Sediment Storage in the Colorado River

Science Center Objects

The sandbars exposed along the shoreline of the Colorado River represent only a small fraction of the sand deposits in Grand Canyon, most of which are on the bed of the river in eddies and the channel. Current management practice includes efforts to maintain and build sandbars by releasing high flows from Glen Canyon Dam that are timed to coincide with periods of fine-sediment supply from tributaries (High-flow Protocol Environmental Assessment). The success of this approach to build sandbars depends on the maintenance of a sufficient supply of sand within the channel. The purpose of the sediment-storage monitoring project is to track long-term trends in sand storage and thereby provide a robust measure of whether or not the supply of sand available for building sandbars is increasing, decreasing, or remaining stable over time-scales of years to decades.

Background & Importance

The sandbars exposed along the shoreline of the Colorado River represent only a small fraction of the sand deposits in Grand Canyon, most of which are on the bed of the river in eddies and the channel. Current management practice includes efforts to maintain and build sandbars by releasing high flows from Glen Canyon Dam that are timed to coincide with periods of fine-sediment supply from tributaries (High-flow Protocol Environmental Assessment). The success of this approach to build sandbars depends on the maintenance of a sufficient supply of sand within the channel. The purpose of the sediment-storage monitoring project is to track long-term trends in sand storage and thereby provide a robust measure of whether or not the supply of sand available for building sandbars is increasing, decreasing, or remaining stable over time-scales of years to decades.

USGS researcher collecting data on a sandbar along the bank of the Colorado River

USGS researcher collecting data on a sandbar along the bank of the Colorado River (May 2014)(Credit: GCMRC, USGS. Public domain.)

General Methods

We measure changes in sediment storage by making repeat topographic maps of the river bed and banks. The maps are made by surveying exposed sediment deposits with conventional total station. These measurements involve the use of a survey instrument set on a known elevation (also called a benchmark) to measure the location and elevation of points on the ground selected by a rodman equipped with a reflective target. Most of the sediment is underwater and is measured with sonar. Multibeam sonar is the most efficient method to measure this sediment, because it is capable of mapping wide swaths of the riverbed. Singlebeam sonar measures depths directly below the instrument and is used to map areas too shallow for the multibeam equipment, but too deep for conventional survey. All of the sonar measurements are positioned by shore-based robotic total stations that track boat position in real time. GPS is not used for any of the measurements, because satellite signals are not sufficiently reliable in the deep canyon environment. Complementary measurements of changes in sediment storage are also made by measuring sediment concentration in the water.

Important Results

Initial results indicate that sand storage did not decline between 2002 and 2009. This period was one of average to above average tributary sand inputs and average to below average release of water from Glen Canyon Dam. These findings are based on a period that was favorable to sand accumulation. Periods when dam release volumes are greater and tributary sediment inputs are less frequent will likely result in less sand accumulation. Recent results also demonstrate that measurements of channel change made in short reaches (less than a few miles in length) can be used to track changes in deposits and transfers of sand among the storage locations within the short reaches. The results, however, cannot be extrapolated to long segments of the river (over 10 miles in length), because the size and distribution of sand storage locations is highly variable.

Additional Information

More infomation about sediment can be found at the Grand Canyon Monitoring and Reserch Center website: https://www.gcmrc.gov/research_areas/sediment_geomorphology/sed/Sed_Geom....

Sediment data is located at the Grand Canyon Monitoring and Reserch Center website: https://www.gcmrc.gov/dasa/default.aspx.

 

Boat in Colorado River using sonar to survey river bed

Boat equipped with multibeam sonar surveying the bed of the Colorado River in May 2009. Instrument in foreground is robotic total station that automatically tracks boat location and radios the position to a computer on the boat 20 times per second. (Credit: GCMRC, USGS. Public domain.)