PHOENIX For the third time since 1996, officials plan to unleash a man-made flood in the Grand Canyon next month in an effort to restore an ecosystem that was altered by a dam constructed on the Colorado River decades ago.
The Glen Canyon Dam, completed in 1963 upstream from the Grand Canyon, permanently changed the Colorado River, transforming it from a warm, muddy, unpredictable force of nature into a cooler, clearer, tightly controlled water-delivery system.
Without spring floods to flush the system and help rebuild beaches and fish habitat, native species suffered even as non-native fish thrived. The shift helped speed the extinction of four fish species and push two others, including the endangered humpback chub, near the edge.
In 1996, the government staged the first artificial flood in the canyon, opening Glen Canyon Dam's bypass tubes for several days in an attempt to replicate natural cycles. A second test in 2004 taught scientists the importance of sand and sediment.
The dam traps almost all the sediment that once flowed down the river, which is why beaches and habitats have eroded. A good monsoon season can wash significant quantities of sand down the Paria and Little Colorado rivers, which empty into the big Colorado below the dam.
Next month's flood will scour and reshape miles of sandy banks on the floor of the Grand Canyon.
What scientists and environmentalists want to see is what will happen to the fish and the canyon when the gates close at Glen Canyon Dam and the staged flood recedes.
Federal officials insist they have progressed with long-term plans to offset the effects of the dam on the river and the Grand Canyon. The chub, the fish at the center of much of the dispute, has recovered some of its lost numbers since the last flood. Scientists also think they better understand when to trigger future floods.
"Our ultimate purpose is to learn whether or not this is a viable strategy for creating sandbars and habitats for native fish," said John Hamill, chief of the Grand Canyon Monitoring and Research Center, part of the U.S. Geological Survey.
Environmental groups argue that the upcoming flood again delays long-term changes to the river's management, further jeopardizing the canyon's health. They want federal officials to permanently alter the dam's operation instead of repeating the same test, adopting a seasonally adjusted plan that better mimics nature.
Nikolai Lash, senior program director for the Grand Canyon Trust, a group that has long fought the government over its management of the dam, said the March flood was hastily planned after the trust sued the government last year for failing to protect the river.
He said the experiment was purposely designed as a single test, even though most scientists think floods must occur regularly.
"They're trying to make it appear that they're doing something beneficial when they're just doing it for appearances," he said. "It's being manipulated to be a 'one and done,' even though we know that doesn't work."