. . . the new report concludes that Conowingo and two upstream dams, Safe Harbor and Holtwood, “are no longer trapping sediment and the associated nutrients over the long term.” Instead, the dams delay a portion of the sediment and nutrients coming down the river during dry years, only to have that material flushed into the Bay during years with higher than average rainfall.It's astonishing, and somewhat maddening to me that this is even regarded as news. When I arrived in the Chesapeake Bay region as a sort of young scientist in 1985, at the first regional conference I attended, the head of the EPA Bay Program at the time said that the biggest threat looming to the Bay was the filling of the pool behind the dam with sediment.
Dealing with the Conowingo issue is one of the key issues for consideration in a midpoint assessment under way of the cleanup plan, formally known as the Chesapeake Bay Total Maximum Daily Load, or TMDL. The plan established nutrient and sediment reduction goals for each state.
The midpoint assessment, conducted by officials from the Environmental Protection Agency and all of the states in the watershed, will use information gained since 2010 to update and revise goals through 2025, when all needed cleanup actions are to be in place. “This report gives us even better clarity into the water quality impacts of pollutants that flow through the dam,” the EPA said in a statement.
But the report also indicates that the filling of the reservoirs will make attaining the 2025 cleanup goal more difficult.
Much of the public has focused on the impact of sediment from the dam, Extreme events like Tropical Storm Lee in 2011 can “scour” built-up sediment from behind the dam, resulting in stunning images of brown sediment-laden plumes stretching far down the Bay.
But the report said the greatest threat to Chesapeake water quality comes not from sediment scoured during large events, but rather from nutrients coming down the Susquehanna that are no longer being trapped. When they reach the Bay, those nutrients spur algae blooms that cloud the water and — when they die — rob it of oxygen needed by aquatic life and contribute to summertime dead zones.
Even if all currently planned nutrient control efforts are in place by the 2025 deadline, the report found that parts of the mid-Bay — the area with the most severe “dead zone” — would not meet water quality goals because of the dam’s diminished nutrient-trapping capacity.
Computer modeling done for the report showed that meeting water quality goals without the dam's help would require an additional 2.4 million pounds of nitrogen reductions and an extra 270,000 pounds of phosphorus reductions from the Susquehanna each year.
That’s problematic because Pennsylvania —which supplies the vast majority of nutrients in the Susquehanna basin — is also lagging far behind in meeting its Bay cleanup goals, making additional nitrogen and phosphorus reductions from the river even more difficult.
More recently, this has become an issue between the EPA Bay Program and bay area farmers, as farmers emphasized the threat from Conowingo and down play their own contributions, while the does the opposite. The farmers may yet regret that as EPA tries to take the dams share of the problems out of their hides.
I'd like to see a study of what would happen if they blew up, breached the dam, and allowed the Susquehanna to run free through the upper Bay. Certainly, there would be an initial pulse of mud and nutrients, but the Bay might be better off in the long run.