Sand Deposition in the Colorado River in the Grand Canyon from Flooding of the Little Colorado River

Abstract

Methods for computing the volume of sand deposited in the Colorado River in Grand Canyon National Park by floods in major tributaries and for determining redistribution of that sand by main-channel flows are required for successful management of sand-dependent riparian resources. We have derived flow, sediment transport, and bed evolution models based on a gridded topography developed from measured channel topography and used these models to compute deposition in a short reach of the river just downstream from the Little Colorado River, the largest tributary in the park. Model computations of deposition from a Little Colorado River flood in January 1993 were compared to bed changes measured at 15 cross sections. The total difference between changes in cross-sectional area due to deposition computed by the model and the measured changes was 6%. A wide reach with large areas of recirculating flow and large depressions in the main channel accumulated the most sand, whereas a reach with similar planimetric area but a long, narrow shape and relatively small areas of recirculating flow and small depressions in the main channel accumulated only about a seventh as much sand. About 32% of the total deposition was in recirculation zones, 65% was in the main channel, and 3% was deposited along the channel margin away from the recirculation zone. Overall, about 15% of the total input of sand from this Little Colorado River flood was deposited in the first 3 km below the confluence, suggesting that deposition of the flood-derived material extended for only several tens of kilometers downstream from the confluence. concern prompted BOR to initiate an investigation of the sand resources in the river between Lake Powell and Lake Mead, and since 1983, the BOR has coordinated a comprehensive program of investigations, the Glen Canyon Environmental Studies (GCES), to determine the effects of releases from GlenCanyon Dam on the riparian and aquatic resources in the park. As part of GCES, the U.S. Geological Survey (USGS) began in 3579 3580 WIELE ET AL.: SAND DEPOSITION IN THE COLORADO RIVER 1989 to develop a suite of flow and sediment-transport models for predicting the response of sand in the canyon to dam releases. ApproachAccurate evaluation of sand resources by direct measurements of the volumes of sand deposits is not feasible because of the great length of the system (380 km), its inaccessibility, the high cost of making necessary field measurements, and the high degree of variability in channel geometry and sand storage. The frequency and density of field measurements in the Grand Canyon is further limited by the need to limit intrusion into the fragile and valued riparian system and into areas held sacred by Native Americans. As a result, we have made a limited number of field measurements designed to be used in conjunction with a model that provides a physically based method of interpolating changes in bed topography both in time and space. The field measurements consist of initial bed topography from which a gridded model ...

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“…Models of channel change in mobile bed streams are based on the evaluation of this sediment imbalance (e. g., Wiele et al,1996;. The hillslopes are a source of fine sediment, which is eroded by shallow overland flow (sheet, rill and gully erosion) and delivered to the drainage network.…”

Origin of the Colorado River experimental flood in Grand Canyon

“…Remaining sand bars were substantially smaller than they had been in 1986. The February 1993 Little Colorado River flood more than doubled the discharge of the Colorado River below their confluence to about 990 m 3 s" 1 and contributed an estimated 4.2 x 10 6 1 of sand (Wiele et al, 1996). The accumulation of sediment on sand bars and in the channel within the first several kilometres downstream of the confluence was equal to the quantity of sand contributed by the Little Colorado River flood.…”

Fine bed material in pools of natural gravel bed channels

“…Similarly, tracer experiments show a rapidly weakening size dependency of virtual transport velocities with decreasing particle sizes less than the median size of bed material [Hassan and Church, 1992;Ferguson and Wathen, 1998]. Variations in channel topography and boundary shear stress can lead to sorting of bed material and formation of surficial patches of fine bed material that locally cover the gravel framework [Lisle and Madej, 1992;Seal and Paola, 1995;Wiele et al, 1996]. Fine patches contain some of the first bed material to be entrained during rising stages, the most erodible at bank-full stages, and the last to be deposited during waning stages [An• drews, 1979;Lisle, 1979;Meade, 1985;Sear, 1996].…”