Sediment Storage in Grand Canyon

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 for rebuilding sandbars. 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.

Monitoring Sediment Storage

Boat equipped with singlebeam sonar for mapping the bed of rivers

Boat equipped with singlebeam sonar for mapping the bed of a river at shallow depths along the shoreline of the Colorado River. (Credit: USGS. Public domain.)

We measure changes in sediment storage directly 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 benchmark to measure the location and elevation of points on the ground selected by a rodman equipped with a reflective target. However, most of the sediment is underwater and is measured with sonar. Multibeam sonar is the most efficient method, 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 (Discharge, Sediment, and Water Quality Monitoring).

Recent Findings

Animation of river bathymetry on Colorado River in Grand Canyon

Visualization of map of riverbed and canyon walls near Navajo Bridge, 4.5 miles downstream from Lees Ferry, Arizona. River bathymetry was measured with multibeam sonar and topography was measured with a boat-mounted laser scanner. The data from this survey collected in April 2016 will be used to measure changes in sand storage on the river bed and to model streamflow and sand transport. (Credit: USGS, Public domain.)

Initial results from efforts to monitor long-term trends in sediment storage indicate that 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 have also demonstrated 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, but the results 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.

Resources

Web application for viewing maps of the river bed: https://grandcanyon.usgs.gov/portal/home/webmap/viewer.html?webmap=a599ee6e49be4adcbb7f042bcf0fb544

 

Perspective view of digital elevation model of the bed and banks of the Colorado River

Perspective view of digital elevation model of the bed and banks of the Colorado River about 44 miles downstream from Lees Ferry, Arizona. Between 2009 and 2012, it is possible to see erosion of sediment from the bed of the river in the channel and erosion of sediment from the sandbar on the bank. The direction of streamflow is from the upper left to lower right and the river is about 450 feet wide at the widest point in this view. (Credit: USGS. Public domain.)