For years, the sediment building up behind Conowingo Dam has been referred to as a ticking bomb — one day in the future the reservoir behind the 100-foot dam would fill and huge amounts of sediment and nutrients would flow, unfettered, into the Chesapeake Bay.

A recently released draft study suggests that day is now here. The reservoir is essentially filled, increasing the flow of sediment and nutrients from the Susquehanna River into the Bay, though the results may not be as devastating as what was once thought.

That added pollution — primarily the nutrients — would keep portions of the Upper Chesapeake from achieving cleanup goals, likely forcing states to make additional nutrient reductions to make up the difference, according to the draft Lower Susquehanna River Watershed Assessment.

The study, which cost $1.4 million and was led by the U.S. Army Corps of Engineers and the Maryland Department of the Environment, also concluded that dredging built-up sediment from behind the dam would have huge costs and provide little water quality improvement. Dredging, the report summed up, “yields minimal, short-lived benefits at high costs.”

That runs counter to an argument put forth by some rural Maryland counties that have called dredging behind the dam, located just 10 miles upstream from the Chesapeake, an essential part of Bay cleanup efforts.

Historically, the dam’s 14-mile reservoir has trapped a portion of the sediment, nitrogen and phosphorus that washes down the Susquehanna, the Bay’s largest tributary. But the reservoir has been filling since the dam was built in 1928–29.

The three-year study, largely based on monitoring data and computer model simulations of sediment and nutrient movement, concluded that the reservoir has reached a state of “dynamic equilibrium.” That means the sand, silt and clay washed down the Susquehanna may accumulate behind the dam in some years, only to be flushed out during severe storms that are likely to hit every few years.

In effect, the dam helps to improve downstream water quality during dry periods, but increases pollution during heavy storms when built-up material is “scoured” from behind the reservoir. The dam has always done that, but now it stores less during low flows, and sends more downstream during high flows.

Nutrients are a Bigger Problem

But the impact of the reservoir filling may be less of a “time bomb” than thought. Officials were surprised when computer modeling done for the study showed that the nitrogen and phosphorus associated with the stored sediment pose a greater threat to Bay water quality than the sediment. Nitrogen and phosphorus, both nutrients, spur algae blooms that block sunlight needed by important underwater plants. And, when the algae die, they decompose in a process that draws oxygen from the water, contributing to so-called “dead zones.”

The additional nutrients associated with the increased sediment flow would likely keep some deepwater areas of the Upper Bay from meeting Bay cleanup goals for dissolved oxygen, according to the study.

“The water clarity effects of sediment essentially decline once the particles settle,” said Anna Compton, a biologist with the Corps who was the study’s director. “However, nutrient pollution has a lingering effect that leads to algae blooms and dead zones that have the potential to suffocate and stress aquatic life.”

The study also concluded that most of the sediment and the nutrients that reach the Bay are not coming from the reservoir behind Conowingo, but from the watershed upstream.

Compton said that between 2008 and 2011, only 13 percent of the sediment entering the Bay from the Susquehanna originated from the reservoir. Even during Tropical Storm Lee in 2011, only about 20 percent of the 14.5 million tons of sediment that reached the Chesapeake were scoured from behind the dam, she said. The rest originated from farther upstream and was carried past the dam by the river without ever being deposited.

Still, the reservoir’s loss of sediment-trapping capacity has increased the amount of sediment going downstream, the study found. The report estimated that had Tropical Storm Lee occurred in 1996, it would have sent about 10 percent less sediment downstream, or 13.1 million tons, because of the reservoir’s greater storage capacity at that time.

The study also said the impact of a storm varies greatly depending on when it occurs. A storm in June has the greatest impact on water quality, while a similar magnitude storm in late fall or early winter has substantially less impact.

Limited Benefit from Dredging

The study expressed doubt that dredging sediment from behind the dam is cost-effective. It estimated that removing 3 million cubic yards of sediment from the dam annually — a bit less than what comes down the river — would cost between $48 million and $267 million every year. Those costs would likely escalate, the report cautioned, as sediment storage sites near the dam begin to fill, and the material has to be transported farther.

Dredging also has a limited impact: Benefits from removal would be short-lived as the sediment is replaced each year by new material washed down the river.

“We saw very minor, temporary, improvements to water quality in Chesapeake Bay from even these large amounts of sediment being removed,” Compton said.

