The Shawmut Dam on the Kennebec River between Fairfield and Benton is pictured on Aug. 20, 2021. Credit: Linda Coan O'Kresik / BDN

AUGUSTA, Maine — Four Kennebec River dams can continue operating while a lawsuit accusing their owners of illegally killing vulnerable fish goes forward, a federal judge ruled Thursday.

The ruling was a setback for environmental groups that say Brookfield Renewable is violating the Endangered Species Act. While U.S. District Court Judge Jon D. Levy said the groups would likely succeed in proving Atlantic salmon are harmed by the dams, but he doubted forcing them to sit idle until they improved fish-passage standards would be an effective remedy.

It leaves a narrow path forward for environmentalists in the latest development in a long controversy over the dams between Waterville and Skowhegan. The administration of Gov. Janet Mills dropped a plan early last year that could have led to the removal of all four dams and softened her stance further as the year went on after backlash from Fairfield millworkers.

The lawsuit brought by the Conservation Law Foundation, Maine Rivers and the Natural Resources Council of Maine, which had been aligned with the Democratic governor on the dam issue, centers on whether Brookfield has the right to harm the fish in its operations, a situation referred to as “taking” fish. That activity requires a permit from the federal government.

The company has been trying to get an expired species protection plan renewed by federal regulators. It shut down three of its four dams last year after regulators warned it they risked violating the law by operating during salmon migration season.

Levy ruled that the environmental groups had not provided any evidence that temporary measures taken at some of the dams last year to allow young salmon to pass through had actually helped the fish or that the public would benefit from the temporary stoppage.

The dams, particularly the relicensing of the Shawmut Dam in Fairfield, have been at the center of a multi-year fight between conservationists, the dams and the communities they affect. The Mills administration wants the dams to upgrade their fish-passage standards to allow for almost all of the salmon to pass; Brookfield has called such a standard unreasonable.