WASHINGTON - It's too early to know whether releasing extra water into the Missouri River is helping to boost the population of an endangered fish, the U.S. Army Corps of Engineers said Wednesday.

But the agency's first report on last year's artificial spring rise found it caused no downstream flooding or damage to sensitive areas along upstream lakes where American Indian artifacts are buried.

The spring rise was designed to mimic rises that used to occur naturally before construction of dams on the upper Missouri River. Government scientists believe a spring rise is critical to encourage spawning by the pallid sturgeon, which would help restore the fish's dwindling numbers.

Missouri officials, long opposed to the spring rise, have questioned whether the plan is based on sound science.

That dispute won't be resolved anytime soon because the corps' report says there's not enough data yet to conclude whether the spring rise is having an effect on spawning.

While the corps had planned for two releases last year - the first in March and a second in May - the drought left water levels high enough for only a single release in May.

That change in plans "did not produce the clean experiment that was anticipated" by officials studying the impact of the spring rise on the pallid sturgeon, the report said.

Also, size of the May pulse was smaller than expected because of low water levels upstream. That made it difficult to isolate the effect of the flow on the fish, the report said.

"Since 2006 was the first year for the artificial spring pulse, it's too early for definitive results, especially biological responses," said Paul Johnston, a spokesman for the corps' northwestern division office in Omaha, Neb.

Last week, the corps announced it was scrapping both of the spring pulses for 2007 because water levels in upstream reservoirs remain too low. But there is no indication the agency will consider bowing to pressure from interest groups that want the practice ended altogether.

The report said the initial spring rise in 2006 provided a baseline set of data that scientists will analyze to help determine how to conduct future rises.

Careful monitoring of groundwater levels downstream also showed that "no manmade flooding was induced by the spring rise," the report said.

In preparation for the release, the corps bumps up water levels in upstream reservoirs.

Data from Lake Francis Case in South Dakota and Lewis and Clark Lake in Nebraska showed some evidence of erosion, the report found, but not enough to expose the remains of sensitive Indian burial sites. Indian tribes had expressed concern that the spring rise might disturb artifacts buried around the lakes and subject them to looters.

The report concluded that the corps should not allow water levels in those lakes to get too high.