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Grand Canyon floods

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A sandbar downstream of Glen Canyon Dam, at the 30-mile mark on the Colorado River, gained area and volume after an experimental flood last November. [For comparison, note the gap between bushes above the beach.] Courtesy of Mark Schmeeckle, Arizona State University.
In the spring of 1996, geologists experimented with a controlled flood on the Colorado River. They wanted to see if they could rebuild sand bars and beaches that had been eroding since the Glen Canyon Dam was built in the 1960s to control river flow through the Grand Canyon. But the flood, after dropping a load of sediment, continued for too long and scoured out the newly deposited sand. Scientists have had to wait until this year for a second attempt, and this time around, they are hoping they got it right.

On Nov. 21, the Department of the Interior approved a release of water from the Glen Canyon Dam, at 41,000 cubic feet per second to last over four days. The flood is an attempt to rebuild the beaches and other sedimentary environments that provide homes to plants, land animals and fish native to the Colorado River, including the endangered humpback chub. Although the results are preliminary, some sand may have been deposited this time.

The original premise behind the dam management plan established in the early 1990s was that during regular dam releases, sediment should have dumped into the Colorado River and built up over time. A flood would presumably remobilize and redeposit it in sandbars and beaches.

But analyses of the first controlled flood — which, over eight days, released about twice the amount of water from the Glen Canyon Dam as the recent flood — showed that some of the assumptions of how the system worked were wrong, says Scott Wright, a hydrologist at the U.S. Geological Survey in Flagstaff, Ariz., who participated in monitoring this year's flood. They had released water for too long, without sufficient sediment in the Colorado River system for the flood waters to deposit. Eventually, the peak flow eroded away the sand that had been deposited in the first few days of the trial.

Wright says that continuous real-time monitoring stations set up after the 1996 flood allowed researchers to see that overall, erosion dominates the Colorado River. "The assumption back then was that under normal [dam] operations, sand and fine sediment would accumulate in the Colorado River channel over a number of years. You could wait and do the high flow whenever you wanted to [and] sand would be stored in the channel," he says. But researchers diagnosing the 1996 flood "found that wasn't the case," and that the river quickly carries its sediment load 280 miles downstream to Lake Mead.

The monitoring also showed that flash floods from the Paria River and other tributaries below the dam were responsible for dumping the largest amounts of sediment into the river in concise pulses. "When you get a big input from a tributary, you have to take advantage of it," Wright says. Researchers determined that the success of any future floods would hinge on a surge of sediment to the system, particularly from the Paria and the Little Colorado tributaries.

River managers decided that once they measured 800,000 metric tons of sediment in the Colorado River delivered by its tributaries, they would be able to conduct the next experimental flood. That level was reached last fall, says Jeff Lovich, head of the Grand Canyon Monitoring and Research Center (GCMRC).

Despite drought conditions in the region, Lovich says, the total amount of water released fits into the requirements for water-use agreements for the Colorado River, "though the timing is a little different." The Western Area Power Administration lost $2 million in revenue while it paused its electric power generation at the Glen Canyon Dam, and Lake Mead gained 2.5 feet in water level, at rates well above average flow.

By Nov. 20, the end of a public comment period, GCMRC mobilized field researchers to track the flood, in addition to the flow meters, sediment monitors and gauging stations already in the river. Wright says he saw very fine sediment — clay and silt — in the first mad rush of the river, which was then replaced by larger sand grains as the flow continued.

A visual check shows that some beaches did get rebuilt along the river, at least for now. "Based on what happened in 1996 and the preliminary photos I've seen, I believe that [the flood] will build some beaches," Wright says. "The question that is more important is how long those will last?"

Naomi Lubick

Links:
USGS Flow Reports for flooding from Glen Canyon Dam, 2004
USGS Information on the flood in 1996

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