As Conowingo Dam fills with runoff, it loses its ability to filter chemicals from the water that eventually flows to the Chesapeake

For most of the past 90 years, the Conowingo Dam has served a purpose not envisioned by its builders. The dam bestrides the Susquehanna River just south of the Maryland-Pennsylvania line and was completed in 1928 as a hydroelectric and flood control project. But the 9,000-acre reservoir it created has also acted as a sort of treatment process for water entering Chesapeake Bay.

When excess fertilizer from farms and communities in the Susquehanna watershed washes into the river, much of the fertilizer's nitrogen dissolves and moves with the water through the dams. But most of the phosphorus stays with silt in the runoff, and the dam retains a portion of this silt, thus keeping the phosphorus out of the Chesapeake's waters. This matters because whatever does reach the bay continues to act as a fertilizer there, promoting growth of algae. As the algae die, they decay and consume oxygen, creating large dead zones.

This sort of sequestration has been a valuable weapon in the federal and six-state effort to restore the health of the bay. But now there's a problem: The accumulated sediments behind the dam have already affected its performance to an extent not anticipated to occur until a decade or two later. Scientists who have studied the situation—including William P. Ball, a professor of environmental health and engineering in the Whiting School of Engineering—believe Conowingo may have effectively reached a state that's long been recognized for two other upstream Susquehanna dams—"dynamic equilibrium." That is, so much silt has filled in behind the dams that the average amount that flows through the gates over a decade is now roughly equal to the amount that comes into the reservoir, with no additional net settling to the bottom. Evidence suggests the amount of removal may be reduced even at normal water flow and that "scouring" of stored deposits—more on that in a moment—is occurring more frequently and to a greater extent in the most recent decade. This is a big deal, since the Susquehanna accounts for roughly 45 to 50 percent of the fresh water that enters the bay.

At the heart of the issue are "scouring events," storms big enough to send so much water down the Susquehanna that it churns up the silt from the bottom of the reservoir and sends it through the dam and into the Chesapeake. Although the biggest effect of these storms has always been extensive upstream erosion that will not settle under such turbulent conditions, the additional scoured solids from the reservoir are also a concern. Ball, who is the director of the Chesapeake Research Consortium, says the organization has decided to model the reservoir as if the Conowingo Dam was no longer there. "To the best of our ability to tell, the dam is definitely losing its performance and pretty darned close to a point where the 12-year average removal of silt and phosphorus is zero," he says. "In other words, it might remove it for five years, but then a year comes along where everything it has removed is lost."

Scientists knew this situation would arise; all dams trap silt and eventually lose their trapping capacity. But on the basis of evidence available at the time, bay managers assumed in 2010 that the problem would not become acute for another 10 to 15 years. Researchers have updated their statistical tools, however, and it's these improved tools that alerted them to how rapidly the reservoir's performance has been changing.

The obvious question is what's to be done so that the substantial progress in the restoration of the bay is not hampered by new and uncontrolled quantities of silt and phosphorus getting past the dam. Maryland Gov. Larry Hogan announced in August that the state would be accepting bids to test whether removing the silt from the reservoir might be effective. "[Some] counties are saying, 'Oh, they should just dredge the dam because the dam is suddenly spewing all this crap out,'" Ball notes. "'We just dredge it back to the way it used to be and life will be good again.' But that costs billions and billions of dollars." (An assessment prepared last year by several groups, including the U.S. Geological Survey, estimated the cost of dredging to sufficiently reduce the amount of sediment flowing into the bay would be between $1.5 billion and $3.5 billion.) "And 70 years after that, actually much less since the 'sufficient' dredging still does not mean complete dredging, the situation will be back to where it started."

Ball adds, "So in the view of many of us, you get so much more by using more sustainable practices upstream in agricultural as well as suburban and urban development." For example, more efforts at restoring riparian "buffer zones" of forested land and wetlands would help keep agricultural runoff from ever reaching the Susquehanna. Better handling of human and animal wastes and effective recycling of nitrogen and phosphorus would help prevent excess nutrients from reaching the reservoir in the first place.