Manmade flood from US dam unleashed in Grand Canyon

Amanda Lee Myers, Associated Press Yahoo News 7 Mar 08;

Four arcs of water unleashed from a dam coursed through the Grand Canyon on Wednesday in a flood meant to mimic the natural ones that used to nourish the ecosystem by spreading sediment.

More than 300,000 gallons of water per second were released from Lake Powell above the dam near the Arizona-Utah border. That's enough water to fill the Empire State Building in 20 minutes, said Interior Secretary Dirk Kempthorne.

"This gives you a glimpse of what nature has been doing for millions of years, cutting through and creating this magnificent canyon," Kempthorne said after he pulled the lever releasing the water from Glen Canyon Dam, upstream from Grand Canyon National Park.

The water gushed from two of four giant steel tubes in parallel arcs into the Colorado River. By afternoon, water poured from all four tubes, creating a churning pool beneath the sheer, sandstone canyon walls rising hundreds of feet.

The water level in the Grand Canyon rose 2 to 15 feet in some places. After the flood ends Friday, officials hope the water will leave behind sediment and restore sandbars as it goes back to normal levels. Officials have flooded the canyon twice before, in 1996 and 2004.

Before the dam was built in 1963, the river was warm and muddy, and natural flooding built up sandbars that are essential to native plant and fish species. The river is now cool and clear, its sediment blocked by the dam.

The change helped speed the extinction of four fish species and push two others, including the endangered humpback chub, near the edge.

Shrinking beaches have led to the loss of half the camping sites in the canyon in the past decade. Since Glen Canyon Dam was built, 98 percent of the sediment carried by the Colorado River has been lost, Grand Canyon National Park Superintendent Steve Martin said.

Martin said manmade floods need to occur every time there's enough sediment to do so — about every one to two years depending on Arizona's volatile monsoon season.

"The science is really clear that's what we need to do," Martin said.

The Grand Canyon Trust, a Flagstaff-based group that has been critical of the federal Bureau of Reclamation's management of the dam, also is calling for more regular high flows.

"The power industry is driving the Bureau of Reclamation more than anything else, as opposed as to what's best for the canyon," trust spokesman Richard Mayol said.

Scientists will document habitat changes and determine how backwater habitats are used by the chub and other fish. Another study will look at how higher water flows affect the aquatic food base.