A federal judge has rejected an effort by sportsmen and conservationists to block a cutback in the amount of water released from Palisades Reservoir into the South Fork of the Snake River.

The U.S. Bureau of Reclamation last reduced the flow below Palisades Dam from 1,000 to 750 cubic feet per second to allow more water to build up in the reservoir for future irrigation needs.A coalition of sportsmen and environmental groups, including the Upper Snake River chapter of Trout Unlimited, already had asked U.S. District Judge Marion Callister to issue a permanent injunction against the reduction. It argued the cutback would threaten fish and other wildlife along the South Fork.

Callister ruled against the request, but asked for briefs so he can rule on other aspects of the coalition's lawsuit against the Bureau of Reclamation.

It had sought a minimum stream-flow of 2,000 cfs in the 67-mile stretch of river until the agency completes an environmental impact statement on the reduction, but the coalition has agreed to settle for a minimum flow of 1,000 cfs.

"If this action continues, a fishery will be decimated," John Radin, the attorney for the sportsmen and conservationists, said in closing remarks at a hearing before Callister.

Up to 600,000 brown and cutthroat trout are believed to have died when flows in the South Fork were cut to 750 cfs last year, Radin said.

But attorneys for the Bureau of Reclamation and water users convinced Callister that irrigators, not anglers, control water in the Upper Snake River Valley, and that allowing a higher streamflow would hurt the farm economy more than the fish.

The agency's 80-year-old Mini-doka Project, a system of dams and reservoirs along the Snake River, delivers water to 1.2 million acres in southern Idaho.

Roger Long, an agricultural economics professor at the University of Idaho, testified that increasing flows from 750 cfs to 2,000 cfs in the South Fork could mean a loss of up to $385 million for the agricultural sector if Idaho's two-year drought continues.

"There would be very serious im-pacts, probably greater than what I've stated here," Long said.

Sportsmen argued there would be enough water for winter flows if irrigators took additional conservation measures, such as lining canals. They cited a 1983 study indicating that 686,000 acre-feet of water - or about half the storage capacity of Palisades Reservoir - is lost to canal seepage each year.

But the Bureau of Reclamation discredited the study, which was conducted by Biowest Inc., a Utah-based consulting firm.

"The major conclusions reached in that report were not supportable," said Max Van Den Berg, manager of the agency's Minidoka irrigation project.

He said Congress had authorized a minimum streamflow of 500 to 1,000 cfs on the river.

Another witness testified that canal lining is cost-prohibitive. Charles Brockway, a professor of hydrology at the University of Idaho, said it would cost $102 million to line ditches operated by Twin Falls Canal Co. alone.

"I'm not an economist, but I don't believe the Twin Falls farmers could pay for that," he said.

Brockway added that canal lining also would reduce outflows at critical river recharge sites at the Fort Hall Bottoms near Springfield and the Thousand Springs in the Hager-man Valley.

More than 100 canal companies are located in the Upper Snake River Basin.