Compton said the study analyzed the impact of dredging sediment in the reservoir to 1996 levels, but found it would cost between $496 million and $2.8 billion and still not provide long-term water quality improvements.

Because the greatest water quality impacts were associated with nutrients, not sediment, officials suggested it would be more cost-effective to offset the dam’s increasing water quality impact through additional nutrient reductions than by removing sediment from behind the dam.

“Further reductions in nutrients will have a greater impact on meeting our water quality standards than reducing sediments,” said Bruce Michael, of the Maryland Department of Natural Resources, and a member of the study team.

Initial estimates presented in the report indicate it would take an additional 4.4 million pounds of nitrogen reductions and 410,000 pounds of phosphorus reductions Baywide to offset the impact of Conowingo being in “dynamic equilibrium.” As of the end of 2013, the Bay region needed to achieve 55 million additional pounds of nitrogen reductions to meet the 2025 target of roughly 192 million pounds, according to EPA figures. So offsetting the filling of the reservoir could require roughly an additional 8 percent of nitrogen reductions Baywide.

Alternatively, the water quality goals could be accomplished with an additional 2.4 million pounds of nitrogen reductions, or 270,000 pounds of additional phosphorus reductions, from the Susquehanna, the study said. The Susquehanna needs to achieve an additional 34 million pounds of nitrogen reductions from 2013 levels to achieve its 2025 target of about 85 million pounds. So it would need to achieve about a 7 percent additional reduction.

The required reductions are less if they come from the Susquehanna, rather than being spread Bayside, because it has a more direct impact on Upper Bay water quality than other parts of the watershed. But that could be problematic as nutrient reductions from the Susquehanna are far behind schedule, primarily because of shortfalls in Pennsylvania, according to EPA figures.

No Decisions Yet

State and federal officials say no final decisions about how to address the dam’s impact on the Bay will be made before 2017. That is when the EPA and all seven jurisdictions in the watershed are scheduled to complete the Mid-Point Assessment of progress toward meeting cleanup goals set forth in the Chesapeake Bay Total Maximum Daily Load in December 2010.

The assessment, roughly the halfway point between 2010 and 2025 — when all actions needed to clean the Bay are to be implemented — will also examine a host of other factors that could influence cleanup progress, such as new scientific information; land use and population changes; climate change impacts; new pollutant reduction technologies; and more.

Each of those factors could result in changes that would require more, or less, nutrient reductions to meet the goals.

“The system is so interconnected, you can’t take that one piece [Conowingo] and isolate it from everything else,” said Rich Batiuk, associate director for science, analysis and implementation with the EPA’s Chesapeake Bay Program Office.

Research issues raised by the study will also influence the Mid-Point Assessment.

The study focused primarily on the impacts of large events — those where the Susquehanna’s flow exceeds 400,000 cubic feet per second, which happen every four to five years on average. New monitoring, to take place over the next two years, will try to better assess the impact of more common storms — those that are likely to happen two or three times a year. Recent research has indicated storms with river flows as low as 175,000 cubic feet per second may send more material downstream than previously thought.

These smaller storms, particularly those in the spring and summer, “may be adding another pulse of nutrients that could be helping to fuel algal blooms leading to more low-oxygen conditions and further block light,” Batiuk said. “We need to take a careful look at that the water quality effect of these smaller storms.”

New monitoring will also take a closer look at the types of nitrogen and phosphorus scoured from behind the dam during storms. Modeling for the study found more nutrients — especially nitrogen — were being released than previously thought. Some scientists, though, have questioned whether the forms of nutrients associated with stored sediment are as easily used by algae as the nutrients are washed directly down the river and into the Bay. The findings of that research could indicate whether those scoured nutrients have more, or less, of an impact on water quality than estimated by the study.

“That is something we don’t have a good handle on, and we want to try to find out,” Michael said.

Monitoring for the additional research is funded by Exelon, which owns the hydroelectric dam.

Good and Bad Sediments

The findings showing increased Bay water quality impacts from nutrients don’t mean that sediments are not causing a problem. Studies have suggested that as the reservoir fills, heavy sand is more likely to be stored while fine silt and clay particles are more likely to spill downstream. That fine material is more likely to carry particles of phosphorus and is also more likely to cause water clarity problems as it floats in the water longer and is more easily resuspended by wind and waves.

