Effects of Glen Canyon Dam on Sediment in the Grand Canyon

Sediment can be considered a basic resource, linked in some way to most of the resources within Glen and Grand Canyons.
--"Glen Canyon Dam Beach/Habitat-Building Test Flow," published in January 1996 by the U.S. Bureau of Reclamation.

Aerial view of sandbar used as camping beach along the Colorado River.

Glen Canyon Dam, which controls the flow of the Colorado River through the Grand Canyon, has affected sediment in the Grand Canyon in a number of ways. Here are three examples:

Loss of Sand from Camping Beaches

Most of the sand that used to be delivered yearly to the Grand Canyon by the Colorado River now gets trapped behind Glen Canyon Dam.

This drop in sand input, combined with erosive daily flow variations used to meet power demands--variations that have produced as much as 10-foot differences in daily river levels--has caused a net loss of sand from sandbars used as campsites along the river.

Buildup of Rapids at Tributary Mouths

The rapids that make the Grand Canyon so popular with white-water rafters are created by debris fans: piles of rock fragments, from clay size to boulders, that tumble down the tributaries during intense rainfall. Rapids form where the debris fans extend into the river and constrict its channel.

Fresh debris fans used to be cleaned out yearly--small fragments flushed away and some boulders moved--by large floods of water, commonly reaching 100,000 cubic feet per second (cfs), that flowed through the canyon during spring snowmelt in pre-dam years.

Glen Canyon Dam dramatically reduced flows through the canyon, which have rarely exceeded about 30,000 cfs since the dam was built. This drop in water flow has reduced the ability of the river to move rock debris at tributary mouths. In the absence of floods, there will be a continuing buildup of boulders and smaller rock fragments on many rapids, which could become more dangerous to navigate.

Silting Up of Fish Habitats

Elimination of yearly flooding in the Grand Canyon has also allowed silt to build up in backwater channels used as habitats by native fish.

Controlled Flood Expected to Redistribute Sediment

A controlled flood was held in the Grand Canyon during late March and early April of 1996, and again in November 2004. Researchers hope to find that the flood moved sand from deep pools in the river channel to sandbars along its banks and flushed silt from backwater channels. The 1996 test flow--45,000 cfs for seven days--may also have moved boulders from some rapids and made them more navigable.

Scientists from the Western Coastal and Marine Geology team of the U.S. Geological Survey are contributing to research on the test flow by studying how it redistributed sand in the river system.

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