GRAND CANYON NATIONAL PARK, Ariz. -- Scientists unleashed flooding at the Grand Canyon yesterday to restore beaches and save fish and plants that have been disappearing since sediment-free water began flowing from a manmade dam 40 years ago.
A torrent of gushing water raced down the Colorado River and into the canyon, carrying badly needed natural sediment with it, as four large steel tubes at the base of Glen Canyon dam were opened.
''The sediment, sand, mud, and silt play an important role in the ecosystem," said Chip Groat, director for the US Geological Survey.
An estimated 800,000 metric tons of sediment were expected to be stirred up during the 90-hour run. The release should reach a peak early this morning, when 41,000 cubic feet of water -- enough to fill more than 11 tractor-trailers -- will be moving through the dam every second.
Four decades ago, before the dam was built, natural flooding built up backwaters, eddies, and sandbars with silt distributed from the Colorado's tributaries.
The construction of Glen Canyon dam upstream forever altered the canyon: Four of eight native fish species have disappeared and prospects for the fifth, the endangered humpback chub, are grim. Only about 7 percent of the sediment before the dam was built remains.
About 50 scientists will study the immediate effects of the high-flow test on the canyon today. A small group of researchers began rafting down the river yesterday as part of the analysis.
Interior Department scientists will continue to study the effects on the river for the next 18 months.
In 1996, officials flooded the canyon in an 18-day water release, although only about five of the days produced high floods. But scientists overestimated the sediment levels in the beds of the tributary rivers that flow into the Colorado below the dam, and sediment redeposited by some of the flooding was only eroded away by other flood waters.