An artificial flood does good in the Grand Canyon

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.

An advisory group to the US Depart­ment of the Interior, which owns and operates the dam, was set up in 1996 to balance the competing interests of preserving the Grand Canyon with modern demands for water and power. The Adaptive Man­agement Working Group convenes 25 representatives from five federal agencies, the seven Colorado River basin states, native American tribes, environmental and recreational groups, and power-purchase coordinators. Here the scientists’ findings are fed into planning the flows to meet contract obligations for water delivery encoded by the “Law of the River,” an elaborate collection of state, federal, and international agreements governing the water rights.

Are scientific findings being weighed adequately? “The science has become extremely clear about what needs to happen at Glen Canyon Dam,” says Nikolai Lash, water program director at the Grand Canyon Trust, namely, “high flows done on an annual basis or near-annual basis followed by months of steady flows.” But Mr. Lash says that when votes are taken to set the flow levels, hydropower interests win every time. His organization is suing the federal government to follow the sediment-management guidelines spelled out in the Grand Canyon Protection Act.

“The dam is actually being operated illegally,” according to Lash.

Why windblown sand is important, too

Not all sand transport in the Grand Canyon is by water. Windblown sand movement, called aeolian transport, plays an big role in preserving thousand-year-old native American cultural sites. Windblown sand puts a protective sand layer on upslope terraces where tribal activities once occurred. The US Geological Survey (USGS) is studying how the operation of the Glen Canyon Dam affects the erosion of sand from the higher-elevation terraces.

“If dam operations contribute to erosion on archaeological sites, then the Bureau of Reclamation would have more responsibility for mitigating impacts above the pre-dam high-water line,” says Amy Draut, a USGS geologist leading the research.

Dr. Draut and her team erected 11 solar-powered weather stations and sand traps along the canyon. The traps, one-meter-tall towers with four wedge-shaped boxes that orient themselves into the wind, collect sediment that Draut uses to see how windblown sand changes with the size of a nearby sandbar and wind direction. “If you have a high-flow [event] build up a sandbar, you can cause more [aeolian] transport if the local wind direction is right,” she says. After the 2004 high-flow experiment, her team saw target sandbars and wind direction combine to double aeolian transport. Their findings after this year’s high flow may confirm that dam operations are risking the cultural heritage of the Grand Canyon.

Previous

1 | 2