Michael Helfrich stands near a wall of weather-beaten concrete 10 stories tall and nearly a mile long that holds back the force of the Susquehanna River – the largest source of fresh water in the Chesapeake Bay.
Helfrich, the Lower Susquehanna Riverkeeper, explains that the Conowingo hydroelectric dam, built in 1929, has been both a curse and a blessing to the nation’s largest estuary. It blocks the passage of migratory fish upstream. But until recently, it has also been blocking about half of the soil, fertilizer and other heavy pollutants washed by rain from Pennsylvania farms and towns down into the Bay.
"The dam has accumulated about 185 million tons of sediment and pollution that otherwise would have entered the bay," Helfrich said.
Suddenly, as he spoke, a siren sounded beside the dam. "Luckily, we’re not down by the river, because there’s the alarm saying that they are going to open some more turbines and the water is going to come up," he said, as a frothing surge of water boiled and grew near the base of the dam. “That siren is the warning."
Alarms have been going off all over Maryland because of the Conowingo Dam. Some have called it a pollution "time bomb" that could rattle bay cleanup plans because the Conowingo Reservoir, behind the dam, is now just about full with sediment. The dam's days as a pollution filter are done. And so now major storms scour millions of tons of sediment – loaded with phosphorus fertilizer, as well as more exotic chemicals-- and flush them over the dam down into the bay.