ESTIMATING SAND THICKNESS FROM RIVERBED TO BEDROCK WITHIN THE COLORADO RIVER IN THE GRAND CANYON

The Glen Canyon Dam has dramatically altered the flow regime, geomorphology and ecosystem of the Colorado River in Grand Canyon. The dam has trapped the incoming sediment load and altered the flow regime by eliminating spring snowmelt floods. As a result, the average sandbar size within the Grand Canyon has decreased. High flow experiments have been conducted since 1996 to maintain sandbar volumes by redistributing sand from the channel to eddies, where most sandbars form. A major question is whether sandbars can be maintained given the limited sediment supply.

The goal of this study is to quantify the volume of sand stored on the riverbed in order to better estimate total sand storage in the system, and to more thoroughly understand the channel-eddy sand exchange system. The primary method of determining sand thickness is CHIRP sonar, which is supplemented by bathymetric and sediment classification maps derived from multibeam sonar data, geomorphic classification maps, and aerial imagery. The CHIRP generates and acoustically-derived image of the subsurface in cross section, which allows delineation of sand/gravel and sand/bedrock interfaces. Interpolating between successive cross sections provides an estimate of the total volume of sand stored within the riverbed. The bathymetric and sediment classification maps are used to constrain sand volume estimates to where sand is present on the riverbed surface, and the aerial imagery is used to link subaqueous with subaerial sand deposits.

Multibeam and CHIRP sonar data were collected in April 2017 between river miles 166 and 225 of the Colorado River. Isopach maps of sand thickness were created from over 2000 cross sections collected over the ~60 mile reach. Sand is mostly concentrated in pools (eddies and eddy adjacent channel segments) with a pool averaged mean sand thickness of about 0.72 meters. However, a significant amount of sand is also present in channel segments not adjacent to eddies, with a channel averaged mean sand thickness of about 0.34 meters. Within individual pools, sand thickness varies between 0 and 8.5 meters. Unsurprisingly, the dominant factor controlling the volume of sand stored within a pool is the pool area, but expansion ratio (upstream constriction width divided by downstream pool width) and pool depth also influence sand storage volumes.