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GRAND CANYON NATIONAL PARK, Ariz. - A manmade flood that gushed between these cliffs a little more than a week ago churned up coffee-creamer-colored clouds of sand from the bed of a normally green river, heaving new beaches ashore the way ancient floods did every year.

At the same time that it seeded sheltering sandbars for an endangered fish, the 60-hour deluge drained $4 million from households that tap the river's hydroelectric power, from Phoenix to the Salt Lake Valley. Water blasting through four release tunnels raised the Glen Canyon Dam's output from a routine 14,000 cubic feet per second to a galloping 41,000.

It was the third in a series of government tests to restore sand movements that all but halted with man's taming of the Colorado River via the 1963 dam construction. Ecologists and park managers hailed this run as the most prodigious bulldozer of sands yet, setting up a battle for controlled flows that might better approximate nature's way.

"We know we need to keep studying, but we also know this is going to work," Park Superintendent Steve Martin said last week while smiling at a massive new sand mound burying the trunks of tamarisk trees. He's pressing the U.S. Interior Department for routine spring floods whenever Utah's Paria River dumps enough sand below the dam.

"It's clear that we need to do these flows every time we have the sediment." Under typical weather conditions, that would be about every two years - enough for the highly erosive Paria to dump about two-tenths of sand that the pre-dam Colorado would have collected in a single year.

But those who manage the dam don't agree that the rationale is proven. Randall Peterson, Upper Colorado resource manager for the U.S. Bureau of Reclamation, puts the hydropower plant's troubles in terms of a homeowner still trying to pay off the investment.

"You're working less hours but still have to pay your mortgage," he said.

Previous tests in 1996 and 2004 had mixed results for different reasons. The 1996 test surprised scientists when it showed they needed new sand and couldn't rely on years of accumulation, which might have washed away first. The 2004 test showed that the Paria's average annual dump was only enough sand to fortify the upper reaches.

Thanks to months of storms, this year the river had twice the new sand, and it showed.

Slicing through the current on a raft during the flood, Martin dipped an oar in murky waters that in most springtimes he would see straight through. The submerged wood disappeared from view. It was a good sign.

"Looking at this, it's hard not to be optimistic," Martin said.

But the Park Service's fortunes are at least as murky as the floodwaters. The agency and environmentalists would like not just springtime floods but steady flows for months afterward to calm spawning and rearing fish - a costly proposition when hydroelectric turbines need rapidly ramped-up flows every afternoon to meet fluctuating air-conditioning demands.

Aligned against them are competing federal interests that built the dam to win the West. Their power surges from 400,000 household electric meters across the Intermountain and Southwestern states.

National Park Service managers point to their congressional mandates to preserve park resources and endangered creatures such as the humpback chub, a species of fish. Water and power interests counter that Congress mandated something else entirely when it authorized the Glen Canyon Dam construction.

"There's a significant legal question as to whether they can bypass the turbines for any purpose," said Don Ostler, Utah's representative to the Upper Colorado River Commission, the board that administers watershed allocations. Congress in the 1950s mandated maximum low-cost power generation.

Lesley James of the Colorado River Energy Distributors Association said that either routine floods or steady summer flows are out of the question. "It is not feasible to continue to advocate for one of these bypass tests every year or every time there is a tributary input."

It's easy to divine this clash's wellspring. The government's Western Area Power Administration is charged with recovering dam construction and operation costs through power sales. Many of the communities that qualify for Colorado River power as nonprofit utilities are rural towns and Indian reservations, but metropolitan Phoenix also benefits. Such substantial Salt Lake suburbs as Bountiful, Murray and Tooele rely on the dam.

Every time a test dumps water in springtime, when there's limited power demand, it's a wasted economic opportunity. Spread among the households that use the power, it comes to $10 apiece.

Likewise, if the government decides to even out summertime daily flow fluctuations, utilities will have to pay more for coal-fired power to make up the difference. The last time the Bureau of Reclamation calculated those costs, in 1996, it figured a tab of $100 million a year. Spread among all the homes using Colorado River power, power bills would be bumped to about $21 a month.

Then there's the chub, a silvery little dagger with the bulbous shoulder of a spawning salmon.

Perched on a slope of boulders above the canyon confluence where the free-flowing Little Colorado River spills mud into its clearer namesake, park officials watching the flood observed a few of the foot-long members of the minnow family leap and roll in the soup. This tributary is the cradle of the chub's survival, owing largely to its warmer water. The dam, backing Lake Powell about 180 miles into Utah, cools water by submerging up to 9 trillion gallons away from the desert heat.

