Environmental Science

When the Conowingo Dam opened to fanfare nearly a century ago, the massive 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 effectively 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...