The largest nutrient and sediment control device in the Chesapeake Bay watershed is likely to stop functioning in the next 15 to 20 years under current conditions, which could dramatically increase the amount of pollution to the Upper Bay.

The device is the Conowingo Dam, located 10 miles upstream from the mouth of the Susquehanna.

When the reservoir behind the 100-foot-high dam is filled, the millions of pounds of phosphorus and millions of tons of sediment now being trapped each year will reach the Upper Bay unless action is taken.

The latest study by the U.S. Geological Survey estimated that the reservoir behind the dam has about 30 million tons of remaining storage capacity, but it is being filled at an average rate of about 1.5 million to 2 million tons of sediment annually. Its total storage capacity is about 174 million tons.

"The reservoir continues to fill," said Mike Langland, a USGS scientist who resurveyed the sediment buildup last year. "There haven't been any real surprises as to where and how it is being filled."

The survey showed that sediment deposition in the lower part of the reservoir increased 8-10 feet compared with the last survey, which was completed in 1996.

Scientists have warned about the inevitable filling of the reservoir and its impact on the Bay since the early 1990s. But the problem has always been so far in the future-and so costly to deal with-that it has been largely ignored.

Now, it's a problem that can no longer be avoided. The states and the EPA have set 2025 as their new Bay cleanup deadline-a date that overlaps the projected filling of the reservoir.

The EPA is also requiring a cleanup plan, known as a Total Maximum Daily Load, to guide nutrient and sediment control efforts. The TMDL is a pollution budget that determines the maximum amount of pollution a water body can receive and still meet its water quality standards-essentially a pollution cap.

That means state implementation plans required as part of the TMDL will have to account for the reservoir filling to keep from exceeding the cap.

"We would expect to see a credible plan that looks at any major changes to major sources," said Jon Capacasa, water division director for EPA Region III.

The EPA expects states to update implementation plans every two years until the cleanup is complete.

"If the science is telling us that there is going to be a major change in the sediment dynamics behind the dams, I think a credible implementation plan will have to deal with that," Capacasa said. "If not in the initial plan, then in the updates over time."

It's a massive issue to resolve. 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 of the sediment moving downstream. (The dam traps little nitrogen because it is water soluble.)

In addition to sediment and phosphorus reductions already needed to meet Bay cleanup goals, a TMDL would require that all additional sediment and phosphorus be offset once the reservoir stops trapping it.

A filled reservoir also poses a greater threat to the Upper Bay in the event of another storm like Hurricane Agnes, which in 1972 scoured huge amounts of sediment from behind the dam and smothered much of the Upper Bay in several inches of sediment, obliterating underwater grass beds and other habitats.

A similar storm today would flush even more material downstream because of the additional sediment buildup. "There is more there to be scoured, and it is probably in areas that would be more rapidly scoured-closer to the dam," Langland said.

The Army Corps of Engineers' Baltimore District this year got $57,000 to begin looking at the reservoir issue. Funding for next year is uncertain. The Senate has proposed another $200,000 in next year's federal budget, although the House included no money in its legislation.

Those funds, though, would only set the framework for a study. A full-blown review by the Corps would likely cost a few million dollars, and take two to three years, said Dan Bierly, who oversees studies for the Baltimore District. It would also require a non-federal "sponsor"-such as a state agency, river basin commission or nonprofit organization-to share the cost. The sponsor will likely share 25 percent of the study costs, though the entire amount can be in-kind services or a combination of services and cash.

A full study would likely look at the practicality of controlling upstream sources of sediment, dredging sediment from the reservoir to increase capacity, and sediment impacts on the Bay downstream, Bierly said. In addition, the sediment needs to be examined for possible contaminants.

"It may be very appropriate to determine impact and potential benefits before anyone really wants to spend the big bucks," Bierly said. Dredging sediment from the reservoir, for instance, could cost tens of millions of dollars.

But the cost of doing nothing could be huge, said Michael Helfrick, the Lower Susquehanna Riverkeeper, who called for a study that would assess the likely consequences for the Bay in the event of another catastrophic storm such as Agnes.

"Until we have a document that tells us how much we have to lose, it is going to be hard to get anybody to pay to not lose it, because the amount we have to pay is outrageous," Helfrick said. "But it may not look that outrageous when you look at the cost of losing everything."

While people search for solutions, Langland said more efforts should be made to promote actions that prevent erosion along streams in the watershed. If the amount of sediment is reduced, it will buy more storage time for the reservoir.

"You're not only improving the capacity of the reservoirs," Langland said, "but you are also improving the quality of the water upstream."

The filling of a reservoir is not necessarily visible to most people. Sediment builds up on the bottom, and once a reservoir is filled, currents prevent more from settling there.