Runoffs, trapped pollutants imperil famed estuary

CONOWINGO, Md. — When the Conowingo Dam opened to fanfare nearly a century ago, the wall of concrete and steel began its job harnessing water power in northern Maryland. It also quietly provided a side benefit: trapping sediment and silt before it could flow miles downstream and pollute the Chesapeake Bay, the nation’s largest estuary.

The old hydroelectric dam spanning the lower Susquehanna River is still ably producing power, but its days of trapping sediment in a 14-mile-long reservoir behind its walls are over. Behind the 94-foot-high barrier lies a massive inventory of coal-black muck — some 200 million tons of pollutants picked up over decades from farmlands, industrial zones and towns.

With Maryland pushing to curb pollution in dam discharges, the issue has become a political football as Conowingo’s operator seeks to renew its federal license to operate the dam for 46 more years after its old license expired.

The estuary famed for its blue crabs and oysters has been rebounding under a federal cleanup program launched in 1983 that put an end to unbridled pollution. But the 200-mile-long bay is being ravaged by runoff-triggering downpours, including record-setting rainfall in 2018 and this year’s soggy spring.

Intense cycles of downpours are washing pollutants into the Chesapeake from municipal sewer overflows, subdivisions and farms.

Maryland politicians and watermen who make their living off the bay’s bounty portray the sediment stored behind the Conowingo as an existential threat.

But bay-area scientists say nutrient pollution, not sediment, is the major threat, noting that most of the pollutants flowing into the Chesapeake come from upstream, particularly in Pennsylvania.