Through the decades the now-endangered humpback chub have retreated about 60 miles downstream from Lee's Ferry, where there is more hospitable habitat. Bighorn sheep, pictured in a panel of ancient rock art not far from the dam, still roam the riparian habitat and side canyons near the Colorado. Permanent and migratory waterfowl can be seen all along the 15 miles of shore between Lee's Ferry and the dam.

After the release, which is expected to cause Lake Powell's already low level to drop 1 to 2 feet, the lake is anticipated to eventually rebound this year by about 50 feet, mostly due to spring runoff from a well-above-average snow season in the surrounding area. In the meantime, tourists are encouraged to witness a part of ecological and environmental history.

"Obviously it's a really exciting event to watch these four tubes shooting a spray of water," said Kevin Schneider, spokesman for the Glen Canyon National Recreation Area. "People can drive down and take a look."

The spectacle of the dam's four jet tubes releasing water should be visible today and Friday, with the high-flow part of the experiment expected to end sometime late Friday or early Saturday.

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E-mail: sspeckman@desnews.com