This paper presents data collection methods and side-scan sonar data collected along the Colorado River in Grand Canyon in August and September of 2000. The purpose of the data collection effort was to image the distribution of sand between Glen Canyon Dam and river mile 87.4 before and after the 31,600 cfs flow of September 6-8. The side-scan sonar imaging focused on pools between rapids but included smaller rapids where possible.
Channel expansions are sites of deposition in bedrock canyons and alluvial rivers.Within these areas, deposition is commonly focused at the separation point, reattachment point, eddy center, or along the shear surface that separates the recirculation zone from the downstream flow in the main channel.This study examined the internal structure of reattachment bars, separation bars, and natural levees along the Colorado River in Grand Canyon, Arizona.The recirculation deposits (separation and reattachment bars) are characterized by a rotary flow pattern that includes upstream flow.Flow patterns within channel expansions vary with discharge.Increases in discharge generally increase the length of the recirculation zone.Both the rotary flow pattern and lengthening of the recirculation zone have been documented from internal structures in the bars.The character of climbing-ripple structures in the bar deposits demonstrates that recirculating flows pulsate erratically; field and lab current-meter measurements have demonstrated that these erratic pulsations occur even when discharge in the main channel is steady.Depositional processes and internal structures differ for the three kinds of bars that were studied.During a single flood, deposits of reattachment bars are thickest.Reattachment bars typically produce fining-upward sequences, because circulation over the bars eventually weakens as upward growth restricts flow from the main channel.Separation-bar flood deposits are relatively thinner and are characterized by transgressive deposits that are commonly dominated by wave-generated structures.Levee deposits originated where bankward-directed flow transported sand onto narrow floodplains along the channel.Flow behind the levees was relatively weak and was typically directed downstream.Depositional rates were determined for a variety of sites by examining sedimentary structures in deposits that survived subsequent flows.During the large flood of 1983 (with a peak that approached 100,000 cfs), deposition on some bars exceeded several meters, corresponding to a depositional rate of a few tens of centimeters per day.During the weaker floods of 1984, 1985, and 1986 a total of approximately three months of flow that approached 50,000 cfs deposition was limited to a few tens of centimeters, corresponding to a depositional rate of approximately 1 cm per day.During flows within the range of power-plant operations (not exceeding approximately 30,000 cfs), depositional rates range to approximately 5 cm per day.In all flows, the range of depositional rates can be expected to have varied considerably, from sites that experienced net erosion or nondeposition to sites that exceeded the observed rates.Three kinds of effects must be considered when evaluating flow alternatives on camping beaches: submergence/emergence of bars, erosion/deposition of bars, and net sediment transport through the canyon.A relatively large annual fluctuation and small daily fluctuation allow deposition at high elevations for a short time and allow emergence through most of the year; low daily fluctuations allow camping on bars that otherwise would be inundated daily.
Experiments in the 1960's demonstrated that the rate of sediment transport represented by migrating bedforms gives a more accurate measure of bedload transport than rates predicted from flow measurements. Rotating side-scan sonar can be used in the field to measure the rate of bedform migration and to calculate bedload transport rates. A rotating side-scan sonar system was deployed in the Colorado River in Grand Canyon for this purpose. For two sites where total load was measured using a depth-integrating sampler, approximately 5% and 0.3% of the sand transport was bedload involved in bedform migration; the other 95-99.7% occurred as suspended load that bypassed the bedforms.
