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r Debris-flow deposits were observed in 36 tributaries of the Colorado River during this study. Twenty-one of the 36 tributaries have evidence for recent. debris flows. Three tributaries were selected for more detailed study on th. basis of previous reports of debris flows. The tributaries studied in detail were the Lava-Chuar Creek, Monument Creek, and Crystal Creek drainages. The fieldwork for this project was completed in March and April 1986. The frequency of past debris flows was determined from analysis of preserved stratigraphy in the tributaries. Sediments from discrete debris flows were traced longitudinally using characteristic color, lithology, and particle sizes of the preserved sediments. Radiocarbon dating and analysis of scarred troes and historical photographs provided a control on dating the ages of events. Evidence for all events was not necessarily preserved in the tributaries; therefore, estimates provide a minimum frequency of debris flows in the tributary canyons. S1.plified hydraulic formulas were used to calculate flow velocities and discharges for debris flows (Pierson, 1985). Velocities were calculated fro. runup evidence of the. velocity head, which is preserved in sites where an obstacle is oriented perpendicular to the flow direction. Superelevation evidence, which is found where the flow surface is elevated on the outside of bends due to centrifugal forces, was also used to calculate velocities. Thellethods provide a conservative estimate of the actual velocity of debris flows (Pierson, 1985). Dischargewas calculated uthe product of velocity and cross-sectional area. Project personnel colle~ted 5- to IO-pound samples of debris-flow matrix for reconstitution of the water content of the debris flow using methods described in Cooley and others (1977) and Gallino and Pierson (1985). Uncertainty in the reconstituted water content by volume for each sample was 1 to 2percr.nt. Particie-size distributions were obtainad by combining sieve data with point-count data obtained in the field. The two methods yield mlllerically. equivalent particle-pi:!e distributions (Kellerhalsand Bray, 1971). DEBUS n.ovs nOli TRUE ftIBUTAllIES Evidence for atleaat five prehistoric debris flows and three historic debris flovs is preserved in the Lava-Chuar Creek drainage. Historic debrisflovs occurred between 1916 and 1966, in December 1966, and be~een . _1973ancL19S11. Debrbc-flows---have---reached--ehll-C-o-tora-ao-Rrver'on-' an average of every 200yellr. during the last 1,500 years and every 20 to 30 years .ince 1916. Debris flows may reach the Colorado River IIOre frequently because .0.. prehi.toric debris flows may not have overtopped the terrace. to leave depositional evidence. The debrisflov of 1966 in the Lava-Chuar Creek drainage began .. .lope faUure. in the PemianHerait Shale and Peraian and Pennsylvanian Supai Group ancI traveled 6.5 .i downstre.. to the Colorado River. The debris flovbad a velOCity of 12 ft/s and a total discharge of about 4,000 ft'l. near the Colorado River. The average water content of the flow wa. e.t~ted to be 22.5 percent, hence the peak .ediment and water di.charge. are e.t~ted to be 3,100 and 900 ft'/s, re.pectively. The debris flow was -3-