sediments behind the Conowingo Dam pose a growing threat to Chesapeake Bay health if they get washed out in a major rainstorm, during normal Susquehanna River flows the contaminants aren’t having a large effect on the estuary.
Looking at four decades of data, scientists at the University of Maryland Centers for Environmental Science confirmed fears that when Susquehanna waters swell, increasing amounts of sediments and nutrients are flowing through the dam and into the upper Chesapeake. They think that’s because so much of the material has built up behind the dam, more of it is washed away with the current, and less gets trapped behind the 91-year-old structure.
But the rest of the time, they found, the potential pollutants are mostly inert, getting trapped in grass beds at the river’s mouth and settling at the river bottom. The Chesapeake ecosystem is resilient enough to withstand the ordinary flow of contaminants, said Cindy Palinkas, lead author of a study published this week in the journal Estuaries and Coasts.
“There’s always been this idea that sediment is bad and there’s going to be all this sediment going into the bay and it’s going to kill everything, and I just don’t think that’s true,” Palinkas said.
Still, the data does support fears that a major storm like Tropical Storm Lee in 2011 or Hurricane Agnes in 1972 could send a “catastrophic” amount of pollution down the bay.
“None of us know if or when that will happen, so I think the thing is to just keep doing the restoration work that we’re doing and we give the bay its best chance of dealing with an event like that when it comes,” she said.
Sediment, and the nutrient particles it carries, can spoil bay ecology, blocking sunlight from reaching underwater plants and fueling massive algae blooms that strip oxygen from the water when they die.
The Susquehanna carries a heavy influence on the bay’s health because it drains more than 40 percent of the Chesapeake watershed. Since it was built in 1928, connecting Harford and Cecil counties 10 miles north of where the river meets the bay, the Conowingo and other dams upstream have been collecting much of the pollution that washes into waterways across much of Pennsylvania and parts of New York and Maryland.
One set of findings was not surprising to those concerned about the dam’s role in the Chesapeake’s health: that more sediment is being scoured away and washed downstream, and that lower flows are required to trigger that scouring than in the past. The researchers didn’t quantify those changes, because they vary by storm, but the trend was clear, Palinkas said.
More surprising, perhaps, was evidence that during normal river flows, material flowing down the Susquehanna doesn’t typically pass beyond the upper Chesapeake. Because waters are mostly fresh there, and because warm and salty water is needed to activate phosphorus attached to the sediments, the pollutants have little effect, getting trapped in beds of underwater grasses or sinking to the bay bottom, Palinkas said.
Beth McGee, director of science and agriculture policy for the Chesapeake Bay Foundation, said the findings are encouraging and help show that broader efforts to reduce water pollution across the bay watershed are more important than focusing on the dam’s role.
“There’s good news that what we’re doing upstream is helping,” she said.
But, McGee added, there is still a looming threat that the amount of sediment passing through the dam will increase as climate change makes rain and storms more intense. Modeling from the federal Chesapeake Bay Program suggests that by 2050, the Conowingo will be releasing more sediment, nitrogen and phosphorus into the bay than it holds back.