The filling of the dam therefore creates a “double whammy,” said Mark Bryer, director of The Nature Conservancy’s Chesapeake Bay Program, which helped fund the study. “There is a disproportionate amount of bad sediment flowing over the dam while the good sediment — the sands and coarser material like gravel — are stuck way up at the top of the reservoir system.”

Coarse sediment that helps build downstream habitat for wetlands, grass beds, mussels and spawning fish is trapped upstream, while the downstream habitat needs them, Bryer said.

The study examined the potential for excavating coarse material from behind the dam and moving it downstream, but found that to be a costly alternative.

While the sediment impacts for habitat may be profound, the study found less of a sediment impact on water quality. During storms, much of the fine sediment is flushed past sensitive habitats into deepwater areas of the Bay, where it has less impact.

“It isn’t that there is no impact from sediments,” Michael said. “We did see an impact downstream from sediment, but it didn’t keep us from meeting water clarity standards in the Upper Bay, as opposed to nutrients, where we definitely don’t meet our dissolved oxygen standards.”

The Clean Chesapeake Coalition, a group of Maryland counties that has argued their cleanup efforts would be overwhelmed if the Conowingo reservoir was not dredged, said the report affirmed that the dam was affecting the Bay, and that dredging should be considered.

“The problems for the Bay, for our seafood industry and for our local economies related to Conowingo Pond being full deserves our continued attention,” said Ron Fithian, coalition chairman and Kent County commissioner. “We should not accept as the new normal that all the reservoirs are full, that Susquehanna River pollution is no longer being trapped, that more storms and scour are expected and dredging is off the table.”

Environmental groups said the report showed the Conowingo needed to be addressed, but also reinforced the importance of states and localities fully implementing their nutrient reduction plans to meet Bay goals.

“The results of this extensive report reveal that the Conowingo Dam does indeed harm the Chesapeake Bay, but that harm is not nearly so large that it justifies giving up all of the other work needed to restore the Chesapeake Bay,” said Michael Helfrich, Lower Susquehanna Riverkeeper. “The next step is to conduct further research to better understand how the added pollution from behind Conowingo should be addressed.”

Alison Prost, Maryland executive director of the Chesapeake Bay Foundation, said the report shows problems stemming from the dam “are not as bad as scientists previously thought. The dam is one of many sources of pollution throughout the Bay’s drainage area. To clean up the Bay, we must clean up our local streams, creek and rivers that feed it.”

The study’s findings regarding dredging are similar to those reached by the Federal Energy Regulatory Commission in its draft environmental impact statement released in early July. That document also concluded that large-scale dredging would be “challenging and complicated” and that it was “premature” to conclude that dredging would be an environmentally acceptable solution. It was also unclear who would bear the burden of such costs — the estimated dredging costs far exceed what Exelon makes from operating Conowingo.

FERC is considering Exelon’s request to renew its operating license for the dam. Its 30-year operating license expired Sept. 1, but FERC issued a one-year extension while officials figure out how to mitigate a host of environmental issues, such as fish passage, in the new license, which could last for up to 46 years.

As part of the relicensing, the Maryland Department of Environment also has to issue a certification that downstream water quality problems caused by the dam are being addressed.

The state has indicated that certification, due Jan. 30, will likely be denied until water quality issues are addressed.

Deadline to comment on study is Jan. 9

The Lower Susquehanna River Watershed Assessment was completed by an inter-agency team composed of the Corps’ Engineering Research and Development Center, U.S. Geological Survey, Susquehanna River Basin Commission, The Nature Conservancy, U.S. EPA Chesapeake Bay Program Office, Maryland Department of the Environment, Maryland Department of Natural Resources, and Maryland Geological Survey.

It is available for public comment through Jan. 9, 2015. A final report is expected in summer 2015.

The full report with appendices, and associated graphics, is at http://bit.ly/LSRWA.

A public meeting and webinar is scheduled for 7–9 p.m. Dec. 9 at Harford Community College in Bel Air, MD. Details on the public meeting and log-in information for the webinar will be posted on the website, as well as other meeting materials.

Public comments may be made through Jan. 9 via:

  • E-mail to LSRWAcomments@usace.army.mil.
  • Letter postmarked by Jan 9, 2015, to: U.S. Army Corps of Engineers, Baltimore District, Attn: Anna Compton , P.O. Box 1715, Baltimore, MD 21203.