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Experimental floods from Glen Canyon Dam have proved effective at rebuilding environmentally critical Grand Canyon sandbars when timed properly, but could killendangered fish when they're not, according to a new U.S. Geological Survey report.

The latest of three such floods, unleashed in March 2008, created lasting Colorado River sandbars to benefit fish, wildlife and river-running campers, said this week's report. It also caused an explosion in nonnative rainbow trout, which eat and compete with the endangered humpback chub.

The results will help river managers craft a plan to govern more experiments in the coming decade, and the researchers say it might make sense to release a burst of water at least once a year whenever tributariesbelow the dam deposit enough sand. In dry years, when Utah's Paria and Arizona's Little Colorado don't add enough sediment, scientists say it might not make sense to do any so-called grand flushes.

Sandbars and beaches have dwindled since the government finished Glen Canyon Dam in 1963. Where still present, they help form warm pools for native fishes(refuges from the cold waters released from Lake Powell's depths) and beaches for wildlife and recreation. The sands also help protect archaeological sites from erosion or vandalism.

At least where sands are concerned, short floods from the dam can help. Of 34 river study sites, researchers found, 31 were larger just after the spring 2008 flood than they had been just before the first flood in early 1996. Summer fluctuations in dam releases, largely timed to hydropower needs, eroded those sandbars, but, by October 2008, 25 still were larger than before.

"It does seem to be somewhat encouraging," said Ted Melis, deputy chief of the Geological Survey's Grand Canyon Monitoring and Research Center and editor of the report.

For endangered fish, though, flood timing may be a life-or-death factor. After the 2008 flood, researchers say, the rainbow trout that draw anglers just below the dam to Lees Ferryhad a great spawning year and, consequently, trout numbers downstream in a crucial area for endangered chubs grew by eight times. It's unclear what that did to the chub population, but the report says it likely was bad.

That's one reason why, the report suggests, the Bureau of Reclamation should consider fall floods, like the 2004 experiment, more often in the future. Another is that the Paria, the most important source of sand below the dam, usually floods during summer and fall monsoons, making fall the best time to push sediments downstream.

Scientists have found that floods must be repeated routinely when enough sands have accumulated below the dam. Otherwise, erosion undoes the work. On Interior Secretary Ken Salazar's orders, Reclamation is preparing a protocol to spell out conditions under which it will repeat the floods. It's also preparing a plan to keep rainbow trout in check.

Some hydropower customers, including in parts of rural and suburban Utah, could be among the losers if the bureau opts for frequent flushes. The floods bypass the dam's turbines, meaning lost generation capacity and potentially higher electric rates.

"We have serious concerns regarding the potential for [artificial floods] to harm humpback chub," said Leslie James, executive director of the Colorado River Energy Distributors Association. "In addition, [floods] that exceed the capacity of the hydropower generation facilities at Glen Canyon Dam will force utilities that rely on that hydropower to spend millions of dollars to replace the power that wasn't generated."

The Upper Colorado River Commission, which includes Utah water managers, also has reservations. The USGS report shows success in sandbar construction, Executive Director Don Ostler said, but it could be disastrous to rush into a regimen of frequent floods without first ensuring the trout problem is fixed.

"I'm hearing the USGS saying we should test that," Ostler said, "which means we're kind of running a test on endangered fish."

Grand Canyon National Park officials hope for more flushes, assuming river managers also control trout.

"It's our feeling," said acting Park Superintendent Jane Lyder, "that if we can have a high flow whenever there's a sedimentation trigger, then we will be much better off than we have been in the last 15 years."

The Bureau of Reclamation's protocol will set conditions for floods in two potential windows each year, March-April and October-November, said Beverley Heffernan, the agency's Upper Colorado Region environmental resource manager.

The USGS report will help inform the plan, she said, and it appears to indicate that the fall window will be best in most years. In other years, though, there may be enough springtime sediment from tributaries to justify a flood then, or even two floods in one year.

In the meantime, the agency is considering how to control trout. Until last year it occasionally sent out boats to stun fish with electricity in the upstream sections dominated by trout, and then kill the ones that float up. That stopped, Heffernan said, when tribes including the Navajos, Zunis and Hopis protested.

The new plan will consider whether to resume killing the fish but sharing them with the tribes, she said, or perhaps making live transfers away from the Colorado.

About the river plans

• To see the Bureau of Reclamation's study of potential future flood plans, go to http://tinyurl.com/4u3yzjt

• To comment by the Feb. 28 deadline, write to Bureau of Reclamation, Environmental Resources Division, 125 S. State St., Room 7218, Salt Lake City, UT 84138, or e-mail protocol@usbr.gov

• To read the USGS report about three experimental Glen Canyon floods, go to http://tinyurl.com/4tet3m3