From: Beth DeFalco, Associated Press
Published November 22, 2004 12:00 AM

Controlled Flood Set at Grand Canyon

PHOENIX − Environmentalists have complained for years that the Colorado River below a manmade dam was washing away natural sediment in the Grand Canyon, wiping away beaches and native fish and plants.


On Sunday, a simulated flood will allow scientists to see whether the Glen Canyon Dam -- the root cause of many of the problems -- can also help fix them.


Officials plan to release a controlled flood, opening four giant steel tubes at the base of the dam and sending a torrent down the Colorado and into the canyon. An estimated 800,000 metric tons of sediment will be stirred up during its 90-hour run.


"We're trying to mimic the role of all that sediment that used to be there before the dam," said Dennis Fenn, director of the Southwest Biological Science Center, under the U.S. Interior Department. "Water that goes through the dam is clear, and sediment-free. The sediment is trapped behind the dam and doesn't come down like it used to."


Fenn said only about 7 percent of the historical sediment before the dam was built is still there.


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Glen Canyon Dam, built 40 years ago upstream from the Grand Canyon, forever altered the landscape. Four of the canyon's eight native fish species have disappeared and prospects for the fifth, the endangered humpback chub, are grim.


Before Glen Canyon Dam's construction, natural flooding built up backwaters, eddies and sandbars with silt distributed from the Colorado's tributaries -- landscape features within the river considered essential to native plant and fish species, including the humpback chub and the razorback sucker.


Officials have unleashed high flood waters before to see how the environment responds.


The Interior Department began studying the effects of the dam on the Grand Canyon in the early 1980s -- and soon found beaches were washing away. In 1996, officials flooded the canyon with an 18-day water release, although only about five of those days produced high floods.


"We learned a lot from that study," Fenn said.


He said a major problem was that scientists overestimated the sediment in the bed of tributary rivers that flow into the Colorado River below the dam. The initial high flood waters redeposited sediment in the Grand Canyon. But steady, lower flood waters began undoing the good, eroding the moved sediment.


Source: Associated Press


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