SPOKANE - The latest effort to save North America's largest freshwater fish from extinction begins this week when water is spilled over Montana's Libby Dam to encourage the ancient fish to spawn for the first time in more than three decades.

The wild Kootenai River white sturgeon, a toothless beast from the days of dinosaurs, has a large head, armor-like scales, can reach 19 feet long and top 1,000 pounds. It takes 20 or 30 years for white sturgeon to mature and reproduce.

An isolated population of the bottom-feeding behemoths lives along a stretch of the Kootenai that passes through Montana, Idaho and British Columbia, Canada. The construction of Libby Dam in 1974 stopped the river from flooding Bonners Ferry, Idaho, but also prevented the high water flows that triggered the sturgeon to move upriver and spawn.

Before the dam, there were an estimated 10,000 Kootenai sturgeon. Fewer than 500 mature adults of spawning age remain.

The effort this week will spill up to 10,000 cubic feet of water per second over the dam in a huge waterfall for up to seven days, in what scientists hope will push the sturgeon to more productive spawning grounds in Idaho. The water will spill from Koocanusa Reservoir into the Kootenai River, where scientists hope the sturgeon will swim to the Bonners Ferry area.

The U.S. Army Corps of Engineers on Tuesday began sending more water through the dam's turbines, in preparation for opening spill gates on Thursday.

"The idea is to recreate more of the natural, spring conditions," said Michael Milstein of the U.S. Bonneville Power Administration, which markets the power from the dam. "That is believed to be a factor that led sturgeon upriver to spawn."

It's not known if the ploy will work. Scientists will track the sturgeon to see if they are moving and spawning, said Nola Leyde of the Army Corps.

Biologists say the wild fish could become extinct within the next decade unless a fix is found. But sturgeon will not disappear from the river.

Idaho's Kootenai Tribe of Indians has stocked the river since the 1990s with thousands of hatchery-raised sturgeon. But the Endangered Species Act requires a species to reproduce naturally in order to be considered recovered.

"The goal is to have a self-sustaining population," Milstein said.

Even with the increased spillover from the dam, the Kootenai River would rise to less than half its historical levels.

There are 24 species of sturgeon worldwide; most are threatened with extinction. The plan to save the fish in the Kootenai came out of a 2008 settlement with environmentalists who sued the federal government for failing to take action.

Noah Greenwald, endangered species director for the group behind the lawsuit, the Center for Biological Diversity, said they hope the spill will work.

"Time is really running out for the sturgeon," he said. "They have not spawned in any real numbers since 1974."