The spillway at Libby Dam will be opened on Thursday in an effort to improve white sturgeon spawning success in the Kootenai River above Bonners Ferry, Idaho.

The dam will be ramping up to powerhouse capacity flows of 25,000 cubic feet per second during the week, and at about 7 a.m. Thursday, releases over the spillway will be incrementally increased to 10,000 cfs.

Female sturgeon tagged with sonic transmitters have been in position downstream from Bonners Ferry for about three weeks, and proper water temperatures at the dam are available for the spill test.

"The temperatures are right, the fish are in position, and this is the time to pull the trigger on this experiment," said Brian Marotz, a biologist with Montana Fish, Wildlife and Parks and a member of the interagency White Sturgeon Recovery Team.

The spill is aimed at determining whether the higher flows encourage sturgeon to move into optimal spawning habitat upstream from Bonners Ferry.

"We need to get them in areas where they can spawn on unconsolidated cobble where eggs can hatch and hide," Marotz said.

Biologists believe natural spawning success has been minimal to nonexistent since Libby Dam was built 36 years ago, leading to the listing of Kootenai River white sturgeon as an endangered species.

"These are ancient creatures that were doing well until not too long ago," Marotz said. "They've been in the Kootenai River since the Wisconsin Ice Age, which is about 12,000 years ... This is potentially a very recoverable fish and it will be important for future generations if we can keep them going."

Even with the higher flows, the Kootenai River is expected to be running several feet below flood stage. Because the spill will elevate gas levels in the river below the dam, two Montana Fish, Wildlife and Parks crews will monitoring gas levels.

They also will be on the lookout for signs of fish being distressed by elevated gas levels. Protocol for the test calls for the spill to be stopped if harm to fish below the dam exceeds a defined threshold.

The spill is scheduled to last for seven days and excessive impacts on fish below the dam are not expected, Marotz said, but there will be impacts on kokanee salmon in Lake Koocanusa.

"The public should expect to see a good number of dead and dying kokanee because they make the wild ride over the spillway," he said.

Biologists with the Corps of Engineers, Idaho Fish and Game and the Kootenai tribes of Idaho will monitor the movements of sturgeon in the Bonners Ferry area.

The spill test is the result of litigation pursued by the Center for Biological Diversity that led to a settlement in 2007. That agreement allowed for a trial period to prompt improved sturgeon spawning without spilling extra water from Libby Dam.

"They tried it in 2008 and 2009 and the success criteria were not met," said Noah Greenwald, endangered species program director for the Center for Biological Diversity. "As a result of the settlement agreement, we are moving to the spill test."

Greenwald said the spill will more naturally mimic historic spring flows on the river.

"We hope this works," he said. "We hope that we get some successful spawning from the sturgeon because they are running out of time."

Fewer than 500 adult sturgeon are believed to survive in the Kootenai River.

Reporter Jim Mann may be reached at 758-4407 or by e-mail at jmann@dailyinterlake.com.