But there may be an even more tangible reason for unleashing 224,000 acre-feet of water from Lake Powell over 2 1/2 days - as river stewards did last week.

"It's about the people," said Kanab river guide Arthur Thevinin.

More than 22,000 visitors get drenched every year riding the river's roller-coaster currents through Grand Canyon National Park in northern Arizona. So anything that buoys up those boat trips will inflate public and private coffers. The river runners already pump more than $20 million a year into the area's economy.

But did the Great Gush of 2004 - unlike a previous attempt in 1996 - accomplish its mission? Scientists won't know for sure until next year. But initial signs are encouraging as the water begins to recede.

"Anecdotal information is coming back that we are seeing sand on beaches that could serve as a basis for fish habitat and a healthy ecosystem," said Jeff Cross, director of the Grand Canyon's Science Center for Natural and Cultural Resources.

Crews have begun gathering data from 22 experiments along the river. After the Sunday-through-Tuesday water release, the National Park Service filled three pontoon boats with scientists, officials, reporters and photographers for a three-day trip down the river - from Lee's Ferry, Ariz., to just east of Lake Mead - to check on those experiments.

Scientists hope to re-establish the shriveling sandbars to create backwaters for endangered fish like the humpback chub and give thrill-seeking river runners more places to camp, dry off and soak in the grandeur of the Grand Canyon.

Those sandbars have been vanishing because Glen Canyon Dam traps 92 percent of the sand and sediment that would normally flow down the river. Instead, the tributaries below Lake Powell contribute the bulk of the sandbar-sculpting silt.

The largest source of that sediment comes from southern Utah's Paria River - which joins the Colorado at Lee's Ferry, the launching point for commercial and private boaters 15 miles below the dam.

The Paria churned more than a million metric tons of silt and clay into the Colorado this fall. But the flows were not strong enough from the dam to carry the sediment down the Colorado River.

Enter: the Bureau of Reclamation, which opened up Glen Canyon Dam's bypass tubes on Nov. 21 to release 306,680 gallons of water per second. Scientists hope that's enough to redistribute the sediments injected into the Colorado by the Paria.

The volume of the release dropped the level of the already-low Lake Powell by 2 1/2 feet. While that may seem alarming at a time of drought, the water is part of the 8.23 million acre-feet required to be released from Powell to meet user needs downstream.

Even so, the deluge practically buried the yellow tips of the invasive tamarisk that grow along the banks and transformed the Colorado's emerald-green flow to a shade of milky coffee.

Bennett Raley, assistant interior secretary, checked out the flush's aftermath firsthand. "Unless we depopulate the West, which is becoming one of the most vibrant and challenging places to live, we need to manage its resources in a way that no one is thrown over the cliffs," said Raley, decked out in cowboy hat and a life vest autographed by journalists he has clashed with over water issues.

Initially, Raley was skeptical of the experiment but became convinced after running the river two years ago with Cross and other scientists.

Raley called the experiment an effort to unite all the Colorado River stakeholders under the three C's - collaboration, cooperation and communication - stressed by his boss, Interior Secretary Gale Norton. Power officials, farming representatives, recreation enthusiasts and environmental groups all signed off on the study.

Ted Mellis, the integrated-science program manager who leads a team of soil experts and hydrologists for the U.S. Geological Survey, said it's vital to learn how and where sediment is deposited into sandbars during high flows and what flows are needed to maintain them - if it can be done at all. "You can't go beyond what you export," he said. "It's like a finance manager who has to keep accounts in the black."

During the flush, crews lowered a 122-pound metal "fish" into the river to collect samples containing sediments. They also submerged laser devices at various points to scan the size of particles as they drifted by.

At one point, Mellis' team followed a single patch of high water for 15 straight hours as it made its way down the Colorado.

Experts also are evaluating the river's topography from before and after the artificial flood.

All the data will be presented during an October symposium in Phoenix.

Besides shoring up sandbars, scientists hope to buoy up the humpback chub, a native fish listed as endangered since 1967. Sandbars create backwaters where the fish can grow to maturity.

The chubs, which can live up to 30 years and grow to 18 inches in length, have been declining in recent years - from 8,000 to 4,000 over a 60-mile strip between Lee's Ferry and where the Colorado hooks up with the Little Colorado River.

Arizona Fish and Game biologist Bill Persons said the sinking chub populations could be the result of lower water levels and higher water temperatures. An Asian parasite or increases in rainbow and brown trout - which feed on chubs - also could be factors.

Six times a year for the past two years, biologists have been killing trout in the river to decrease their numbers and studying their stomachs for signs of predation.

Michael Yard, a consulting biologist and self-proclaimed "fish zapper," said crews lower electrodes capable of emitting 350 volts into the water at night to stun the trout, which then are scooped up in nets when they pop to the surface. Their stomachs are then tested. Any chubs that get stunned are returned to the river.

The dead trout are ground up for fertilizer and delivered to the Grand Canyon-based Hualapai Tribe.

"We don't know yet if the trout are the source of mortality for the humpback," Yard said. "The results are still outstanding."

Biologists also are studying whether the flood-induced sandbars will help nurture vegetation for the endangered southwestern willow flycatcher. They also lifted habitat sections for the endangered amber snail above the brief high water levels to keep them from being washed away during the artificial flood. The habitats will be returned when the water recedes.

If nothing else, said David Burnheart, Norton's deputy chief of staff, at least the water release spawned a new cooperative spirit.

"There is a lot of science being conducted here from diverse interests that will help in making challenging public policy decisions," Burnheart said.

Raley was confident that the effort will pay off.

"I've made a lot of mistakes," he said. "This is not one of them."

mhavnes@sltrib.com

WHAT HAPPENED?

On Nov. 21, the Bureau of Reclamation increased the flow of water from Lake Powell into the Colorado River by five times for 60 hours. The level of Lake Powell dropped by 2 1/2 feet.

WHY FLUSH THE CANYON?

The flush should help rebuild shrinking Colorado River sandbars, replenish fish habitat, help the ecosystem and provide backwaters and landing spots for water tourists.

HOW DO THEY MEASURE SUCCESS?

Samples were collected by a device lowered into the river to measure particle size. The experiment will be evaluated by comparing topography before and after the artificial flood.