Conowingo Dam: A liability for the Chesapeake?

For an earlier story about the challenge of cleaning up the Chesapeake Bay, I talked to Michael Helfrich, the Lower Susquehanna riverkeeper. Among the many problems facing the bay, he called the load of sediment stalled behind the Conowingo Dam one of the many elephants in the bay that officials aren't paying enough attention to.

The dam is on the Susquehanna River just upriver of Havre de Grace, Md.  Built in 1928, it means that one of the last acts of the Susquehanna and all its tributaries is to generate electricity.

But the dam also slows the flow of the river, and sediment settles out -- about two million tons a year, one of the bay's three big pollutants. Another million tons of sediment flows over the top.

The U.S. Geological Survey study has found that the dam will reach its sediment capacity in 15 to 20 years. So  at that point, two million more tons of sediment a year will course downriver and into the bay.

Storms could buy more time for the reservoirs by rinsing out the sediment, but that's a problem, too.

In 1972, during a four-day downpour brought by Hurricane Agnes, the raging Susquehanna scoured the dam impoundments and carried years' worth of additional sediment downriver.

When it got to the bay, Helfrich said in the earlier story, "it smothered everything. " It killed crabs. It killed bay grasses. "It was the most damaging event in the written history of the bay. "

It could happen again. In mock tribute, one of his colleagues has nicknamed Conowingo "Katrina-wingo. "

Or, now, how about Lee-wingo? A Washington Post story by Darryl Fears, published Sunday, says that during Tropical Storm Lee, officials had to open the floodgates, and four million tons of sediment rushed through in about four days.

It showed satellite images in which the bay's blue-green water looks instead like coffee with cream.

Fears reports that environmentalists are saying the sediment dump during Lee could spawn another mammoth, oxygen-depleted "dead zone" in the bay, much like last summer's.

What to do? Officials are looking at conservation measures such as buffer plantings, cover crops and other devices on land that could absorb the flow of stormwater, which erodes soil and carries it away as sediment.