Dredging Conowingo little help to bay, study finds

Dredging millions of tons of sediment from the Susquehanna River upriver of the Conowingo Dam would potentially cost billions of dollars and do little to help the Chesapeake Bay compared with cleanup efforts already underway, a new federal-state study found.

Gov.-elect Larry Hogan and critics of the current pollution-fighting strategies contend that removing massive amounts of sediment flowing down the river would help restore the Chesapeake more than controversial measures to levy stormwater fees, restrict septic-based development or limit farming practices.


The study, to be released Thursday by state officials and the Army Corps of Engineers, concludes that the sediment washed into the bay from the Susquehanna after extreme weather mostly hurts a small part of the upper bay. The study was launched following Tropical Storm Lee three years ago.

Nutrient pollution from sewage plants, farms and urban runoff across the six-state watershed is the main threat to the bay, the study found, and wide-ranging cleanup efforts must continue if the Chesapeake is to be restored to health.


"We get very minor, temporary improvements to water quality in the Chesapeake Bay from these large amounts of sediment being removed," said Anna Compton, a biologist with the Army Corps of Engineers Baltimore District.

Hogan said Wednesday that he had not read the report but questioned its findings, calling the Army Corps a biased source and accusing it of neglecting sediment above the dams for decades.

"It's obviously a major problem," Hogan said. "We can argue about what percentage it is."

The Susquehanna, which drains much of Pennsylvania and reaches into New York, is the bay's largest tributary. It's also the largest single source of the nutrient and sediment pollution that feeds massive algae blooms in the bay and creates a sprawling "dead zone" every summer where fish and crabs struggle for oxygen.

Conowingo and two other hydroelectric dams on the Susquehanna have been shielding the bay from pollution for decades, "trapping" sediment as it settled to the bottom of their reservoirs.

Experts expected that in another decade the dam reservoirs would fill to the point they no longer retain any sediment. They believed that would give the bay cleanup effort a chance to reduce pollution sufficiently by its federally imposed 2025 deadline to shrink the "dead zone."

The study, which relies on multiple computer-driven mathematical models, found that the dams are no longer trapping most of the silt coming down the river. Big storms add to the problem by "scouring" out what's settled above Conowingo and sending it to the bay.

Still, the study found the Conowingo plays a relatively small role even in those cases. Tropical Storm Lee flushed 3 million tons of sediment from behind the dam, the study estimated. But four times as much reached the bay from flood-aggravated erosion and storm runoff farther upriver.


Big pulses of storm-driven sediment from the dam could still keep bay cleanup efforts from succeeding in the northernmost portion of the bay, said Bruce Michael, director of resource assessment with the Maryland Department of Natural Resources, one of the state agencies that participated in the study. For that reason, he said, something more needs to be done to achieve pollution reduction targets there.

Dredging just enough sediment to keep the situation from getting worse — roughly 3 million cubic yards a year, or 1.5 million pickup truckloads — would cost $48 million to $267 million, the study estimated.

To restore sediment levels to what they were in the mid-1990s, the study estimates, 25 million cubic yards of silt would need to be excavated and disposed elsewhere. That could cost up to $3 billion. And unless the flow of sediment coming down the river is throttled back, the pond would gradually fill in and need to be dredged again.

With the exception of the northern end of the bay, Michael said, most of the Chesapeake's rivers aren't heavily affected by sediment or nutrient pollution from the Susquehanna. So to clean up the rest of the bay and its tributaries, he said, communities across the state need to continue to upgrade sewage plants and reduce polluted runoff from cities, suburbs and farms.

Hogan, a Republican, said during the campaign that he would seek to shift the burden of bay cleanup from Maryland's businesses, farmers and homeowners to Exelon Corp., Conowingo's owner, and to the states up the Susquehanna. He vowed to fight the Environmental Protection Agency's bay cleanup mandates imposed on Maryland and to press Pennsylvania and New York to do more.

Exelon has been seeking a new license to operate the hydropower facility at Conowingo, but Maryland and federal environmental officials have been reluctant grant it, citing concerns over migratory fish passage and the sediment buildup. Hogan has said he opposes granting the company a long-term renewal of its license until it agrees to help remove the sediment.


The company recently got a one-year license extension from the Federal Energy Regulatory Commission. The dam also needs a permit from the state, and Matthew Rowe of the Maryland Department of the Environment said the dam's role in sending sediment to the bay would be a factor in the state's evaluation.

Exelon has pledged to underwrite $3.5 million worth of water quality monitoring above and below the dam over the next two years, Michael said.

More study is needed to better understand the sediment and nutrient pollution issues, the Army Corps study concludes, and the most cost-effective remedies.

Environmentalists welcomed the findings.

"The dam is one of many sources of pollution throughout the bay's drainage area," said Alison Prost, Maryland executive director of the Chesapeake Bay Foundation. "To clean up the bay, we must clean up our local streams, creeks and rivers that feed it."

Prost said Exelon should "be held responsible for its share of the problem," and upriver states need to do their part. But she said the most cost-effective means of cleaning the bay is reducing pollution at its source — our farms, cities and in our own yards, to name a few."


But Charles Macleod, a lawyer representing the Clean Chesapeake Coalition, a group of rural and suburban county governments, shared the governor-elect's skepticism about the study.

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"That these agencies and environmental groups want to continue to downplay the magnitude of the problem is fascinating, because we have been focusing on much smaller solutions to the bay," Macleod said.

He pointed to the O'Malley administration's moves to reduce pollution from septic systems as an example. Those measures are aimed at reducing pollution from another nutrient, nitrogen, but Macleod said many believe sediment dredging would be more effective in cleaning the bay.

"Find me a person to say it's better to spend $3 billion regulating septics in Maryland or dredging Conowingo," Macleod said.

A public meeting on the study is scheduled Dec. 9 at Harford Community College in Bel Air, and public comments are being taken until Jan. 9. A final report is expected to be issued next summer.

Baltimore Sun reporter Scott Dance contributed to this article.