Post-Dam Floodflows Through the Grand Canyon

The Grand Canyon has experienced floodflows--releases from Glen Canyon Dam in excess of the 33,200 cubic feet per second (cfs) that can flow through the power plant--only a few times since the dam was built:

• In 1965, excess water was released to balance the reservoir.
• In 1980, when Lake Powell filled up, excess water was released to test the spillways.
• In 1983, unusually heavy spring runoff raised the level of Lake Powell so high that a temporary 8-foot wall was built on top of the dam and water was released as fast as possible to keep the lake from overtopping the dam. Floodflow discharges were maintained for about a month, with a peak discharge of almost 100,000 cfs.
• In 1984, 1985, and 1986, Lake Powell was still so full that floodflows of 40,000 to 50,000 cfs had to be maintained for about a month each year.
• In 1990 and 1991, discrete research flows were conducted to provide data for the Glen Canyon Dam Environmental Impact Statement.
• In 1996, the Bureau of Reclamation released water at a rate of 45,000 cfs for seven days, from March 26 to April 2. The purpose of this controlled flood was to help researchers determine if periodic large releases can help restore habitats that have deteriorated since the completion of Glen Canyon Dam in the early 1960's.

The 1983-86 floodflows (similar to pre-dam spring peaks) transported sand stored within the river channel, eroded low-elevation sandbars, and built up high-elevation sandbars in wide reaches. Researchers hope to find that the controlled flood held in late March and early April of 1996 had similar effects.

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