IMPACTS OF THE FLOODS OF 1938, 1969 AND 2005 ON THE OLD MT.BALDY ROAD, SAN GABRIEL MOUNTAINS CALIFORNIA

Six members of Cal Poly Pomona’s GIS Applications course conducted a detailed field survey to deduce the history of interaction between old Mt. Baldy road and various Holocene flood channels of lower San Antonio Creek. The original road, situated on active alluvium, provided access to Mt. Baldy village prior to completion of a new road along the west canyon wall in 1955. We utilized hand-held Garmin GPS receivers and ArcGIS software to map various paved road segments affected by the floods of 1938, 1969, and 2005. We walked approximately 4 miles of the canyon between Shinn Road and Mt Baldy School, noting places where bridges, pavement and culverts have been buried, undercut or annihilated by historic landslides and floods. Waypoints and field observations were integrated with ArcMap software to create a multi-layered color-coded map. The 1957 Mt. Baldy topographic quadrangle and Google Earth satellite photos provided base images for geo-referencing.

Floods associated with the 2005 record rainfall season scoured away sediments or heavily vegetated soil to uncover previously buried portions of the old Mt. Baldy Road. We encountered photogenic crosscutting relationships in which the oldest paved road segments are overlain by 1938 flood deposits, and younger road segments are eroded and locally buried by 1969 flood deposits. The two generations of road (pre-1938 and pre-1969) may be distinguished by different methods of road construction. In some places damaged segments of the pre-1938 road were simply repaved, whereas in other cases the younger road was routed on the opposite side of the canyon. There are several localities where 1969 flood deposits are covered by landslide deposits associated with an intense rainstorm of January 7-11, 2005 (26.14 inches total, with 16.52 inches precipitation recorded between January 8 and 9). Unstable canyon walls previously weakened by the wildfire of 2003 contributed sediment for this debris flow event. The dramatic field relationships in Lower San Antonio Canyon provide an excellent natural laboratory for studying the interactions between natural hazards and human infrastructure. Precise constraints on the timing of historical disturbances make this site ideal for field trips involving Engineering Geology and Natural Disasters courses.