Since the early 1990s, scientists have warned that the Conowingo Dam loomed as an ominous threat to the Chesapeake. When the reservoir behind the massive 100-foot dam filled, more sediment and nutrients would begin pouring down the Susquehanna River.

For nearly as long, dealing with the issue has been largely put off; the reservoir issue has always been considered a problem for the future.

But the future may be here, according to new research.

“It’s not a decade out,” said Bob Hirsch, a research hydrologist with the U.S. Geological Survey. “It’s now.”

To be sure, the giant dam, located near the Maryland-Pennsylvania border 10 miles upstream from the river’s mouth, is still trapping much of what washes down the Bay’s largest tributary. But it appears to be trapping less than it used to, particularly during high flows, according to Hirsch.

Hirsch’s analyses, presented at a recent Chesapeake Bay Program scientific meeting, suggests that more sediment and phosphorus have been reaching the Chesapeake over the last decade as the reservoir behind the dam has neared its storage capacity.

If correct, that may means that portions of Pennsylvania, New York and Maryland upstream from the dam may need to take additional steps to control sediment and phosphorus runoff to achieve the Chesapeake Bay Total Maximum Daily Load, or “pollution diet.” The TMDL established the maximum amount of nutrients and sediment that can enter the Bay from each major tributary.

Scientists estimated in the past that the 12-mile-long reservoir traps about two-thirds of the sediment coming down the river, along with more than a third of the phosphorus — which is often attached to sediment particles.

The storage isn’t always permanent. During extreme storms, fast-flowing water eats into the built-up sediment and carries it, along with the associated phosphorus, beyond the dam and into the Bay where both degrade water quality.

Sediment clouds the water and smothers habitat for clams, oysters and other bottom-dwelling creatures. Phosphorus feeds algae blooms, which also reduce water clarity and use up oxygen needed by fish and other aquatic species. (Relatively little nitrogen, another nutrient which degrades water quality, is stored behind the dam as nitrogen is more water- soluble than phosphorus.)

Scientists previously estimated that such scouring events take place when river flows reached 400,000 cubic feet per second. Such events are relatively rare: In the past 44 years, flows reached that level 11 times, or an average of once every four years.

But a review of USGS monitoring data by Hirsch and colleagues indicates that that has changed during the last decade. As the reservoir has continued filling, scouring may be taking place during lower river flows, perhaps between 200,000 and 300,000 cubic feet per second.

Those events happen more frequently. Flows at Conowingo exceeded 300,000 cfs 22 times, or once every two years, in the last 44 years, and they exceeded 200,000 cfs 60 times, an average of once every 270 days.

That might explain a reversal in phosphorus trends on the river. A steady decline in the phosphorus concentrations monitored at the dam halted in the mid-to-late 1990s, and concentrations have slowly risen the last decade.

Hirsch said that increased scouring of sediment stored behind the dam during high flows likely explain that increase. An analysis of water quality monitoring data indicates that phosphorus concentrations have declined during low and moderate flows, but have increased during high flows.

“My guess it is has a lot to do with this process of filling up the reservoir,” he said.

As the reservoir fills, its depth is reduced, which causes the velocity of the water to increase for any given amount of flow. This means that velocities capable of scouring the sediment occur more frequently than they had in past decades. The net result is that runoff control efforts are increasingly being overshadowed by material scoured from behind the dam.

In addition, extreme high-flow events — those of more than 400,000 cfs — are carrying sediment and phosphorus concentrations significantly higher than those seen in the past, an indication that high flows are scouring more material than they used to.

In 2004, a phosphorus concentration of 1.17 milligrams per liter of water was observed during an extreme event. That was so much higher than concentrations seen during other high-flow events since 1978 — which never exceeded 0.4 mg/l — that scientists initially thought it was an error. But that mark was exceeded twice last year, once during March, and again after Tropical Storm Lee in September when concentrations hit an unprecedented 2.3 mg/l.

“Clearly, there is a change in the way this system is behaving,” Hirsch said.

Those figures may understate the high concentrations that happen during floods. Because monitoring at Conowingo Dam becomes unsafe during extreme events, the monitoring data don’t include samples taken at the very highest flows, Hirsch said.

When built in 1928, the reservoir behind the dam had about 152,000 acre-feet of storage capacity. USGS surveys indicate that only about 6,000 acre-feet of capacity is left, said Mike Langland, a USGS scientist who has routinely resurveyed the sediment in the reservoir the last two decades. (An acre-foot is one acre covered to a depth of one-foot, or 43,560 cubic feet.)

Langland said that the reservoir would never fill to the point where the dam cannot produce electricity. Rather, it would reach an equilibrium where roughly the same amount of sediment passes the dam as is trapped — something that has already happened at two smaller upstream dams, Holtwood and Safe Harbor. “The data suggests things are starting to approach this equilibrium, faster than perhaps was suggested,” Langland said. “Functionally, we are definitely approaching the final phase.”

The huge price tag associated with removing stored sediment has complicated dealing with the pending problem. Past studies have suggested it could cost tens of millions of dollars annually just to keep pace with what’s washing in.

Last September, the Army Corps of Engineers, Maryland Departments of Natural Resources and of the Environment, the Susquehanna River Basin Commission and The Nature Conservancy launched a three-year study to devise solutions to the problem and evaluate potential options for extending the sediment storage capacity behind the dam, including potential beneficial uses of the stored sediment.

Bruce Michael, director of the Maryland Department of Natural Resources’ Resource Assessment Service and a member of the study team, said the new analysis adds urgency to that work.

“There was kind of a perception historically that we have 15 or 20 years of storage capacity left behind the dam before we see dramatic increases in sediments and nutrients entering the Bay, and then we’ve really got to do something,” Michael said. “And that is not the case. As it fills up, we are starting to see a greater negative impact on water quality, underwater grasses and living resources when we have these storm events that generate scouring behind the dam. We are starting to see a lot of sediment and phosphorus being released into the Bay.”

If studies confirm that more sediment and phosphorus are reaching the Bay than previously thought, Pennsylvania and New York, both of which are upstream of the dam, along with a small portion of Maryland, may need to take additional actions to meet their existing phosphorus and sediment targets, said Rich Batiuk, associate director for science with the EPA Bay Program Office.

The TMDL established a maximum sediment and phosphorus “load” that could enter the Bay each year without degrading water quality. That figure would not change. But if the dam is starting to lose its storage capacity, it would mean more control actions are needed upstream to offset the difference, Batiuk said.
“We don’t want to get behind the curve with regard to the rate of phosphorus and sediment pollution load reductions,” Batiuk said. “The situation with the dam is only going to get worse as it continues to lose its pollutant trapping capacity.”

Earlier this year, the watershed states submitted two-year milestones that spelled out specific pollution reduction and prevention actions they would take this year and next. Batiuk said the EPA could reset the milestones to require Pennsylvania and New York to account for the increased pollutant loads coming over the dam and begin planning for how to address those increases.

The TMDL, which was released in December 2010, had assumed “the current trapping capacity will continue through the planning horizon for the TMDL” which is 2025. But it also stated that adjustments would be needed if the situation changed.
“We were thinking we had another five, 10 or more years to address this, but these new scientific findings tell us it’s happening now,” Batiuk said.