Image: Eroded sandbar
U.S. Geological Survey via AP
In the top photo from March 10 a sandbar created by a manmade flood along the Colorado River is shown. The bottom photo of Aug. 24 shows the same sandbar, which has shrunk to its pre-flood size.
updated 9/10/2008 11:06:15 AM ET 2008-09-10T15:06:15

Newly built-up sandbars crucial to wildlife in the Grand Canyon have rapidly eroded in the last four months, some shrinking back to the size they were before a costly manmade flood.

Torrents of water were released from the Glen Canyon Dam on the Arizona-Utah line in March to mimic natural flooding and rebuild sandbars along the 277-mile river in the Grand Canyon, where the ecosystem was forever changed by the dam's construction more than four decades ago.

Officials had expected erosion following the three-day flood, but they hadn't expected so much so fast.

"Circumstances conspired against our being able to protect the beaches as long as we had hoped," Grand Canyon National Park Superintendent Steve Martin said Tuesday. "Substantial erosion has occurred."

The accelerated erosion is the result of a requirement to release extra water from Lake Powell above the dam into the Colorado River, said John Hammill, chief of the Grand Canyon Monitoring and Research Center.

The requirement says that when Lake Powell has extra water, some of it needs to be released and go to Lake Mead on the Arizona-Nevada line. The requirement is designed to ensure that Colorado River states all get an equal share of water, a precious and limited resource in the West.

Lake Powell rarely has extra water because of extended drought, but a wet winter led to the highest water level in the reservoir in six years. That triggered the requirement in April, a month after the three-day flood in the Grand Canyon.

Between April and Sept. 1, officials increased flows from the Glen Canyon Dam by 20 percent. Time-lapse videos taken by the U.S. Geological Survey of two sections of the Grand Canyon show that the three-day flood created sandbars as large as football fields. But the sandbars began shrinking in April and by August appear to be much smaller and about the same size they were before the flood.

Four to five times the normal amount of water was released from the dam during the flood, picking up sediment and depositing it in sandbars.

The 20 percent higher flows this summer then washed away much of it. Compounding the erosion is the practice of sending more water through the dam in the daytime to maximize power generation, then trimming output at night.

Martin said some benefits remain from the three-day flood, and despite the erosion, more floods should be released into the canyon whenever there's enough sediment to be deposited along the shoreline — about every one to two years.

Since 1963, the Glen Canyon Dam has blocked sediment from the Colorado from flowing 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.

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

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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