One of the most effective pollution control devices in the Bay watershed is the 100-foot-high Conowingo Dam on the Susquehanna River.

On average, the dam traps about 3.5 million pounds of phosphorus and 2 million tons of dirt every year-about a third of the phosphorus and more than half the sediment heading downstream.

But the reservoir behind the dam has slowly been filling since it was completed in 1928. Once at capacity, the amount of phosphorus and sediment reaching the Bay-just 12 miles downstream-will rise sharply.

The last study of sediment behind Conowingo was completed in 1996. Based on those findings, scientists estimated the reservoir's trapping capacity would be filled around 2025.

At the Chesapeake Executive Council meeting in December, Pennsylvania Gov. Ed Rendell pledged to support a study taking a new look at the dam because of its potential to be a major problem in the future.

"It's been 12 years since that type of analysis has been done," said Tom Beauduy, deputy director of the Susquehanna River Basin Commission, which has long supported further study at Conowingo.

"Everyone who has a stake in this issue has been working from assumptions that were developed as a result of studies in the 1990s," Beauduy said. "It is time to verify what has happened in the interim with storage behind the dams so those assumptions can be either validated or refined."

Already, when flows hit 400,000 cubic feet per second-something that happens every few years on the Susquehanna-the river actually scours sediment from behind the dam, sending a brown wave downstream into the Bay.

In extreme events, such as Tropical Storm Agnes in 1972, huge amounts of sediment can be gouged from the reservoir, with devastating results.

A study proposed by Mike Langland of the U.S. Geological Survey would use bathymetry to remap the sediment behind the reservoir and compare results to what scientists found in 1996.

That would reveal whether the reservoir was filling faster, or more slowly, than thought.

Exactly what officials would do with that information is a big question mark.

The Army Corps of Engineers estimates it would cost about $2.5 million just to explore sediment management options. Actually trying to dredge and remove the accumulated material could be hugely expensive, easily costing tens of millions of dollars.

As a result, some question whether that would be the best use of limited Bay cleanup funds.

Harry Campbell, a scientist with the Chesapeake Bay Foundation's Pennsylvania Office, said a more effective use of that money is working to control and stabilize eroding sediment in streams before it gets to the dam.

"It would be so cost-intensive to do that work [behind the dam], we generally have felt that it is more beneficial, holistically, to look at our small streams and tributaries and addressing those as a priority," he said.

On the other hand, Michael Helfrich, the Lower Susquehanna Riverkeeper, said prevention efforts alone will not keep the Conowingo reservoir from filling because so much eroded sediment is already in streams and working its way toward the dam.

"I'm 100 percent in favor of fixing the problems upstream, but we can't fix that problem fast enough," he said. "I'm not saying to perpetually redredge the dams for the rest of eternity. I'm saying we possibly may need to dredge it to gain some time so we can do the work upstream that we need to do."

Langland, of the U.S. Geological Survey, said the impact to the Bay depends on the type of sediment making it past the dam. While heavier particles such as sand are effectively trapped at Conowingo, a larger proportion of finer material, such as silt and clay particles, pass through the dam.

The finer material is the bigger problem for the Bay. It travels farther down the Bay and is more responsible for clouding the water.

But some fine material is trapped behind the dam, Langland said, and during high-flow events, those materials may be disproportionately eroded and sent into the Bay.

He said that it's possible a targeted solution could be found, such as identifying and dredging only areas of the reservoir with large amounts of fine particles. But such action would likely still be costly-and tricky, he said, as fine particles would be easily stirred during dredging activity.

"Usually when you dredge, you dredge sands, you usually don't go after silt," he said.

Beauduy said this year's study would be a starting point for further discussions about what-if anything-to do to manage an issue some have likened to a '"ticking time bomb."

"That study is far less expensive than some of the other options that have been discussed," he said. "but it may be very helpful in informing our management decisions moving forward."