GRAND CANYON NATIONAL PARK, Ariz. -- Four decades after one of the West's last big dams blocked the free flow of water into the wild recesses of the Grand Canyon, the Colorado still manages to roar through here like the granddaddy of Western rivers. But it has become the Hollywood version -- strikingly beautiful and in vital ways, fake.
With every passing year, the Grand Canyon's stretch of the Colorado River bears less and less resemblance to its former self. The fine, white sand beaches on which thousands of weary boaters unfurl their sleeping bags every summer are disappearing.
So are native fish species that have been in the canyon for millions of years. Millennium-old American Indian burial sites are washing away with the eroding sands.
Without the scouring of regular flooding, the feathery green tamarisk bush imported to the United States in the 1800s is overrunning the river banks, and boulders washed out of side canyons are piling up in the main channel. The river's mythic rapids are growing more difficult to navigate and some may become impassable.
The 1963 completion of Glen Canyon Dam just upstream from the park is best known in environmental circles for drowning stunning canyon lands under the waters of Lake Powell. But its effects have also been traumatic in the downstream river corridor of the Grand Canyon, through the heart of the park.
A warm, muddy, violently unpredictable river that shaped the canyon's ecosystem for millions of years turned cold, clear, steady and aquamarine. It may match the romantic notion of a river, but it is utterly unnatural in this sunbaked cleft in the Colorado Plateau.
The damage has long been recognized. Congress in 1992 passed the Grand Canyon Protection Act, directing the Interior Department to devise ways of making the dam's water releases for generating hydroelectric power less harmful to the canyon environment.
But it is increasingly apparent that the modified flows, adopted eight years ago, haven't worked. The failure has deepened the pessimism of some experts that, short of taking down the dam, humans may not be able to offset the harm done by its construction.
"The Grand Canyon river corridor is getting nuked," said David Haskell, a retired National Park Service career officer who directed the Grand Canyon's science center from 1994 to 1999. "It's in the final stages of having the natural ecosystem completely destroyed and replaced with a man-made one because of the presence of the dam."
That is not exactly the way federal scientists put it in their briefings to a group of some two-dozen water managers, Interior Department officials and journalists who recently spent a week rafting down the river, discussing the drought and federal water policy with Assistant Interior Secretary Bennett Raley.
But the canyon told the tale.
"The beaches continue to erode. The humpback chub (a native fish) continues to decline," said Jeffrey Cross, the current director of the park's science center. "Tamarisk has not only invaded the main stem but has moved up many of the tributaries of the canyon. These are all changes that have happened and have continued to happen."
There were 10,000 humpbacks in canyon reaches in 1992. Now there are 2,200. Of the eight native fish species found in the canyon before the dam, four are now gone.
In the early 1970s, there were about 180 sand beaches roomy enough to allow rafters to pitch a tent. Half that number are left, Cross said. The rest have washed away or are so overrun by tamarisk that camping is impossible.
Lars Niemi, a 42-year-old boatman who has been on the river since he was a teen, has watched the beaches dwindle. "We just used to be able to throw down in a lot of places that aren't there anymore," he said, his hand on the rudder of one of the Raley group's two big pontoon boats.
It was the third trip through the canyon for Raley, the Bush administration's point man on water policy. A Colorado attorney and property rights advocate who has no qualms about dams, Raley is nonetheless drawn back here, not just by the rock-walled grandeur, but by the river's imprint on the Western psyche.
"I don't know how you can come down here and not be humbled," said Raley, who sees political life as a tug of war between idealism and compromise -- one that is reflected on the river. "There's virtually nothing that goes on here that doesn't involve trade-offs or balances."
The rafting party glided by pale red and beige canyon walls that opened onto majestic vistas of mesa and then closed into dark gorges chiseled into a million different faces. The water arched in polished blue-green curls, looking more like the Caribbean than a river named Colorado -- "colored red" in Spanish -- after the ruddy sediment washed into it along its 1,400-mile length.
Geologically, the river functions as a huge watery conveyor belt carrying ancient, eroded bits of the Colorado Plateau to the Gulf of California. Before the Glen Canyon dam, at least 60-million tons of sand and silt tumbled and slid through the Grand Canyon every year, swept along by annual floods four times greater than today's high flows. When the dam went up, it stopped not only the floods, but the sand, which is piling up at the bottom of Lake Powell, the reservoir behind the dam.
Now the canyon's only sand comes from two tributaries below the dam, the Paria and Little Colorado rivers, which contribute less than 10 percent of the river's historic volume of sediment.
Without sand, the Grand Canyon river system is like a body without nourishment. Fine sands and silts are loaded with nutrients for aquatic life that become food for insects that, in turn, become food for fish and birds. The sediment builds spawning beds for fish and sand bars where plants can grow and river rafters can sleep.
"At all sorts of levels the sand is the foundation of the system," said Ted Melis, a U.S. Geological Survey scientist with the Grand Canyon Monitoring and Research Center who has studied the river for years.
The banks are actually more verdant than they used to be because there are no longer any major floods to wash out vegetation. But most of the growth is tamarisk, which is shunned by the canyon's desert big horn sheep and displaces native willow and cottonwood that offer more diverse bird habitat.
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