HELENA, Mont. -- The Bureau of Reclamation said Tuesday it was cutting flows into Montana's Big Horn River because of a later-than-expected spring runoff. State officials warned the move would harm the river's renowned trout fishery.

Bureau area manager Dan Jewell said the reduction would be made overnight Tuesday, and was needed to make sure enough water is behind Yellowtail Dam, a source of hydropower.

The dam also contains Bighorn Lake, a recreational area straddling the Montana-Wyoming border. Those two states have been in a running political skirmish over which of the uses along the Big Horn should take precedence.

Montana wants more water for a healthy trout population; Wyoming more water for the tourists and locals who use the lake.

By this morning, flows on the river were to drop from 1,900 cubic feet per second to 1,500 cfs. They would stay at that level for at least 25 days as the lake fills.

That's long enough to kill 80 percent of the river's newly spawned brown trout, said Bob Gibson with Montana Fish, Wildlife and Parks.

"We understand the complexities of managing for more power, for water skiers on the lake," Gibson said. "It's difficult that Montana fly fishermen are going to bear the brunt of it. There's a world-class fishery that is being damaged."

Jewell acknowledged Tuesday's reductions would hurt the trout fishery, a major economic draw in south-central Montana. But without the cutback, he said the bureau could be forced to make more drastic cuts in the fall and winter, a crucial time for rainbow trout.

A smaller cutback, to just 1,650 cfs, had been proposed last month. That action was suspended after Montana officials said they would accept a deeper reduction in early May if warmer weather did not arrive to melt upstream mountain snow.

"They took that gamble, but the runoff just hasn't come as early as they hoped it would," Jewell said.

Montana's Republican congressman, Denny Rehberg, issued a statement urging Jewell's agency to remain flexible in coming weeks as it continues to assess appropriate flows for the river.