Grand Canyon Sandbar Monitoring

Science Center Objects

Since the completion of Glen Canyon Dam in 1963, the amount of sand supplied to Grand Canyon National Park has been reduced by more than 90 percent. The Paria River, a tributary to the Colorado River 15 miles downstream from the dam, is now the single most important supplier of sand to the Colorado River within the Park. This large reduction in sand supply has resulted in substantial decrease in the number and size of sandbars. Sandbars are important because they serve as campsites for river runner and hikers, provide important aquatic and riparian habitats, and are the source of sand that may help protect archaeological sites. The information collected by this project will be used to determine whether dam operations, including High-flow Experiments, cause increases or decreases in sandbars and associated campsites in Grand Canyon National Park.

The Sandbar Monitoring Data

Currently, topographic maps are made at a set of monitoring sites annually using conventional survey equipment. These surveys are used to calculate the size of each sandbar in terms of the area of exposed sand and the volume of sand contained in the bar. Both of these calculations are relative to an elevation of interest.

Surveying an exposed sandbar along the bank of the Colorado River

Surveying an exposed sandbar along the bank of the Colorado River in May 2014. (Credit: USGS. Public domain.)

Recent Findings

Each of the five High-flow Experiments (HFEs) that has been released from Glen Canyon Dam between November 2012 and November 2018 resulted in deposition at more than 50 percent of 44 long-term sandbar monitoring sites in Marble Canyon and Grand Canyon. That deposition also resulted in small cumulative increases in sandbar volume at those same monitoring sites. Cumulative increases in sand volume between 2003 and 2018 are significant at two sandbar types—reattachment bars and upper pool bars. Hydrograph shape appears to affect sandbar topography for at least some sites. The lower downramp rate used in 2012 resulted in sandbar topography that was less steep compared to the downramp rate used in the 2008 HFE. However, because the adjusted hydrograph with lower downramp rate was tested in only one year and because topographic surveys were only available for three sites, it is uncertain whether this response would be consistent among many sites or repeatable in future HFEs.

 In four out of the five years with HFEs, the sand mass balance for the July 1 to December 1 accounting period for all five account years has been significantly positive and in one year the sand mass balance was indeterminant. Thus, the objective HFEs to cause deposition on sandbars and increases in sandbar size without causing decreases in sand storage in Marble Canyon was achieved or exceeded each year.

Data and Resources

Sandbar monitoring data: www.gcmrc.gov/sandbar (https://www.usgs.gov/apps/sandbar/)

Sandbar monitoring photographs (including responses to High-flow Experiments): https://grandcanyon.usgs.gov/gisapps/sandbarphotoviewer/RemoteCameraTimeSeries.html