RELATION OF INVERSELY GRADED DEPOSITS TO SUSPENDED-SEDIMENT GRAIN-SIZE EVOLUTION DURING THE 1996 FLOOD EXPERIMENT IN GRAND-CANYON

Before Glen Canyon Dam was completed upstream from Grand Canyon, flood s scoured sand from the channel bed and deposited sand on bars within recirculating eddies, After completion of Glen Canyon Dam in 1963, pea k discharge of the mean annual floods dropped from about 2600 to 900 m (3)/s, and 85% of the sediment supply was eliminated, Under the postda m flow regime, sand bars in eddies have degraded, In an experiment to study, in part, the effects of floods in rebuilding these bars, a cont rolled 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-limi tation and grain-size evolution are natural processes that do not requ ire the presence of a dam.