METHODOLOGY AND IMPLICATIONS OF MAXIMUM PALEODISCHARGE ESTIMATES FOR MOUNTAIN CHANNELS, UPPER ANIMAS RIVER BASIN, COLORADO, USA

Historical and geologic records may be used to enhance magnitude estim ates for extreme floods along mountain channels, as demonstrated in th is study from the San Juan Mountains of Colorado. Historical photograp hs and local newspaper accounts from the October 1911 flood indicate t he likely extent of flooding and damage. A checklist designed to organ ize and numerically score evidence of flooding was used in 15 field re connaissance surveys in the upper Animas River valley of southwestern Colorado. Step-backwater flow modeling estimated the discharges necess ary to create longitudinal flood bars observed at 6 additional field s ites. According to these analyses, maximum unit discharge peaks at app roximately 1.3 m(3) s(-1) km(-2) around 2200 m elevation, with decreas ed unit discharges at both higher and lower elevations. These results (1) are consistent with Jarrett's (1987, 1990, 1993) maximum 2300-m el evation limit for flash-flooding in the Colorado Rocky Mountains, and (2) suggest that current Probable Maximum Flood (PMF) estimates based on a 24-h rainfall of 30 cm at elevations above 2700 m are unrealistic ally large. The methodology used for this study should be readily appl icable to other mountain regions where systematic streamflow records a re of shea duration or nonexistent.