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.
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A debris avalanche at Forest Falls, San Bernardino County, California, July 11, 1999