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The effect of unsteadiness of dam releases on velocity and longitudinal dispersion of flow was evaluated by injecting a fluorescent dye into the Colorado River below Glen Canyon Dam and sampling for dye concentration at selected sites downstream. Measurements of a 26-kilometer reach of Glen Canyon, just below Glen Canyon Dam, were made at nearly steady dam releases of 139, 425, and 651 cubic meters per second. Measurements of a 380-kilometer reach of Grand Canyon were made at steady releases of 425 cubic meters per second and at unsteady releases with a daily mean of about 425 cubic meters per second. In Glen Canyon, average flow velocity through the study reach increased directly with discharge, but dispersion was greatest at the lowest of the three flows measured. In Grand Canyon, average flow velocity varied slightly from subreach to subreach at both steady and unsteady flow but was not significantly different at steady and unsteady flow over the entire study reach. Also, longitudinal dispersion was not significantly different during steady and unsteady flow. Long tails on the time-concentration curves at a site, characteristic of most rivers but not predicted by the one-dimensional theory, were not found in this study. Absence of tails on the curves shows that, at the measured flows, the eddies that are characteristic of the Grand Canyon reach do not trap water for a significant length of time. Data from the measurements were used to calibrate a one-dimensional flow model and a solute-transport model. The combined set of calibrated flow and solute-transport models was then used to predict velocity and dispersion at potential dam-release patterns. (KEY TERMS: flow velocity; longitudinal dispersion; steady flow; unsteady flow; tracers; water policy/regulationldecision making; Glen Canyon Dam; Colorado River.)

please include NTIS report number with inquiry. 14 Reattachment deposits 19 Upper-pool deposits 21 Channel-margin deposits 23 Distribution of deposits 23 Aggradation and degradation at Eighteen Mile Wash, 1965-86 25 Hydraulic conditions 25 Aggradation and degradation at Eighteen Mile Wash, 1965-86 Continued Topographic changes of the separation deposit Bathymetric surveys Aggradation and degradation of alluvial deposits, 1965-86 Changes in alluvial sand deposits, 1973-84 Flow characteristics Changes in deposits Changes in alluvial sand deposits, high flows, May 1985 Flow characteristics Changes in deposits Changes of alluvial sand deposits during strongly fluctuating flow, October 1985 to January 1986 Flow characteristics Changes in deposits Comparison of changes in alluvial sand deposits Summary References cited Appendix A Comparison of river mile inventories of 1973 and 1983 from Lees Ferry to Stone Creek

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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