1_README.TXT A Debris Avalanche at Forest Falls, San Bernardino County, California, July 11,1999 By Douglas M. Morton(1) and Rachel M. Hauser(1) 2001 U.S. Geological Survey Open File Report 01-146 Version 1.0 U.S. Department of the Interior Gail A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. (1) U.S. Geological Survey, Department of Earth Sciences, University of California, Riverside, CA 92521 For sale by USGS Information Services Box 25286, Building 810 Denver Federal Center Denver, CO 80225 (888) ASK-USGS ISBN: 0-607-97134-7 Available online at http://geopubs.wr.usgs.gov/open- file/of01-146/ QUICK START For those already familiar with Adobe(2) Acrobat(2) Reader and who have version 5.0 or greater on their computer, go directly to the file of01-146.pdf. On a PC system with Windows(3), the autorun feature on this CD-ROM may launch Acrobat Reader and load the file of01-146.pdf for you. If not, open "My Computer," right-click on the CD-ROM icon, and select "Open." ABSTRACT: The community of Forest Falls, California, is frequently subject to relatively slow moving debris flows. Some 11 debris flow events that were destructive to property have been recorded between 1955 and 1998. On July 11 and 13, 1999, debris flows again occurred, produced by high- intensity, short-duration monsoon rains. Unlike previous debris flow events, the July 11 rainfall generated a high- velocity debris avalanche in Snow Creek, one of the several creeks crossing the composite, debris flow dominated, alluvial fan on which Forest Falls is located. This debris avalanche overshot the bank of the active debris flow channel of Snow Creek, destroying property in the near vicinity and taking a life. The minimum velocity of this avalanche is calculated to have been in the range of 40 to 55 miles per hour. Impact from high-velocity boulders removed trees where the avalanche overshot the channel bank. Further down the fan, the rapidly moving debris fragmented the outer parts of the upslope side of large pine trees and embedded rock fragments into the tree trunks. Unlike the characteristic deposits formed by debris flows, the avalanche spread out down-slope and left no deposit suggestive of a debris avalanche. This summer monsoon-generated debris avalanche is apparently the first recorded for Forest Falls. The best indications of past debris avalanches may be the degree of permanent scars produced by extensive abrasion and splintering of the outer parts of pine trees that were in the path of an avalanche. ACKNOWLEDGMENTS: We appreciate the help and interaction in the field with Wes Reeder, Bob Fleming, Rick Everert, Henry Jones, Anne Kennedy, Rich Minnich, Natalie Morton, Gary Patt, and Eric Riggs. Russ Campbell, Wes Reeder and Cynthia Diamadi provided critical reviews. Wes Reeder kindly provided photographs of the 1997 and some of the 1999 debris flows. Uncredited photographs are by the authors. CONTENTS This CD-ROM contains a Portable Document Format (PDF) file that contains 67 pages with 57 illustrations. 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