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composed of 30 to 3S percent sand and carried boulders that were about 1 to 2 ft in diameter. The largest boulder measured that was transported during the debris flow weighed an estimated 9 tons. Two debris flows occurred in the last 2S years in Monument Creek drainage. A storm on July 27, 1984, initiated an avalanche and subsequent debris flow that reached the Colorado River. Some evidence indicates an earlier debris flow that occurred in the early 1960' s. Older debris-flow deposits were radiometrically dated at about A.D. 1780; however, lack of correlation with downstre..deposits precluded any use of this date for determining frequencies of events. The debris flow of. 1984 in Monument Creek drainage began as an avalanche froll the Pemian Esplanade Sandstone of the Supai Group" 2,000 ft above the channel. A 20-foot-high debris dam resulted and had not been breached as of 1986. The debris flow traveled 2.8 mi to the Colorado River at a velocIty of 11 to 13 ft/s and had a peak discharge of about 3,800 fta/s. The water content of the flow ranged from 27 to 34 percent, and the flow was composed of 30 to 40 percent sand. One boulder that was transported during the flow weighed an estimated 37 tons. The d4bris flow of 1984 created a new fan surface at the Colorado River that significantly constricted Granite Rapid. Volume of sediJlent transported onto the fan and into the river was estimated on the basis of four hypothesized .eenarios of the fan geometry after deposition of sed1llents fro. the . debris flow. The most likely volume of sediment transported onto the fan and in'1:o the river is 300,000 ft'l. The debris fan was completely, devoid of particles less than 16 11m in diameter in 1986, which suggested that alLfiner<particles (including sand).were transported quickly into the, Colorado River . Asswaingan average sand content of 35 percent, the estiaatedvolUlle of sand entering the river is 84,000 ft'lwith a range for all scenarios of 56,000 to 150,000 ft'l. Estimates of the volu.. o~ transported sedillent and the upstre.. discharge indicate that the fanvas created in 1 'to 3 lIinutes during the first pulse of the debris flow. The Crystal Creek drainage averaged allinim~ of one debris flow reach~ng the Colorado River every 50 years. A large dobris flow in DeceJiber 196F. (":ooleyand others, 1977) has been the only debris flow to reach the Colorado River in this century. SlI&l1 debris flows that did not re.chthe Colorado River significantly aggraded the channel and probably de~~i ~~d.~ed~~~~.., ~~..c..~~4_1.r.g.r~debriS-flows" to . reacb. the.-rive-r.---~" The debris flow of Decellber 1966 in the Crystal Creek drainage bepn with 11 slope failures in the HermIt Shale and Supai Group and traveled 13 111 to the Colorado River. Calculated flow velocity ranged frail 10 to 18 ft/s, and the discharge ranged froll 9,200 to 14,000 ft'l/s. Yater content of the debris flow ranged froll 24 to 33 percent. and the sediJlents hacl a sand content of 10 to 15 ~rcent. One boulder transported by the debris flow weighed an e.tillated 47 tons: transported boulders with db_ter. inexce.. of 5 ft were co.-on. Upon reaching the Colorado River, the debris flow created a new fan .urface that significantly constricted the Co10radotiver (Kieffer, 1985). ~'--'^"