LAST YEAR, THE FEDERAL GOVERNMENT DIRECTED the communities surrounding the Chesapeake Bay to take on a difficult challenge: improve the Bay's water quality by reducing the overabundance of sediments and nutrients flowing into it by 2025. But that might prove to be an especially big challenge for residents of Pennsylvania and New York if policy makers don't figure out what to do about the Conowingo Dam on the Susquehanna River.

The Conowingo hydroelectric dam, located ten miles upriver from where the Susquehanna meets the Bay, currently offers a boon to the Bay's ecology, but also a potential threat. The beneficial part is that under normal weather conditions, the dam traps more than half the sediment reaching it — sand, silt, and clay washed into the river from fields and construction sites upriver. The trapped sediment would otherwise end up in the Bay.

The downside of the dam, which was built in 1928 and has been accumulating deposits all these years, is that sediment could completely fill its reservoir within 15 years. After that, all of the sediment reaching the dam would flow through it downstream, more than doubling the current amount reaching the Chesapeake and far exceeding the planned reduction in sediment loads. Also worrisome are findings that the more sediment trapped behind the dam, the larger the volume scooped up and sent toward the Bay during large storms.

Like the dam, the sediment results in two kinds of impacts on the Bay, one benign and another alarming. Too much fine-grain sediment suspended in the water can choke off the growth of the submerged aquatic vegetation that improves water quality and provides habitat for fish. But the Bay's marshes need coarser sediments to survive.

Photograph: Ed Rybczynski

So, in theory, pumping some of the larger-grained sediments from the reservoir might yield two benefits at once: increasing the dam's storage capacity and helping marshes downstream. A new study, begun in 2011, will examine that and other ideas for dealing with the Conowingo conundrum.

"Everyone's concerned that we're spending all this time and effort working out how to control nonpoint sources of sediments and nutrients from flowing into the Bay," says Anna Compton, study manager for the U.S. Army Corps of Engineers, which will conduct the study. "But the big unknown remains the sediments trapped behind the dams in the Susquehanna River."

The Susquehanna is the Bay's largest tributary, and one of its largest sources of sediment. At 464 miles, it is also the longest river on the East Coast that drains into the Atlantic Ocean, and one of the most flood-prone.

The nearly-mile-wide Conowingo Dam, in Cecil County, Maryland, is the southernmost and largest hydroelectric dam on the river. Two smaller dams upriver, Holtwood and Safe Harbor, will also be included in the new Corps of Engineers study. The smaller dams' reservoirs have already completely filled with sediment, leaving the Conowingo as the last manmade stopping point for sediment headed toward the Bay.

It's difficult to paint the Conowingo as an environmental threat when one considers the green energy it generates: those 572 megawatts provide enough power for nearly a half-million homes, with no emission of greenhouse gases.

Then there's the sediment trapped behind the dam. Each year more than 3 million tons of sediment reach the dam from upriver, and the dam captures close to 2 million tons, according to estimates by the U.S. Geological Survey. A figure of nearly 2 million tons can be difficult to grasp, but according to estimates by the Susquehanna River Basin Commission, the material would annually fill more than 22,000 railroad hopper cars.

The estimate for when the Conowingo reservoir will fill, according to a 2009 Geological Survey report, is 15 to 20 years. That lifespan could be extended by another five to ten years if people upriver cut the total amount of sediment reaching the dam by about 20 percent. That's the same amount of reduction, as it turns out, that the U.S. Environmental Protection Agency set in 2010 for the entire Chesapeake in its "pollution-diet" plan for reducing sediment and nutrients.

The capacity could be further stretched out, the estimate said, if the Susquehanna gets hit with more big storms. High water flows in the river scour out sediment from the Conowingo's reservoir at predictable rates. For example, rain from Tropical Storm Lee, which socked the region in September, dug out 4 million tons, the Geological Survey estimates. That bought the equivalent of another two years of storage capacity, but at a price to the environment: the pulse of sediment, flowing over just a few days, was larger than what the upper Bay normally receives in an entire year.

Once the dam's reservoir reaches capacity, Conowingo's role as a friend of the Chesapeake could diminish or end. The flow of sediment past the dam would more than double to some 3.3 million tons annually. (Increased amounts of nitrogen and phosphorus would also move downstream.) The 1.8-million-ton increase in sediment would far surpass the magnitude of the decrease that Pennsylvania is required to achieve under the EPA sediment-reduction plan.

This target is based on the current amount of sediment flowing from the Susquehanna; the plan does not include any exception to the sediment limit if the Conowingo's reservoir fills up. As a result, if the capacity runs out, Pennsylvania would be required to reduce sediments in the river by much more than is called for under the current EPA plan, by greater than 60 percent.

Experts agree this could be extremely difficult to achieve. Reducing sediment would require a variety of land-use restrictions and conservation measures across the Susquehanna's 27,500-square-mile watershed — for example, containing soil that washes off farms and construction sites into streams and storm drains during storms. Residents and elected officials in Pennsylvania and other states have already complained that a 20-percent reduction in sediment looks prohibitively expensive.

That's why federal and state agencies agreed this year that it was time to look for solutions to the concerns raised about the Conowingo Dam.

