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<br />~~"t-ii <br />/~~/ "~~, <br />1(11.,... . ~~. @, <br />l('{t1 ljl'ClI, <br />1'-~~f:"g'9GIS\_5 I! <br /> <br />BuHetin of the <br />Association of Engineering Geologists <br />Vol. XVIII, No.3, 1981 <br />pp. 309-322 <br /> <br />Debris Flows in Small Mountain Stream Channels of <br />Colorado and Their Hydrologic Implications <br /> <br />JOHN E. COSTA* <br />Department of Geography, University of Denver, Denver, CO 80208 <br /> <br />ROBERT D. JARRETT <br />United States Geological Survey, Mail Stop 415, Lakewood, CO 80225 <br /> <br />" <br /> <br />ABSTRACT <br /> <br />Gaging-station records and onsite investigations indicate that debris <br />flows in small mountain streams have been confused with waterfloods. <br />Geomorphic, sedimentologic, and hydraulic evidence attributed to debris <br />flows were found in five of seven areas studied, each with an area of less <br />than 10 square kilometers (4 square miles) and an altitude greater than <br />2,300 meters, (7,500 feet) where floods resulting from intense rainfall had <br />been reported. Evidence used to identify debris flows includes coarse, <br />lobate, poorly sorted, unstratified, pebbly mudstone.like unconsolidated <br />deposits with well.defined levees and terminal lobes, and the extent of <br />ground-litter disruption below high-water marks. Debris-flow deposits are <br />more poorly sorted than water-flood deposits. Although boulders as large <br />as I meter (3.3 ft) in diameter were transported by these debris flows, <br />small trees on debris fans were not severely scarred, and small willows <br />had diverted the flows. Downstream, high-water marks and gaging-station <br />records indicate that only a small percentage of the debris-flow discharge <br />was water. <br />The distinction between debris flows and waterftoods is important be- <br />cause: (1) Mitigating procedures for waterftoods, such as channelization <br />and damming, may not be effective for debris flows; (2) peak discharge <br />measurements made from evidence of debris flows lead to excessive es- <br />timates of floods for the Rocky Mountains; and (3) because of sparse <br />rainfall data in mountain regions, indirect.discharge estimates have been <br />used by some hydrologists and engineers to determine the amount of rain- <br />fall that occurred during a storm. This may lead to inaccurate estimates <br />of rainfall and flood discharges that are used in the design of flood-control <br />structures, <br /> <br />INTRODUCTION <br /> <br />In 1978 the U.S. Geological Survey, in coopera- <br /> <br />tion with numerous Federal and State agencies, be. <br /> <br />*Also: United States Geological Survey, Mail Stop 415, Lake- <br />wood, CO 80225. <br /> <br />gan to study floods in the foothill areas and moun- <br />tain regions of Colorado. One of the first tasks of <br />the project personnel was to compile a record of <br />recent floods resulting from intense rainfall in the <br />Colorado mountains and to review literature de- <br /> <br />[309] <br /> <br />I <br /> <br />~ <br />