For 82 years the polluted sediment has been piling up in the 14-mile-long reservoir behind the Conowingo Dam in Harford County Maryland.
Now, scientists say its full.
The implications for the Chesapeake Bay are serious. Sediment is a major pollutant. It smothers oyster beds and carries excess nutrients that cause algae outbreaks that kill fish and destroy water quality.
After decades of trapping polluted sediment at the dam, scientists say its now simply passing through.
The Conowingo dam is the last stop on the Susquehanna River before it pours all its power and pollution into the top of the Chesapeake Bay in northeastern Maryland.
The river is badly polluted during its journey through upstate New York and Pennsylvania on its way to the bay.
Maryland Governor Larry Hogan has proposed a small-scale test project to dredge about 25,000 cubic yards of sediment from behind the dam.
Fully dredging the reservoir would require as much as $3 billion dollars, according to the US Army Corps of Engineers. There is enough sediment in the 14-mile reservoir behind the dam to fill 7.7 million dump trucks, by one estimate.
The Chesapeake Bay Foundation is among the environmental groups pointing out that the reservoir is bound to fill with sediment again if it is dredged. The groups believe in the long run, the money would be better spent by reducing the pollution load in the Susquehanna to start with.
The dam is owned by Exelon energy which uses it to generate hydroelectric power. Hogan believes Exelon should contribute to dredging the reservoir. The issue comes as Exelon is applying for permits from Maryland to continue operating the dam for another 46 years.
Excelon issued a statement saying: "...protecting the vitality of the Bay is a multi-stakeholder, multi-state issue, and we continue to work with all parties, including Gov. Hogan and his administration, to ensure the Lower Susquehanna River retains its important environmental and recreational benefits."