NYTimes article on Grand Canyon flood
Pretty good summary of what they are doing and why:
November 23, 2004
In Bold Experiment at Canyon, a River Rips Through It
By SANDRA BLAKESLEE
A bold experiment is under way to restore the sandbars and banks of the Colorado River where it runs through the Grand Canyon.
At dawn yesterday five scientists, an engineer and two river guides clambered into a 24-foot rubber raft to ride the leading edge of a man-made flood designed to fling tons of sediment out of the river and onto its eroded banks.
Another 50 scientists were stationed at various points downstream, waiting to witness the transformation.
A previous flood experiment in 1996 turned out to be based on faulty assumptions.
The current $3.5 million venture, supported by more than a dozen government agencies and interest groups, is planned differently and will take advantage of some new instruments.
It ends Thursday, but how well it works will not be known for months.
Bennett Raley, the assistant secretary for water and science at the Department of the Interior, said the experiment was "a success story for adaptive management," the process by which scientists try something, learn from their mistakes and try something else.
"Speaking tongue in cheek," he said, "playing God is harder than it looks."
Sand is almost as much a part of the Colorado as water. For thousands of years, snowmelt carried vast amounts of sand downstream, creating bars, backwater channels for spawning fish and sandy habitat for land animals.
More recently, it has provided beaches for rafters and fishermen.
In 1963, the natural flow of sand and water was permanently altered by the construction of Glen Canyon dam just upstream from the Grand Canyon.
The dam now traps all upstream sediment that would have flowed through the canyon. The water that flows through the dam is ice cold, crystal clear and used to generate electricity.
The Colorado River is sand-starved, said Theodore Melis, a sediment expert at the Grand Canyon Monitoring and Research Center in Flagstaff, Ariz., which was established a decade ago to provide scientific advice on river management.
Native fish, which evolved in warm silty waters, are disappearing. Their young are eaten by cold-water trout that now dominate the river. Riverbanks have eroded. Those that remain are choked with non-native vegetation.
Most of the sand now entering the Colorado River below the dam comes from the Paria River, Dr. Melis said.
In 1996, planners assumed that several years worth of sediment from the Paria had settled on the bottom of the Colorado, waiting to be picked up and redistributed by a flood.
Normal dam operations rarely release more than 20,000 cubic feet per second of water.
Working with the dam's operators, the scientists released 45,000 cubic feet per second of water for seven straight days.
At first everything looked as if it was working, said Dr. David Rubin, a sediment expert at the United States Geological Survey in Santa Cruz, Calif. "Three years later we realized we'd made some big mistakes," he said.
The years of sand dumped by the Paria had not stayed on the river bottom. Measurements showed that new sand from the Paria River washes down to Lake Mead, within months of a storm. The sand that was in the Colorado was swept rapidly away by the rapidly moving water. Most of it was gone halfway through the flood.
The new flood is taking place immediately after three heavy rainstorms in September and October pushed an estimated 800,000 tons of sediment out of the Paria and into the Colorado.
Even though the West is suffering from a serious drought, specified amounts of water are released to states downriver from the dam and timing of the releases is flexible.
Dam operators kept water levels low since the beginning of September to preserve any new sand that arrived.
The flood will peak at 41,000 cubic feet of water per second for two and a half days, then drop back to 8,000 cubic feet per second.
According to a model developed by Dr. Stephen Weile of the Geological Survey in Tucson, Ariz., some places along the river might build sandbars in as little as 13 hours.
It's a balancing act, Dr. Rubin said. "This time we're running high water for 60 hours instead of 168 hours,'' he said. The goal is to throw as much sand as you can onto the sandbars without pushing more sand downstream than you need to."