Environmental groups American Rivers and Alabama Rivers Alliance challenged the 30-year relicensing of the river’s hydrogenation system after a review said the dams create “perilously low” dissolved-oxygen levels that at times threaten multiple endangered species.

Even so, FERC earlier concluded that licensing the generation projects “would have no substantial impact either on the river’s ecological condition or endangered species,” according to a three-judge panel of the U.S. appeals court in Washington, D.C., in a July 6 ruling issued as a formal mandate vacating the license on Sept. 7.

FERC’s assessment under the National Environmental Protection Act was “insufficiently reasoned and unsupported by substantial evidence,” said the court, which sent the issue back to the agency for further action.

The previous Alabama Power license was issued in 1957, but hydroelectric power plants on the Coosa date to the 1920s.

Alabama Power has installed multiple systems to increase dissolved oxygen levels at three dams under the requirements of the 2013 license, says Jim Crew, hydro services manager for Southern Co., Alabama Power’s owner.

Dissolved-oxygen levels in the river have to be at least 4 mg/l while hydroelectric systems are generating power, he says.

The construction was completed in May, and the utility has been monitoring dissolved-oxygen levels since then. The debate now centers on whether the company must keep the levels high enough to support fish life when the hydro systems are not generating power.

Crew says the utility only must keep levels high during generation, under a state environmental mandate. “The state’s water quality standard is very clear,” he says.

But fish need oxygen to survive whether power is generating or not, says Frank Chitwood, a member of Coosa Riverkeeper. A

Alabama law requires that dissolved-oxygen levels be no less than 4 mg/l at all times, including during periods of non-generation, the court said, noting that FERC’s interpretation is only during generation.

Allowing Alabama Power to operate without maintaining the minimum level of dissolved oxygen to avoid “acute mortality constituted a significant adverse environmental consequence without reasoned justification,” the court said. Levels fell below the minimum 4 mg/l more than 25% of the time across all Coosa dams, it said.

“We fully expect Alabama Power to maintain at least a 4 mg/l at all times to be fully compliant with the court’s decision and Alabama law,” says Sarah Stokes, an attorney with the Southern Environmental Law Center.

The court also admonished FERC for granting the license without determining if aeration systems proposed by Alabama Power would raise levels to the required standard.

The company has dissolved-oxygen issues only at the three dams with reservoirs on the Coosa. Intakes for the hydrogenation system primarily draw water from the deeper portions of reservoirs, which have lower oxygen levels.

Water release through powerhouses can suppress dissolved oxygen levels in tailraces below those dams. The utility has installed two electric blowers with a 24,000-cu-ft-per-minute capacity at those sites.

The challenge is retrofitting dams to get air to the draft tubes, Crew says. A new technology using a distribution ring to inject air into the main header of the tube at 150 locations creates much better absorption into water, he says.

Alabama Power awaits further FERC direction but expects it to require more environmental study of reservoirs, says Crew. The dams will continue to operate until a new license is issued.