For 60 hours early next month, the Colorado River will reclaim some of its former glory, swelling with floodwaters that will scour and reshape miles of sandy banks on the floor of the Grand Canyon, all in the service of a 3.5-million-year-old fish.

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.

This is the third such experiment along the river in a dozen years, and it could end as the most controversial amid questions about whether the government has shirked its obligations to protect the canyon's natural resources.

At issue is how to manage a structure that stores water and provides electrical power for millions of people across the West, yet has also damaged a wondrous, complex ecosystem.

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 endangered humpback 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.

But 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.

Even some of the government scientists participating in the experiment seem impatient with the continued tests.

"At some point we're going to have to draw a line in the sand and say this is the best way to manage the dam and manage resources in the Grand Canyon," said Martha Hahn, science center director at Grand Canyon National Park. "We can't continue to go on and on just researching. We have to make a decision. We really are losing valuable resources."

Floods to help the fish
Glen Canyon Dam, completed in 1963, forever changed the lower 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 invaders, such as trout and salt cedar, thrived. The shift helped speed the extinction of four fish species and push two others, including the chub, near the edge.

In 1992, Congress passed the Grand Canyon Protection Act, which ordered the Interior Department to manage the dam in a way that better protected resources. Four years later, the government staged the first artificial flood, opening the dam's bypass tubes for several days in a mostly unsuccessful attempt to replicate natural cycles.

A second test in 2004, while still underwhelming, 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.

"There's been a good sand season, if you want to call it that, since about 2006, and we're thinking there's a lot more sand in the system than there has been in past experiments," said John Hamill, chief of the Grand Canyon Monitoring and Research Center, part of the U.S. Geological Survey.

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