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Relation of inversely graded deposits to suspended-sediment grain-size evolution during the 1996 flood experiment in Grand Canyon

¹ U.S. Geological Survey, M.S. 999, Menlo Park, California 94025
² U.S. Geological Survey, M.S. 413, Lakewood, Colorado 80225

Before Glen Canyon Dam was completed upstream from Grand Canyon, floods scoured sand from the channel bed and deposited sand on bars within recirculating eddies. After completion of Glen Canyon Dam in 1963, peak discharge of the mean annual floods dropped from about 2600 to 900 m³/s, and 85% of the sediment supply was eliminated. Under the postdam flow regime, sand bars in eddies have degraded. In an experiment to study, in part, the effects of floods in rebuilding these bars, a controlled flood was released from Glen Canyon Dam in late March and early April 1996. Although fluvial sequences characteristically fine upward, the deposits of the experimental flood systematically coarsen upward. Measurements of suspended-sediment concentration and grain size and of bed-material grain size suggest that the upward coarsening results from the channel becoming relatively depleted of fine-grained sediment during the seven days of the high-flow experiment. Predam flood beds of the Colorado River also coarsen upward, indicating that supply-limitation and grain-size evolution are natural processes that do not require the presence of a dam.

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