In September 2011, Tropical Storm Lee caused flooding in the mid-Atlantic region. The Susquehanna River rose far above its banks, causing disruptive floods in Pennsylvania and Maryland. Near the river’s mouth into Chesapeake Bay, massive flooding threatened to breach the 572-megawatt Conowingo Dam.
With its flood gates wide open, the dam survived the flooding. At peak flows, about 7 million gallons flowed through the dam every minute. That water transported millions of tons of sediment from the Susquehanna watershed out into the bay, along with large amounts of trash and debris.
The impacts of the flood are still being assessed. Under typical operations, the dam builds up about 2 million tons of sediment every year, or about two-thirds of the Susquehanna River's total sediment burden. (Compare the dams currently being removed from the Elwha River in Washington, which had trapped an estimated 24 million cubic yards of sediment.) Overall, four dams on the Susquehanna might hold up to 280 million tons of sediment.
While Lee removed several years' worth of sediment from the Conowingo Dam, more sediment builds up every year. The Army Corps is concerned that the Susquehanna River dams have nearly reached their full capacity to hold sediment, and is launching a project to study what could be done, such as sediment dredging or remediation.