Maryland Gov. Larry Hogan is convening his second annual summit Tuesday on the buildup of sediments and nutrients behind Conowingo dam.  But the agenda and attendees remain a mystery.

Doug Mayer, Hogan’s communications director, said in an email that the summit would be at 11 a.m. in Darlington, MD, near the dam on the Susquehanna River.  But Mayer said the session was closed to the press. He said Hogan would hold a press conference following the summit, but did not respond to a query about why the meeting was not open to the public.

Attendance at the summit, which is to be held at the Darlington Volunteer Fire Company, apparently is by invitation only. A couple of those who received invitations said they were not given any information about the agenda or other invitees.

Hogan, who has made an issue of the dam’s role in the Chesapeake Bay restoration, is expected to announce he is seeking proposals to deal with the buildup of sediment and nutrients behind the impoundment.

The 94-foot high dam, 10 miles upriver from the Bay, was built in 1928 to generate electricity from the river’s flow. It has created a 14-mile long “pond” upriver, where roughly 200 million tons of sediment and nutrients have settled out over the years. A 2014 assessment found that the dam had reached “dynamic equilibrium,” in which the dam stopped capturing sediment or nutrients flowing down the river.

That’s a challenge for the federal-state Bay restoration effort, because the Environmental Protection Agency had earlier estimated that the dam was trapping as much as 20 percent of the nitrogen and 50 percent of the phosphorus coming down the river. Those two nutrients cause algae blooms and “dead zones” in the Chesapeake. 

Earlier studies indicated the dam would continue capturing those pollutants through 2025. But more recent research it's already stopped. A review by the U.S. Geological Survey found that Conowingo has been trapping fewer and fewer nutrients since the 1990s, and sometime in the last few years reached the point where it essentially was no longer retaining nutrients and sediment.

Local officials from rural Maryland counties have complained that they’re being forced to go along with other pollution reduction measures, such as a clampdown on septic-based development, that are far less effective in comparison to the impact that might come from dealing with the buildup behind the dam.  Those complaints have resonated with Hogan, a Republican who during his 2014 campaign for governor embraced the message of the Clean Chesapeake Coalition, a group that represents rural politicians. A study coordinated by the U.S. Army Corps of Engineers found that because of the dam’s diminished trapping capacity, the nutrient reductions called for in the Susquehanna watershed by the EPA's Bay "pollution diet" are no longer enough to meet dissolved oxygen goals in deep waters of the Upper Bay.

But the Corps study also found that dredging the sediment and nutrient buildup from behind Conowingo would be costly and of limited benefit to the Bay. 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. Dredging just enough sediment to keep the pond from filling in again — roughly 3 million cubic yards a year, or 1.5 million pickup truckloads — would cost $48 million to $267 million, the study estimated.

Between 2008 and 2011, only 13 percent of the sediment coming down from the Susquehanna originated from the reservoir, the study found. Even during Tropical Storm Lee in 2011, a major flood, only about 20 percent of the 14.5 million tons of sediment were scoured from behind the dam, the study estimated, while the rest was carried past the dam by the river without ever being deposited.

Hogan, however, has maintained that the dam is a neglected source of the Bay’s water quality woes. And in his newly assumed position as chairman of the Bay Program’s Executive Council, he has leverage to highlight the issue. He has indicated that he holds Exelon Corp., owner of the hydroelectric dam, the federal government and the upriver states of Pennsylvania and New York responsible for the continuing flow of sediment and nutrients down the river.

Last year, following Hogan's first Conowingo summit, Maryland issued a request for information seeking suggestions for dealing with the sediment and nutrients behind the dam. It received at least 11 responses, according to a request for records under the state Public Information Act.  Those proposals argued that the sediment and nutrients could be removed and used beneficially.

It’s not clear who will be attending this summit. The Clean Chesapeake Coalition mentions the upcoming summit on its website, but the group’s lawyer did not return a phone call seeking information.

John Surrick, spokesman for the Chesapeake Bay Foundation, said the Annapolis-based environmental group was not invited to the summit.

And a spokeswoman for the Pennsylvania Department of Environmental Protection said that Secretary Patrick McDonnell would not attend the summit, but would send subordinates in his stead.