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Summary and Synthesis of Geomorphic Studies Conducted During the 1996 Controlled Flood in Grand Canyon John C. Schmidt Department a/Geography aJld Earlh Rtsourr:t.s, Utah Srat~ University. Logan The 1996 controlled flood demonstrated that a discharge of 1274 m3/s was sufficient to form or rework alluvial deposits along the Colorado River in Grand Canyon. This flood was moderately large in relation to other floods that have occurred since Glen Canyon Dam was completed in 1963. The flood also provided the opponunity to make measurements of physical processes that occurred. The flood caused widespread deposition of sand to elevations 3-5 m above the stage of the administratiyely-determined minimum daytime dam release of 227 m3/s and caused an increase in the area and volume of the high- elevation pans of sand bars, thereby increasing the number of new campsites. The low-elevation pans of the same eddies were eroded, as were adjacent channel pools. Recently-aggraded debris fans were reworked. Suspended- sediment transpon measurements showed that suspended-sand concentrations decreased with time and that transpon rates are highest when the channel bed is retatively fine and has a higher proponion of sand. The controlled flood was most effective during the first 4 days when mainstem transpon rates and eddy deposition rates were high; debris-fan reworking entirely occurred during the first 4 hrs at one site. Modeling studies strongly indicate that resultant eddy-bar topog- raphy depends greatly on the concentration of mainstem suspended sediment and the initial topography of eddies. Large-scale erosion occurred on different days at different sites and had the potential to evacuate large proponions of newly- deposited sand from eddies. The final flood-formed topography of eddy bars was thus highly variable from site to site and from reach to reach. I. INTRODUCTION riyer was sufficiently different immediately after the flood that many r;yer users and managers termed the flood a resource-management "success"[Collier et al.. 19971. The flood also proyided the opportunity to measure physical processes that cause sediment entrainment, transport, and deposition during flood in a river where channel conslric- tions and lateral zones of flow separation are common. These geomorphic measurements led research scie",isls 10 revise some fundamental ideas about geomorphic processes in Grand Canyon and other rivers where fan-eddy complexes, as described by Schmidt and Rubin [19951. are The 1996 controlled flood was an opportunity 10 demon- strale the role or floods in forming and reworking alluvial deposits along the Colorado River in Glen, Marble, and Grand canyons. The slatus of physical resources along the The Controlled Flood in Grand Canyon Geophysical Monograph 110 Copyright 1999 by the American Geophysical Union 329