Accumulating River Sludge Threatens Bay

Harford Dam Could Become Too Full to Confine Muck

The Conowingo Dam generates hydroelectric power, but it also catches dirt flowing down the Susquehanna River. (Matthew S. Gunby -- AP)

CONOWINGO VILLAGE, Md. -- Just upstream from here, piled up for miles along the Susquehanna River bottom, is 200 million tons of environmental conundrum.

It's muck, for lack of a better word: dirt, coal dust and particles of manure brought down by the Susquehanna and trapped behind the massive Conowingo Dam. The muck has been building up here for more than 75 years, stopped just at the doorstep of the Chesapeake Bay.

Now, the gargantuan muck pile has become a hot topic among scientists trying to fix the bay. They worry that in the next two decades, it could fill all the space behind the dam, forcing any new sediment -- and the pollutants it contains -- straight through to the bay. Floods and hurricanes could exacerbate the problem, dumping huge amounts of sludge over the dam in a single stroke.

Many scientists have concluded that it would be better for the bay's fragile ecosystem if the muck was gone. But all their efforts keep coming back to the same problem: There's enough crud to fill the MCI Center 219 times.

"What do you do with it?" asked Jean Kapusnick, a civil engineer with the U.S. Army Corps of Engineers, which handles dredging at harbors nationwide.

The Conowingo, which generates hydroelectric power, is one of three major dams on the lower Susquehanna. The blockages have their environmental sins -- they hinder the migration of fish, for one -- but they do one thing that has made them some of the bay's best friends.

They catch dirt.

This happens because the dams cause the river to slow down and pool. As the current slackens, scientists say, much of the dirt in the water settles out. In all, scientists estimate, at least 55 percent of the Susquehanna's sediment is trapped in this way before it can make it to the Chesapeake.

But at some point, the buildup becomes a problem. Once the space behind the dam is filled, new material will get pushed through the dam's turbines or floodgates and head downstream.

"What comes down is going to go over, basically," said Mike Langland, a hydrologist with the U.S. Geological Survey.

If that happens at the Conowingo Dam, scientists say large increases in two key pollutants would flow down the Susquehanna to the bay.

The amount of dirt, which could bury oyster beds and block out sunlight needed by underwater grasses, could double.

And the levels of phosphorus, which feeds algae blooms that deplete the bay's oxygen, could go up by perhaps 70 percent, according to scientific estimates.

The first Susquehanna dam to hit its capacity was Holtwood, in Pennsylvania. It was built in 1910 and by 1920 could hold no more dirt.