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. Linkage Between Grain-Size Evolution and Sediment Depletion During Colorado River Floods David J, Topping u.s. Geological Surve)\ Denver, Colorado David M. Rubin us. Geolog;cal Survey. Menlo Park, California Jonathan M, Nelson, Paul J, KinzellIl, and James p, Bennett u.s. Geological SUI1Jey, Dell vel; Colorado Suspended-sediment concentrations decrease and suspended sediment and bed sediment coarsen significantly during fioods or seasonal fiood periods in the Colorado River in Grand Canyon. We present evidence of these processes for the 1996 controlled fiood, examine their role in producing the sedimentologic signa- tures of both modem and historical fiood deposits, and suggest how these processes may be exploited to optimize fiood releases to achieve management objectives. In general, the processes that cause grain-size evolution in the Colorado River arise because of sediment supply limitation and a mismatch between the timing of tributary sediment supply to the Colorado River and high sediment-transporting events in the mainstem, The system fines immediately following large tributary sediment inputs. and the system coarsens as the bed sediment is winnowed during subsequent mainstem fiows, I, INTRODUCTION The Colorado River in Grand Canyon falls somewhere in the middle of the speclrum between classical self-formed alluvial rivers and bedrock rivers, Like purely alluvial rivers. Ihe morphology of Ihe bed and bars is somewhat adjus.table and is coupled to Row in the river~ on the olher hand. this reach also displays the effects of sediment-supply limilalion and heJrock control. This l11ix.ture- of behaviors The Controlled Flood in Grand Canyon Geophysical Monograph 110 This paper not subject to U.S. copyright Published in 1999 by the American Geophysical Union makes predicting the geometric rearrangement of s~dimenl in the system during floods particularly difficult. because any such prediction muSL be based on bOLh a detailed un<jer- slanding of how the Row interacts with the available sediment to produce Ihe observed bed and bar morphology and also on what volume and size classes of sediment are available on the bed for transport by the flow, Wiele el ai, [Ihis volume] have shown that Ihe response of Ihe channel bed and h.u morphology during Hoods is highly sensitive to Ihe supply of sedimen!. Moreover. it is difficult and time consuming (0 acquire the detailed local infonnalion about s~dilllenl volumes and sizes on the bed that is required 10 make accurate predictions of the topographic response of the system 10 high nows. Furthermore, even if that infor- 71