Large stream boulders in 16 drainage basins within the Upper Santa Ana basin were studied; 9 of these basins were in the Eastern San Gabriel Mountains and 7 were in the Western San Bernardino Mountains. A total of 43 sites were sampled. Field measurements included boulder dimensions, lithology for specific gravity (and particle density) determination, bed roughness, channel slope, and width. The five largest boulders at each site were selected. Field data were used in paleohydraulic calculations to determine, average velocity, average flow depth, and estimated discharge that would accompany boulder movement in a fluvial system. Tractive competence, the largest particle that can be moved in a stream bed, is velocity dependent, and flow depth is a function of velocity. Therefore, by selecting the largest stream boulders for study, flow velocity and depth at the time of boulder transport can be estimated. This information, used in conjunction with the stream channel cross section, can then be used to estimate conditions that only occur during the highest magnitude flood event: the probable maximum flood (PMF). The largest flood this century in the study area occurred in 1938. This study indicates that the PMF for the selected streams is approximately three times the discharge of the 1938 flood. This is generally in line with the peak flood discharge estimated by the slope-area method for the “Great Flood” of 1862.