WINSLOW – More than four dozen people gathered Monday afternoon on the shores of the Sebasticook River, just feet from the Fort Halifax Dam.

They also made their presence felt in Kennebec County Superior Court, filing suit Monday against the state in a bid to save the nearly 100-year-old dam from being breached.

The Fort Halifax Dam is a hydroelectric facility.

Its defenders maintain that an environmental disaster will occur if it is breached.

Some rivers and fisheries advocates said Monday they dispute the validity of the lawsuit.

The impoundment behind the dam covers 400 acres, state Rep. Kenneth Fletcher, R-Winslow and opponent of the breaching said.

He owns a home near the impoundment. “If the dam is breached, the river will lose 90 percent of its volume. Breaching the dam will remove a renewable energy supply and destroy the four-season recreational area the ‘lake’ has become,” Fletcher said. “What will be left will be a shallow, narrow stream.”

The dam’s owner, FPL Energy of Juno Beach, Fla., was granted permission by the state Department of Environmental Protection to breach the dam after the company determined that the estimated $4 million cost of installing a fish lift was prohibitive.

Save Our Sebasticook, an organization of about 30 people, filed the lawsuit Monday morning against the State Planning Office and the state Inland Fisheries and Wildlife and Marine Resources departments.

SOS is seeking the invalidation of a 1998 agreement between the state and FPL Energy that requires the owner either to install a fish lift or breach the dam.

Nick Bennett, staff scientist for the Natural Resources Council of Maine, and Jeff Reardon, New England conservation director for Trout Unlimited, speaking on behalf of the Kennebec Coalition, said SOS statements have been “incorrect and misleading.”

“It is irresponsible and inaccurate to reference any part of this project as a threat to the natural environment,” Reardon said.

“The Maine Department of Environmental Protection and the Federal Energy Regulatory Commission have thoroughly reviewed the environmental impacts of the project and found that dam removal will be beneficial. SOS is simply ignoring this record.”

Members of the Kennebec Coalition include American Rivers, the Atlantic Salmon Federation, the Natural Resources Council of Maine and Trout Unlimited and its Kennebec Valley chapter.

“The reality is that this project has strong environmental benefits,” Bennett said.

“Fish and wildlife habitat will be enhanced and water quality will be improved. The fact is, all of the [SOS] arguments have already been heard and rejected. A few local landowners are running to court to try to stop a project that has enormous benefits for the natural resources in the region.”

Fletcher said he believes the 1998 agreement allowing the breaching violated Maine’s Freedom of Access Law and other statutes, requiring among other things adequate public notice. “Our voice should be heard,” Fletcher said. “We are in favor of restoring fish [to the Sebasticook] but we believe there are creative options beyond the fish lift.”

Fletcher maintained that options such as fish ways and fish pumps were not even part of the 1998 agreement.

Charles Dow of the state Attorney General’s Office, which represents the state in the case, said his office had not seen the lawsuit by Monday afternoon and that he could not comment.

SOS has also filed an appeal at the federal level against FERC’s decision to allow FPL Energy to proceed with the dam removal process. Also on Monday, SOS filed an appeal with DEP regarding that department’s order to remove part of the dam.