The Conowingo Dam, located five miles below the Pennsylvania-Maryland border, is the last dam on the Susquehanna River before it empties into the Chesapeake Bay. The dam's original storage space has shrunk over the years since it was built, as shown in the graph above. The remaining capacity could run out by 2024. Source: Graph, Michael Langland, U.S. Geological Survey; map, redrawn from a U.S. Geological Survey map.

The Corps of Engineers announced in September 2011 that it was starting a three-year, $1.4 million study to review possible solutions and their possible costs. Partners include the state of Maryland, the Susquehanna River Basin Commission, and the nonprofit Nature Conservancy.

The study is intended to provide a more detailed and accurate understanding of sediment flow in the Susquehanna and the role of the three dams in storing the material. The project is expected to provide analysis and estimated cost ranges for management options other than dredging.

The new analysis will differ in several ways from previous studies of the Conowingo. It's the first by the Corps of Engineers, a national leader in projects to dredge sediments. A key player in the study will be the Corps Army Engineer Research and Development Center in Vicksburg, Mississippi, which is known for its expertise in studying the transport of sediments down the Mississippi River to its delta.

It's also the first study to examine how the upper Chesapeake's ecology could be affected by varying quantities of sediment delivered from the dams. The Corps will use a model of the Bay's water quality developed by the Vicksburg lab and used by the EPA's Chesapeake Bay Program Office to calculate the sediment limits.

Besides reexamining dredging, the study will also consider, among other options, a method called sluicing. Sediment could be funneled past the dam in a pipe, but in a controlled procedure meant to minimize the impact on the upper Bay's ecology. Of key importance, the work might occur only during the winter months, when aquatic vegetation is dormant and fish are not spawning.

The sluicing approach has attracted interest from researchers who think it could benefit the Bay's ecosystem. The upper Chesapeake is starved for the kind of sandy, large-grain sediment that makes up much of the material trapped behind the dams, says Michael Langland of the U.S. Geological Survey, who has estimated the dam's remaining storage capacity. The sand deficit resulted because different-sized sediments behave in different ways. Sandy sediments fall to the bottom relatively quickly in rivers, water doesn't carry them as far, and they are more easily trapped behind dams. However, much of the sediment flowing through the dam consists of smaller particles like silt and clay, which settle more slowly and so are carried farther.

The construction of the Conowingo largely blocked the natural movement of the bigger sediments down the Susquehanna and into the Bay. Marshes require fresh supplies of sandy sediment to replace their own stores that are eroded naturally by waves. They need this sandy material to keep pace with sea-level rise in coming years.

Neither sluicing nor the other options under study by the Corps of Engineers is guaranteed to extend indefinitely the Conowingo's capacity to store sediments, Compton says. But some measures could at least extend its capacity by a few additional years, buying more time for policy makers and people living in the Susquehanna watershed to reduce the sediment load.

A question mark about the Corps of Engineers study is whether Congress will finance the full $1.4 million price tag. So far it has provided only $250,000. However, the project is included in the Corps budget for 2012, and Compton says that the Corps has placed a high priority on it.

Financing the study may prove far simpler than funding any solutions it recommends. Who will pay, and how much? The answer remains to be determined.

The Susquehanna River Basin Commission estimated in 1995 the cost to dredge only the 2 million tons of sediment that is newly trapped behind the dam each year: $48 million, in 2010 dollars. That figure did not include disposing of the material or removing any of the more than 174 million tons that have already accumulated in the reservoir.

For its part, the dam's owner, Exelon Corporation, has been noncommittal on whether it would share in the cost of a fix. In a public document, corporation officials did call dredging "very unlikely" to be viable because of costs and other factors. And the corporation has said that it "cannot bear primary responsibility (both in terms of costs and resources) for addressing the adverse impacts of others," impacts like sediment entering the Susquehanna above the dam.

"[In the river's watershed], there's a lot of people on that land contributing to that 3 million tons, and we're going to need everybody to address that," says Mary Helen Marsh, Exelon's director of environmental operations. "It's a watershed issue; it's not a Conowingo Dam issue."

Exelon's license to operate the dam expires in 2014, and sediment could play a role in the license renewal process now underway. The Federal Energy Regulatory Commission, which controls the license, asked the corporation in 2010 to develop a plan for managing the sediment.

The Corps of Engineers might participate in financing a fix for the sediment problem, Compton says.

If the Conowingo's reservoir is allowed to fill completely and the target level for sediment downstream is exceeded, the consequences could be dire. The EPA has threatened tough enforcement actions, like requiring local governments to adopt and impose new land-use restrictions to control sediment.

Because the Conowingo sits in Maryland but is affected by residents living upriver in other states, finding an answer to the sediment puzzle will likely require the regional cooperation and leadership that have proven difficult in some efforts to clean up the Bay. The Susquehanna River Basin Commission called attention to sediments in major reports in 1995 and 2002. But action didn't follow, and the reservoir has continued to fill with sediment.

"It seems like there's enough interest to get a study and then as soon as it's complete, the study just drops off, because there's not another mechanism to pick it up," says Michael Langland of the Geological Survey. "There just seems to be at times a lack of somebody to step up and pursue the next step."