YANKTON, S.D. (AP) - Over roughly the next two weeks, the U.S. Army Corps of Engineers will raise the water level on the Missouri River in hopes of coaxing the endangered pallid sturgeon into reproducing.

The plan is the result of a 2003 U.S. Fish and Wildlife Service opinion that calls on the corps to protect the fish by attempting to replicate the way melting mountain snow made the river rise each spring before dams were built.

Creating a so-called spring rise on the river has been the subject of court battles for years. The corps is going ahead with the latest plan, despite a lawsuit from the state of Missouri to try to stop it.

The corps' plan had called for two spring rises this year, but the first was canceled because water levels in reservoirs feeding the river were too low. The corps said the reservoirs now have more than enough water.

There is dissent upstream and downstream to artificially increasing the river level. Along its lower banks, people are worried crops could be flooded and barges could be sidetracked.

Most environmental groups see the plan as a good way to protect the river's wildlife.

Recovery programs nationwide, such as the spring rise, are a challenge. Managers must try to save a species. But with imperfect scientific knowledge, failures are likely and costly.

An estimated $33 million has been spent on pallid sturgeon recovery from 1996 to 2004 and more than $11 million will be spent this year.

Critics of the Endangered Species Act say that's too much for a system that does not work quickly enough.

Ambitious changes to the act passed the House last year. They were prompted by Rep. Richard Pombo, R-Calif., who has said that 10 of the roughly 1,300 endangered and threatened species have been recovered - "a less-than 1 percent success rate."

Rep. Stephanie Herseth, D-S.D., voted for the bill, though she was worried it might inject politics into what should be a scientific process. The bill now goes to the Senate Environment and Public Works Committee.

Sen. John Thune, R-S.D., is a member of that committee and said negotiations are at an impasse.

"I believe there should be landowner incentives included in the Endangered Species Act," he said in an e-mail. "Also, private landowners should not be held liable for the viability of those species."

Thune said committee chairman U.S. Sen. James Inhofe, R-Okla., plans to break the blockage by moving his own reform bill.

Sen. Tim Johnson, D-S.D., said the House bill "needs to be improved."

Defenders of the act say much of the criticism is based on unreasonable expectations. Researchers working on the Missouri River are often caught in the middle.

Many of them cringe when they hear "spring rise" because they see it as a phrase too often used out of context and intended to provoke controversy.

They say they are out only to collect data, which they hope will help to ease the controversy.

Several of them gathered at the Gavins Point National Fish Hatchery near Yankton last week to find out whether five pallids are ready to spawn.

U.S. Geological Survey Biologist Mark Wildhaber held down a pallid sturgeon while his colleague, Janice Bryan, applied an ultrasound scanner.

A cluster of tiny circles on the scanner told Bryan this fish was a female with plenty of eggs, perhaps 20 percent of her body weight. That strongly suggested she was ready to reproduce.

At the Garrison Dam National Fish Hatchery at Riverdale, N.D., biologists hope to get 400,000 eggs from nine breeding-age pallid sturgeon.