Bullfrogs were in full-throated cry Tuesday morning as boats pulled up to the landing at Dr. Wagner’s Honey Island Swamp Tours, discharging visitors eager to see the plant and animal life that is nourished by fresh water in the lower Pearl River system.

But according to a national conservation group, the Honey Island Swamp — along with oyster reefs in Mississippi and Louisiana and two endangered species — are under potential threat from a proposed dam near Jackson, Mississippi.

As a result, the group, American Rivers, announced Tuesday that it is putting the Pearl on its 2015 list of the nation’s 10 most endangered rivers.

A combined feasibility study and environmental impact statement for the dam is under review by the U.S. Army Corps of Engineers and a draft will likely be made public this summer, according to Keith Turner, a lawyer for the Rankin Hinds Pearl River Flood and Drainage Control District, the Mississippi agency that is pursuing the project.

The dam, which would create a 1,500-acre lake on the river, is intended to prevent flooding but also is supported by economic development interests.

Pearl River advocates in St. Tammany Parish turned out at the swamp tour launch Tuesday for a news conference about the Pearl’s inclusion on the list of endangered rivers.

Helen Rose Patterson, of the Gulf Restoration Network, who spoke on behalf of American Rivers, said the purpose of the list is to bring national attention to rivers that face key decisions this year about their future health or function.

Paul Trahan, who runs the 32-year-old swamp tour business, said he fears changes in the flow rate will allow salt water to move up the river, killing cypress, tupelo and gum trees and altering the swamp’s ecology.

That’s happened elsewhere in the state, he said, pointing to the slow dying off of trees in St. Bernard and Plaquemines parishes. The Honey Island Swamp is one of the few of its kind that still remain, Trahan said, and while he could still run tours on his boat if its ecology changes, what visitors see would not be the same.

That’s why he doesn’t want to see a dam built upstream. The balance, in his view, is too delicate. “Leave it alone,” he said.

St. Tammany Parish Councilmen Gene Bellisario and Richard Artique also were on hand and pointed to a resolution the council passed in late 2013 opposing the project. That resolution cited fears of lower water flows and reduced oxygen levels that would hurt estuaries and the plant and animal life there, including the ringed sawback turtle, Gulf sturgeon and inflated heel splitter mussel.

Changes in salinity in the Mississippi Sound could hurt the oyster and fishing industries in both Louisiana and Mississippi, Bellisario said.

Patterson, in prepared remarks, questioned the idea of spending money on a dam when Mississippi is spending $50 million on marsh and oyster reef restoration at the mouth of the Pearl — restoration that relies on sustained freshwater flows.

“Mississippi needs to recognize that it squanders state money when it builds projects that cancel each other out,” she said.

Ben Emanuel, of American Rivers, called the dam an “expensive boondoggle” that would cause irreparable harm.

But the Mississippi flood control district argues that the project will not harm the environment. In a prepared statement, the district called it “disappointing” that environmental groups are spreading what it termed “disinformation.”

Studies do not support claims that sediment and water quantity and quality will be affected downstream by a proposed river channel and weir project, the district said. “In fact, the Flood Control District’s work has revealed that most of the lower Pearl River issues are related to local actions and past natural events,” it said.

The dam would provide significant flood protection for Hinds and Rankin counties and result in environmental improvements “while protecting wetlands area and providing recreational benefits,” the statement said.

The district has taken measures beyond those required by the National Environmental Policy Act to reach out to all interested parties and consider all environmental issues on the upper and lower Pearl, the district said.

Public meetings will be held in St. Tammany Parish as well as in Jackson and southern Mississippi once the draft environmental impact statement is through the review process, Turner said.

Follow Sara Pagones on Twitter, @spagonesadvocat.