Conowingo Dam opens the floodgates that allow sediment, nutrient pollution and trash flowing down from Pennsylvania to reach the Chesapeake Bay. (Kenneth K. Lam/Baltimore Sun video)
Hurricane Florence has moved on, but the storm is nonetheless forecast to deliver another potentially damaging slug of floodwaters and pollution to the Chesapeake Bay over the next few days.
Rising waters on the Susquehanna River are expected to crest Thursday night, prompting the operator of the Conowingo Dam to again open floodgates that allow sediment, nutrient pollution and trash flowing down from Pennsylvania to reach the Chesapeake Bay. Exelon Corp. has already collected three times more debris from the rim of the dam over the past nine months than it normally does over an entire year.
Scientists and bay advocates say the latest surge could be enough to ensure that the estuary’s improving health takes a hit. The region’s second-rainiest summer on record has already washed unusual amounts of pollution and debris into the bay at a time when freshwater flows into its waterways are normally at their lowest.
The cumulative impact of all that rain could be a significant setback for underwater grasses, oysters and oxygen levels across the bay, said Scott Phillips, Chesapeake Bay coordinator for the U.S. Geological Survey.
“If we hadn’t had a wet summer, the impacts would be pretty short-lived,” Phillips said. “Coming on the tail of these continual high flows like we had in the summer, it’ll probably have more impact.”
It wasn't just a surge of debris that washed into the Chesapeake Bay last month — the estuary received a record-setting flow of fresh water, scientists say, potentially hinting at long-term impacts on ecosystem health.
The Susquehanna provides the bulk of the freshwater in Maryland portions of the Chesapeake, and therefore it also delivers much of the pollution that disrupts bay ecology. That includes sediment that clouds waters and smothers bottom-dwelling life, as well as nitrogen and phosphorus that fertilize algae blooms and lead to “dead zones” of little or no oxygen in the depths of the bay.
The river’s flow usually drops during the summer, and surges of precipitation from tropical systems are common in the fall. But this year, record rains in May and early June and from mid-July into September have instead kept Susquehanna waters high. A mess of trash and logs that washed into Chesapeake waterways in July prompted Gov. Larry Hogan to demand New York and Pennsylvania “take responsibility” for the state of the bay.
Then the river’s flow hit a record in August, averaging 133,000 cubic feet of water per second, four times normal levels for that month.
“It’s been a crazy year,” said Bill Dennison, vice president for science application at the University of Maryland Center for Environmental Science.
So far, he said, an expanse of grasses at the mouth of the Susquehanna has withstood the onslaught of waters that researchers have likened to chocolate milk. And August storms stirred waters enough to oxygenate summer dead zones.
But there is a risk the surge of pollution could have “lingering” impacts, Phillips said. Plant material washed downstream could remain on the bottom of Chesapeake waterways into next spring, potentially making dead zones worse next year as they decompose and use up oxygen, he said.
Even as Conowingo Dam owner Exelon Corp. is suing Maryland, denying responsibility for pollution that flows down the Susquehanna River, state environmental officials on Monday appealed directly to the company in a letter asking for help in "a critical moment" for the Chesapeake Bay.
And bay grasses, which hit a record of more than 100,000 acres in 2017, could also take a hit, USGS scientists cautioned. The last time Susquehanna waters were stubbornly high, amid tropical systems Irene and Lee in 2011, grass acreage tumbled by 40 percent over the next two years. The grasses help clear and oxygenate waters — and provide vital habitat for young fish and crabs.
Ecologists are also concerned that another influx of freshwater could kill oysters and clams, which need some salinity in the water to survive and reproduce. There have already been reports of clams dying off in upper portions of the bay, said Allison Colden, Maryland fisheries scientist for the Chesapeake Bay Foundation.
The Chesapeake has posted its best grades ever on recent report cards evaluating water quality and ecology, though maintaining progress is expected to require more aggressive pollution-reduction policies and programs in the coming years.
Exelon officials said they expect Conowingo Dam operators to open six to 10 of the structure’s 50 gates Thursday, with the river’s flow expected to peak close to 200,000 cubic feet per second that night. They say their federal license governing operation of the hydroelectric dam, which spans the river about 10 miles upstream of the Chesapeake, requires them to keep water levels north of the dam within a range of 3 feet to maintain the Conowingo’s structural integrity.
That has been particularly challenging given rainfall in recent months across the Susquehanna’s 27,000-square-mile watershed across Pennsylvania and into parts of Maryland and New York.
Exelon and the dam itself have faced criticism for sending debris into the bay, in part because the company is suing Maryland over demands that it do more to trap pollution that builds up against the dam. But company officials maintain that they don’t produce the pollution, and they have been busier than ever working to remove the logs, water heaters, plastic barrels and even a jetski that have washed up along the Conowingo.
“The amount of debris that comes down the river is proportional to how much river flow we see,” said Brandon Commodore, the Conowingo’s operations manager.
The company says in a normal year, it uses a crane, a trash-collecting boat and manual labor to remove about 600 tons of debris from the dam. This year, it has collected 1,800 tons, officials said.
Bay scientists said they don’t blame the dam for the threat of setbacks to Chesapeake health, though. The real problems lie across Pennsylvania farm fields, and in the potency of storms, they said.
“The dam is just controlling how much of the water is coming. It’s not making it rain; it’s not producing nutrients,” Colden said. Impacts on the bay, she added, are “just going to depend on how quickly the rains fall.”