PHOENIX — The results of a manmade flood in the Grand Canyon last week were immediate and substantial, adding as much as football field-sized areas of vital sediment to parts along the Colorado River, officials said.

The three-day flood that ended last Friday was designed to redistribute and add sediment to the 277-mile river in the Grand Canyon, where the ecosystem was forever changed by the construction of a dam more than four decades ago.

The sediment provides a habitat for plants and animals, builds beaches for campers and river runners and helps protect archaeological sites from erosion and weathering.

But since 1963, the Glen Canyon Dam just south of the Arizona-Utah border has blocked the sediment from the Colorado downstream, turning the once muddy and warm river into a cool, clear environment that helped speed the extinction of four fish species and push two others near the edge.

Grand Canyon National Park Superintendent Steve Martin, who returned from a five-day trip down the river Tuesday to see the initial impacts of last week’s flood, said the results are phenomenal.

“On a couple of big sandbars there were already beaver tracks, bighorn sheep tracks,” Martin said. “You could see the animals already exploring new aspects of the old canyon.”

The new sandbars range in size from small nooks and crannies to ones as large as football fields.

“It changes the feeling of the canyon as you see the sediment along the shoreline from a feeling of increased sterility to one of a greater amount of vibrance,” Martin said.

During the flood, flows in the Grand Canyon increased to 41,000 cubic feet per second for nearly three days — four to five times the normal amount of water released from the Glen Canyon Dam. Water levels along the river rose between 2 and 15 feet and left sediment behind when the four giant steel tubes releasing the water from the dam were closed.

Officials released similar manmade floods into the canyon in 1996 and in 2004.

But those floods actually resulted in a net reduction in overall sandbar size because they were conducted when the Colorado River was relatively sand-depleted, according to the U.S. Geological Survey. Officials believe this year’s flood will be beneficial because sand levels in the river are at a 10-year high and are three times greater than 2004 levels.

Whatever benefits come from this year’s flood, however, will be eroded away within 18 months without additional similar floods every year to 18 months depending on the amount of sediment available, according to Martin.

The U.S. Bureau of Reclamation calls for no other high-flow releases other than last week’s flood until after 2012 in its environmental assessment on Glen Canyon Dam releases.

The Grand Canyon Trust, a Flagstaff-based group that has been critical of the bureau’s management of the dam, also is calling for more regular high flows and plans to legally challenge the bureau’s environmental assessment in federal court.

“It’s kind of like when President Bush landed a jet on the aircraft carrier and held up a banner that said ’mission accomplished,”’ said Nikolai Lash, senior program director at the trust. “Reclamation has come in with a lot of show and fanfare from last week’s event and we’re seeing the benefits of doing these high flows. But we know that they’re short-lived and the Grand Canyon deserves long-lived benefits, long-lived restoration.”

Scientists will collect data on the flood’s effects through the fall. Initial reports will be available late this year or early next year. A complete synthesis of the results, which will include comparisons to the 1996 and 2004 floods, will be finished in 2010.

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On the Net:

Grand Canyon: http://www.nps.gov/grca/