Correction: An earlier version of this article incorrectly reported that the University of Maryland participated in a study about Conowingo Dam sediment’s impact on the Chesapeake Bay. The university had no involvement. This version has been corrected.


Gov. Larry Hogan and Exelon Corp. are debating whether the Conowingo Dam is causing a sediment buildup that is polluting Chesapeake Bay. (Linda Davidson/The Washington Post)

Nearly everyone agrees that the Conowingo Dam is a boon for Maryland, generating enough clean energy each day to power 160,000 businesses and homes.

On top of that, the dam, near the Pennsylvania border, is a playground for thousands of people who frolic in its vast reservoir in the Susquehanna River, a few miles upstream from where it enters the Chesapeake Bay. Scores of eagles feed on the fish that are sucked through its generators. And if there’s a blackout in the Mid-Atlantic, the region can rely on the Conowingo to jump-start the power grid.

But the agreement ends there.

To Larry Hogan, Maryland’s new Republican governor, the dam is an environmental hazard. Over the course of nearly a century, more than 170 million tons of sediment has built up in a huge reservoir designed to trap it before it reaches the bay.

But now the reservoir is full, and millions of tons of the gritty material pour over during every major storm, scouring grasses that marine animals rely on to survive. At a news conference in February, Hogan said the dam’s sediment problem has been “ignored for eight years” and vowed to bring attention to it. Hogan has said that a dredging operation costing up to $250 million might be the answer to the bay’s pollution problem, and that the dam’s owner, Exelon Corp., should pay most of that cost.

An American bald eagle soars over the prime fishing grounds below Conowingo Dam in Harford County. (Linda Davidson/The Washington Post)

Exelon calls the governor’s concern misplaced and says dredging isn’t the answer. To back that claim, executives who manage the dam pointed to a November study by the Army Corps of Engineers and the Maryland Department of the Environment said another type of pollution — nutrients from phosphorous and nitrogen that run off farms and municipal sewer overflows — is far more harmful to the Chesapeake.

Sediment at the dam, the study said, is harmful only to isolated pockets of the giant estuary. In fact, the study said, less than a fifth of all sediment in the bay came from behind the Conowingo between 2008 and 2011. Most flows into streams from agriculture and municipalities throughout the Chesapeake watershed. Dredging, the study concluded, would not help much.

For the moment, the two sides are in a standoff. Exelon is hoping for a meeting with the governor. A spokeswoman for Hogan, Erin Montgomery, said that Hogan also wants to meet but that his schedule is full and it could be weeks before he’s free.

What’s clear is that the standoff comes at the worst time for Exelon, which is seeking to renew its federal license to operate the dam for 46 more years after its old license expired in 2014.

Maryland has leverage because the state must certify that the dam meets federal water-quality standards under the Clean Water Act before the Federal Energy Regulatory Commission will approve a license, and both Hogan and state environmental officials strongly question whether those standards are being met.

Because of the state’s concerns, Exelon’s application for a renewal is on hold. At the state’s request, the energy giant is funding and helping conduct a $3.5 million water-quality analysis that will take place over two years.

Conservationists say Hogan and the state should use every tool available to force Exelon to improve not only water quality but also machinery that helps fish such as shad get over the dam’s walls, which block their migration.

“They’re going to get a 40-to-50-year license, at a minimum 30 years,” said John Seebach, senior director of federal river management at the nonprofit group American Rivers. “I probably won’t see the next one. The only time we can get them to do something about these things is right now.”

In spite of what the Army Corps study says, Hogan is focused on dredging. During a public appearance shortly after his election, he said, “The Army Corps hasn’t dredged the dam in 85 years.” Hogan called that “a major part of the problem.” He expressed skepticism over the report, saying that “the guys who have the problem and haven’t fixed the problem are giving the report — so it’s probably not that fair and accurate.”

Dan Bierly, chief of the civil project development branch at the Army Corps, stood behind the research, saying it “brought together the foremost experts in this field” and used “the best and most trusted mathematical models” to arrive at the conclusion that “sediment itself doesn’t really pose a major threat to the bay.”

Nutrient pollution, not sediment, is what causes disease and oxygen-depleted dead zones that kill fish in the bay, Bierly said. And dredging at the dam would remove sediment, but the natural flow of the Susquehanna would replace it, forcing digging every year at a cost of hundreds of millions of dollars.

Not every Republican agrees with Hogan about dredging. Chris Meekins, a spokesman for Rep. Andy Harris (R-Md.), said: “We need to consider all viable options to improve the health of the bay; whether or not dredging behind the Conowingo Dam is fiscally viable is yet to be seen.”

The Army Corps study is hard to swallow for some because it contradicts not only Hogan but also the concerns of his predecessor, former governor Martin O’Malley (D). After Tropical Storm Lee pushed millions of tons of sediment from behind the wall into the bay, turning its waters brown for more than 100 miles, O’Malley ordered a study “to address this threat” and head off “what could be a catastrophic event.”

The sudden reversal on concern about the impact of sediment held by O’Malley, the U.S. Geological Survey and numerous conservation groups was greeted with suspicion by the Hogan administration. “It was kind of surprising to us that all of a sudden the Army Corps was reversing their concern about the dam,” said Adam Dubitsky, policy director for Hogan.

Chip MacLeod, general counsel for the Clean Chesapeake Coalition of 10 rural farming counties such as Frederick, Cecil and Dorchester that support Hogan, thinks he knows why.

MacLeod said turning the focus away from sediment is the goal of a multibillion-dollar federal plan to clean up the bay by limiting nutrient pollution. Run by the Environmental Protection Agency and managed by states in the bay watershed, including Maryland, the “pollution diet” is supported by the Army Corps and regional environmental groups.

But in calculating the amount of pollution that enters the bay, the EPA made a major error, MacLeod said. An EPA study in 2009 said the reservoir behind the dam was about half full with sediment and could continue trapping it for years. Three years later, a USGS report said the reservoir was in fact nearly full.

“We started making this point that, look, that thing is full. The plan was flawed,” MacLeod said. The coalition thinks the study’s conclusions were made in advance, he said.

“They were going to find a way to conclude [sediment] wasn’t the most important thing to focus on,” MacLeod said, to support the “pollution diet and structure” that the EPA and environmental groups favored.

The Clean Chesapeake Coalition supports farmers who complain that the cost of limiting nutrient pollution from chemicals and animal manure that runs off their land hurts their bottom line.

But environmentalists say the coalition’s position ignores the best available science. Beth McGee, senior water-quality specialist for the Chesapeake Bay Foundation, said it was “one of the groups that was concerned and that sediment at the dam was a ticking time bomb.”

The Army Corps study ended the foundation’s concern about sediment and the need to dredge it.

“Honestly, if it were as simple as saying we just need to dredge behind the dam, though it would be expensive, it would be one thing we could do,” she said. “We could have pulled state and federal resources to go after dredging, but it’s not a silver bullet.”

Jenna Johnson contributed to this report.