Sand in the Grand Canyon:
Two U.S. Geological Survey Studies

Table of Contents:

• Background
• Study of sandbar formation
• Study of sand distribution on the channel bed
• Additional studies

Background

Effects of Glen Canyon Dam

Observers in the Grand Canyon of the Colorado River (see map) have noticed a gradual decline in the size and number of sandbars used as camping beaches by river runners and backpackers.

Because most of the sand that was carried yearly to the Grand Canyon by the Colorado River now gets trapped behind Glen Canyon Dam, many people feared that the sandbars would gradually erode away and never be replaced.

But the dam also dramatically reduced the magnitude of floods through the canyon, and today's reduced flow means that sand is not washed out of the Grand Canyon nearly as fast as before the dam was built.

Sand is still being supplied by undammed tributaries--such as the Paria River, which enters the Colorado River at Lees Ferry about 15 miles downstream from Glen Canyon Dam, and the Little Colorado River, whose mouth is about 60 miles farther downstream from Lees Ferry. This sand may be sufficient to replenish sand lost yearly to erosion.

Can Controlled Floods Rearrange Available Sand?

A controlled flood was held during late March and early April of 1996 to help researchers determine if periodic large releases of water from Glen Canyon Dam can rebuild sandbars and restore other habitats that have deteriorated since the dam's completion in 1963.

Researchers now studying the effects of the controlled flood hope to find that it lifted sand out of deep pools in the river channel and deposited it on bars along the banks, possibly adding as much as a few feet to sandbars in the Grand Canyon.

Such redistribution is possible because sand suspended by swiftly flowing water is deposited where the water slows down.

Water of the main current moving swiftly enough to lift sand from the channel bed slows down where it is deflected into eddies downstream from natural obstructions, such as debris fans extending from the mouths of tributaries.

The sand deposited in these eddies forms sandbars.

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Study of Sandbar Formation

During the Controlled Flood--Watching Sandbars Form

David Rubin, George Tate, and their colleagues used the following equipment to study the formation of sandbars while the controlled flood was in progress:

• current meters to measure how fast water was flowing in sandbar-building eddies,
• optical devices to measure how much sediment was suspended in the water, and
• sidescan sonar to record the migration of sand waves and ripples on the surfaces of sandbars.

After the Controlled Flood--Examining Internal Structures

Rubin returned to the canyon in June 1996, after the river had had time to erode cutbanks into the newly built-up sandbars. He and his team studied the internal structures of the sandbars as revealed in the cutbanks and in the walls of trenches--about 3 feet deep and up to 300 feet long--that they dug across selected sandbars.

The internal structures consist mainly of sand waves and ripples formed by water currents from which the sand was deposited. From them, Rubin and his team can reconstruct the events that formed a bar and estimate how fast it was deposited.

Cutbank eroded into a sandbar reveals internal structures:

Sand ripples in close-up view of cutbank:

How the Sandbar Data Will Be Used

The team's estimates of sedimentation rates will be used to verify models U.S. Geological Survey hydrologists have developed to predict what the river will do to sediment at different flow rates. Such models may eventually be used by the U.S. Bureau of Reclamation to regulate discharges from Glen Canyon Dam.

Detailed descriptions of the sandbars' internal structures will help geologists recognize the remnants of such bars in rocks that formed millions of years ago, enabling them to more accurately reconstruct the geologic histories of ancient rivers.

More Information

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Study of Sand Distribution on the Channel Bed

Before the Controlled Flood--Mapping the Channel Bed

In late spring of 1994, Michael Marlow, Roberto Anima, David Hogg, and Kaye Kinoshita mapped the distribution of sand on the channel bed of the Colorado River along

• lines that cross the river and have been surveyed for bathymetry by U.S. Geological Survey hydrologists, and
• lines that parallel the river channel.

They used sidescan sonar and underwater video to view the channel bed.

Underwater Video Guides Sidescan-Sonar Interpretations

The sidescan sonar built up images of the channel bed from reflections of high-frequency sound waves. Comparing the sidescan-sonar records with the underwater video, the researchers saw that different types of sediment--boulders, cobbles, gravel, sand--produced distinctive patterns on the sidescan-sonar records.

The underwater video provided clear footage upstream from the mouth of the Little Colorado River and downstream to about 7 miles below the Little Colorado (see map). Farther downstream, suspended sediment brought in by the Little Colorado and other tributaries made the Colorado River too turbid for useful video footage.

The consistency with which patterns on the sidescan-sonar records could be matched with sediments seen on video footage from the clear reaches gave the researchers confidence that sidescan sonar could be used to map sediment on the channel bed in all the reaches they studied.

Working in Calm Water

Most of the sand on the channel bed rests in deep pools where currents are not swift enough to wash the sand away. To map this sand, the researchers conducted their surveys in calm pools that form upstream and downstream from rapids.

The reaches they studied stretch from above North Canyon Rapid at about River Mile 20 (20 miles downstream from Lees Ferry) to above Lava Falls Rapid at about River Mile 179.

After the Controlled Flood--Looking for Changes

Marlow, Anima, and Hogg returned to the river in June 1996. They used sidescan sonar and underwater video to map the distribution of sediment on the channel bed along the same lines they mapped in 1994.

Comparison of the 1996 map with the 1994 map will show how the test flow redistributed sand on the channel bed and will provide sediment-distribution data for the USGS hydrologists' models. These models may eventually be used by the U.S. Bureau of Reclamation to regulate discharges from Glen Canyon Dam.

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Additional Studies

The studies described above are just two of many studies of the controlled flood planned by various research groups and coordinated by the U.S. Bureau of Reclamation.

For a summary of U.S. Geological Survey studies planned for the controlled flood, see this illustrated fact sheet.

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See suggestions for further reading for the sources of information on this page.