Report: Nutrients, Not Just Sediment at Conowingo, Affect Chesapeake Bay

By Dani Shae Thompson

ANNAPOLIS — The Chesapeake Bay may have water quality issues, but according to a new study, the Conowingo Dam doesn’t seem to be a major cause of them.

A multi-agency report found that the Conowingo Dam is not the biggest culprit for water quality issues affecting the bay, and dredging sediment from the reservoir behind the dam should not be considered a cost-effective solution.

Rather, the report points to nutrients associated with the sediment, washed down from states upstream, and from other tributaries to the Chesapeake, that pass through the dam and are contributing to dead zones in the bay.

The Lower Susquehanna River Watershed Assessment, made public Thursday, details the movement of sediment and nutrients through the river, reports how they may affect the Chesapeake Bay, and offers suggestions for how to best manage the problem.

Suggestions include continued research and monitoring of nutrients, stormwater management, and a recommendation that the EPA integrate findings of this study into their water quality assessment of the bay.

“The overwhelming majority of pollution entering the bay from the Susquehanna River comes not from behind the Conowingo Dam but from the 27,000-square-mile watershed upstream,” Alison Prost, Maryland executive director of the Chesapeake Bay Foundation, said in a statement Wednesday.

According to measurements taken from 2008 to 2011, only 13 percent of sediment pollution came from the Conowingo Reservoir—the other 87 percent came from the greater watershed area, said Anna Compton, U.S. Army Corps of Engineers biologist and manager of the study.

The Susquehanna begins in Cooperstown, New York, flows through central Pennsylvania and into Maryland where it winds its way down to the Chesapeake Bay.

Along the way, the river collects runoff—sediment and nutrients—from farms, cites, yards, and anywhere in between.

Sediment essentially means dirt—clay, silt and sand—but it has the potential to carry nutrients, pesticides, oil residue, manure and other toxic particles.

Dams are designed to hold back water, but they also collect and hold back this sediment—millions of tons of it.

Some of that sediment gets through the dam, and for dams in the greater Chesapeake Bay watershed, this means sediment will continue on to the bay, where it has the potential to harm the aquatic ecosystem.

Before the new assessment was completed, researchers thought that this sediment was causing major harm to the bay because an abundance of particles floating in the water could block out light or bury bottom-dwelling aquatic species.

The study concludes that this is not the case.

“When we ran the model simulations looking at removing a really large amount of sediment, we fully expected to see water quality improvements in the Chesapeake Bay,” said Compton.

“We were surprised. We simply didn’t see it.”

The reason for this is that sediment quickly settles and dissipates without burying bottom dwelling species.

The assessment found that even in a major weather event, like Tropical Storm Lee in 2011, large sediment plumes in the Susquehanna and the Chesapeake dissipate quickly without affecting water clarity for long.

“(The satellite imagery) looks very catastrophic. You think ‘Oh my goodness. This has got to be impacting the bay.’ And it does, there are short-term impacts, but the sediment falls out quickly,” said Bruce Michael, the Resource Assessment Service Director for the Maryland Department of Natural Resources.

The problem with sediment is that it carries nutrients, like nitrogen and phosphorus, along with it.

These nutrients can stimulate the growth of algae, leading to low levels of dissolved oxygen in the water. This can create dead zones—areas uninhabitable for aquatic species that need oxygen to survive.

Since the sediment itself is not harming the Chesapeake Bay, the study suggests, one way to improve water quality would be to reduce nutrient pollution upstream.

“The nutrients are more the driving factor in this and not the sediment alone. Further reductions in nutrients will have a larger impact on meeting our water quality standards,” Michael said.

This includes better management of storm water, agricultural runoff, and runoff from paved surfaces like roads, or residential areas.

For Marylanders, fees to pay for this kind of management have become known as the “rain tax.”

The bill, more formally known as a “stormwater management fee,” was signed into law by Gov. Martin O’Malley in 2012, and requires nine counties and Baltimore City to implement watershed protection programs.

As part of the program, local governments charge landowners based on the amount of impervious surface on their property.

Despite political controversy about the tax, the new study recommends stormwater management as an important strategy to lower nutrients and protect the water quality of the bay.

The assessment (link:, was released for public comment Thursday morning. The 185-page report was three years in the making and cost $1.4 million to complete. It involved several agencies, including the Maryland Department of Natural Resources, the U.S. Army Corps of Engineers, the Maryland Department of the Environment and The Nature Conservancy.

Another recommendation of the assessment is to continue long-term monitoring of the lower Susquehanna River system.

Exelon Generation Co., which owns and operates the Conowingo Dam, has offered to cover the $3.5 million price tag of this enhanced monitoring over the next few years.

Exelon currently leases the Conowingo Dam and Reservoir. The lease was issued on Aug. 14, 1980, but it expired on Sept. 1.

So now Exelon is in the process of negotiating a new leasing license with the Federal Energy Regulatory Commission. The final decision will be made by FERC in January, and a new lease would be effective for another 46 years.

Of course, what comes of the report could also depend largely on the opinion of governor-elect, Larry J. Hogan Jr. , who has pointed to upstream states as complicit in the pollution of the bay, and responsible for their shares of the cleanup.

“I think we can help clean up the bay by standing up for Maryland and fighting back against some of the upstream polluters,” Hogan said in a YouTube video (link: produced by his campaign on Aug. 19.

“We’ve got to push back against the EPA, the federal government has a role to play … and we’ve got to get the other states to pay their fair share.”

Hogan’s staff confirmed they received a copy of the report on Wednesday, but the governor-elect could not be reached for comment as of Thursday afternoon.

The Big Four—Important Findings from the Lower Susquehanna River Watershed Assessment

1. Before the completion of the assessment, it was thought that the Conowingo Dam would continue to trap sediment for 10 to 20 years. But the report found that the reservoir is essentially at full capacity for sediment.

-- The assessment says the dam is in a state of “dynamic equilibrium”—meaning sediment continually accumulates, but every so often a large storm event will push enough sediment from the reservoir through the dam to leave extra room for additional trapping.

-- Because of these storms, the reservoir never reaches a “full” capacity of sediment, but it also can’t be expected to trap much more. This means any additional sediment coming into the Conowingo Reservoir from upstream will pass through the dam and continue on to the Chesapeake Bay.

2. The nutrients carried with the additional sediment passing through Conowingo Dam is affecting the health of the Chesapeake Bay.

-- Sediment contains nutrients that can stimulate the growth of algae, leading to low levels of dissolved oxygen in the water. Low dissolved oxygen levels can cause dead zones—areas uninhabitable for aquatic species that need oxygen to survive.

-- If nothing is done to mitigate the amount of sediment and associated nutrients flowing through the Conowingo Reservoir, water quality standards set for the Chesapeake Bay (intended to be met by 2025) will not be attainable.

3. Upstream sources of sediment and nutrients have more impact on the Chesapeake Bay than the sediment and nutrients collecting at the Conowingo Dam.

-- The Susquehanna River watershed upstream of the Conowingo Dam is responsible for the majority of pollutants, which include phosphorous and nitrogen, associated with negative impacts on the Chesapeake Bay.

4. Dredging (removing) sediment from the Conowingo Reservoir would not be an effective method for improving water quality in the Chesapeake Bay.

-- Dredging would cost between $48 million and $267 million each year just to keep the sediment at its current levels. It would have to be done annually in order to make even a short-term difference in sediment storage capacity in the Conowingo Reservoir. To dredge enough sediment to return to 1996 levels, the assessment estimates it could cost as much as $2.8 billion.

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