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An artificial flood does good in the Grand Canyon

Researchers hope controlled high flows from Glen Canyon Dam will help restore natural habitat.

(Page 2 of 3)



In 1988, three groups – the Grand Canyon Trust, the Wildlife Federation, and the Western River Guides Association – sued to force the first environmental impact study of the Glen Canyon Dam, 25 years after it was finished. This led to the passage of the Grand Canyon Protection Act of 1992. The study that resulted found that the dam was negatively affecting fish habitat and sediment resources in the canyon.

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Any sediment that enters the river below the dam settles to the riverbed and slowly bounces along the bottom. Pre-dam, periodic large floods would lift the sediment above the low-flow waterline and form sandbars, says Professor Schmidt. Part of the new management plan was to restore sandbars and beaches through so-called high-flow experiments.

The March 2008 high-flow experiment was designed to move more than 1 million metric tons of Paria River sediment that could have otherwise dribbled downstream to the lake. For 60 hours, the jet-tubes at the base of the dam gushed around 41,500 cubic feet per second (cfs), 25,000 cfs higher than normally allowed.

Brad Warren, the Colorado River Storage Project manager at the Western Area Power Administration, which markets the power generated at Glen Canyon Dam, said the experiment has cost WAPA $4 million this year because it has had to purchase power to meet a contract – power that it could have generated from the water that bypassed the turbines.

The high flows do not alter water delivery: Irrigation projects and aqueducts that tap the river must stay at set levels, regardless of what happens at the dam.

Scientists prepared for the flood by burying chains vertically in sandbars. The meter-long “scour chains” were driven into the ground with the top link exposed. As the torrent passed, the sand around the chains washed away, toppling the chain link by link until the water receded. The dropped portion of the chain was then buried by newly arriving sand.

Rubin, Schmidt, and their teams relocated the chains and dug into the sandbars. Vertical faces of up to six feet were scraped clean, and the layers revealed if eddies had churned the riverbed and how long water stayed at an elevation. “We’re trying to understand the details of how these bars form,” Schmidt says.

Even changes in grain size can help. A digital camera called the “flying eyeball” was developed to bounce along the riverbed and take photographs of individual sand grains. “It’s a 100-pound wrecking ball that we honed a cylindrical shape right through the middle of for the video camera,” says Hank Chezar, a USGS photographic technologist who designed and patented the instrument with Rubin. Photographs taken before and after the experiment help determine where new underwater sand deposits formed.

Although the scientists are not yet ready to release their findings, others are confident that the high flows, also done in 1996 and 2004, are helping. “These experiments are having a positive effect on the beaches,” says Stephen Martin, superintendent of Grand Canyon National Park. “The deposits exceeded our hopes.” This is good news for creating habitat for the endangered humpback chub and shoring up protective sand deposits on archaeological sites from native American tribes.

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