The trouble comes when flash floods push young fish from the Little Colorado into the mainstem's cold current, theoretically stunning them long enough that they're swept downstream, away from their safe haven. Building sandbars traps water on the river's sides so it can warm to the chubs' liking.

At least until this summer's up-and-down hydropower flows eat into them, the new sandbars appear up to the task, scientists on a six-day recon float trip said last week. John Hamill, chief of the U.S. Geological Survey's Grand Canyon Monitoring and Research Center, pointed to a soccer field-sized sandbar where two shirtless kayakers flung a Frisbee. Off to the bar's side was a long stretch of flat water, impounded on three sides.

"There's no flow-through, so it really just takes on the temperature of the air," he said. That can mean the difference between 60 degrees and 80 degrees. "That's a beautiful sandbar there," he said.

The flood also could protect archaeological treasures. One hundred-fifty ancient sites including camps, cliff dwellings and potsherds are threatened. Runoff from the canyon's rims chisels into them because there's no sand cover. But at one beach where researchers have placed wind-borne sand gauges that spin like weather vanes, studies of the 2004 flood determined that up to 1 1/2 feet of sand blew uphill to offer a buffer, said Helen Fairley, sociocultural manager at the monitoring center.

Elsewhere last week, including near Grapevine Rapids at the head of the narrow and fast-flowing Granite Gorge, waterborne sand piled up on beaches 12 to 15 vertical feet atop what pre-flood photos showed as boulder fields with no tent sites. Upstream at Cardenas Creek, a beach that had atrophied for years looked sweet to 23-year river guide Zeke Lauck as he floated past. Tamarisks and tufts of tall grass poked through a smooth white plain.

Although summer fluctuations will erode some of the gains, Peterson said it shouldn't be as bad as the last time, when the dam churned out up to 20,000 cubic feet per second in the summer. This year the flood will be limited by environmental considerations to 16,000 or 17,000 cubic feet per second.

Peterson and the Park Service's Martin jockeyed for scientific position at each research camp they passed last week. When U.S. Geological Survey research hydrologist Scott Wright told officials that early sampling suggested good sandbar construction, Martin pressed him to say whether frequent floods might be necessary to maintain the gains.

"It will take multiple events over many years," he agreed.

Peterson, in turn, pointed out that the river didn't deposit sandbars far downstream in 2004 even though researchers found it to be carrying lots of sand there. He said there's much to be learned before the next Interior secretary decides on a flow schedule, and there will be protracted lobbying.

Zuni Indian tribal cultural officer Octavius Seowtewa put the political ebb and flow in the eternal terms of geologic and spiritual time. His New Mexico tribe supports the floods as a way to protect sacred sites in the place their oral history calls their earthly origin, but it also favors letting them wash out naturally when time and tide are right.

Crouching by the river each morning before the floating field trip moved out, Seowtewa fed part of his breakfast to his ancestors in the water. It was both a sacrificial offering and a promise to keep watch over the canyon. "I told them I'm leaving now, but I'll be back," he said.

Glen Canyon dam and its environment

* Where's the sand?

Since the dam's construction in 1963, almost all of the sand that once coursed through the Grand Canyon has been trapped on flats on the upstream end of Lake Powell. Moving just a tenth of that sediment to and through the dam would require a $100 million pipeline, according to the U.S. Bureau of Reclamation.

* What about game fish?

The clear, cool water below the Glen Canyon Dam spawned a trophy rainbow trout fishery. Tinkering since then has dropped the non-native species' numbers and sizes, and anglers worry about losing the fishery altogether. "If floods happened every year it would [hurt trout], I'm sure," Arizona Game and Fish Commissioner Michael Golightly said.

* How cheap is the power?

The Western Area Power Administration markets Colorado River power at 2.5 cents per kilowatt hour, compared with 8.3 cents an hour in Salt Lake City for Rocky Mountain Power from a mix of sources, including coal. Most customers of Colorado River power rely on it for only a portion of their supply.

* How are endangered fish doing?

In recent years, the canyon's humpback chub population declined from 12,000 adults to just 4,000 but has since recovered 1,000 or so of its numbers. Environmentalists say the balance is precarious but others say the rebound is cause to maintain current practices.