April 18, 2009 Field Trip #4: Natural Hazards, Past and Impending, in the Eastern San Gabriel Mountains Jonathan A. Nourse Department of Geological Sciences California State Polytechnic University 3801 West Temple Avenue Pomona, CA 91768 Introduction The eastern San Gabriel Mountains present a spectacular outdoor laboratory for studying the causes and consequences of natural hazards that include earthquakes, floods, landslides, and fires. Since 1991 I have utilized the San Antonio Canyon region as a convenient place to educate Cal Poly Pomona students enrolled in my Natural Disasters, Engineering Geology, Structural Geology, Tectonics, Groundwater Geology and Optical Mineralogy courses. The area has also provided world-class field trip sites for the Geological Society of America, the Thomas W. Dibblee Foundation and NAGT. This guidebook includes excerpts from several previous field trip guides for which I have been principle or contributing author (Nourse, et al., 1998; Trent and Nourse, 2001; Trent et al., 2001, Nourse, 2003). The oblique aerial view of Figure 1 highlights the San Gabriel Mountains block, uplifted along the Sierra Madre-Cucamonga frontal thrust system and dissected by left-lateral and right-lateral strike slip faults. Our plan for today is to begin the trip at San Antonio Dam near the intersection of the Cucamonga, San Jose and San Antonio Canyon faults. Working our way up the Mt. Baldy Road, we shall view field evidence of floods, landslides and debris flows that have resulted from interplay between steep topography, severe weather conditions, and major earthquakes. Effects of the devastating fire of 2003 are also prominent. We will discuss impacts of the floods of 1938, 1969, and 2005 on human structures such as dams, roads and buildings. Following lunch at Mt. Baldy Visitor Center, we will take a 1-mile hike up the Icehouse Canyon trail to ponder the natural processes that have reduced the number of habitable residential cabins from 119 in the early 1900’s to 26 today. The trip closes with view stops in upper San Antonio Canyon. A road map showing locations of today’s nine stops is provided in Figure 2. 1 Road log to Stop #1: Drive east on Temple Avenue from Mount San Antonio College Mileage cumulative 2.8 Mileage interval 2.8 Turn right and loop onto the northbound 57 Freeway. The eastern San Gabriel Mountains form the prominent block uplift directly ahead. Cross over I-10, continue north on 57. Mileage cumulative 8.0 Mileage interval 5.2 Angle right onto the 210 Freeway east (toward San Bernardino). Mileage cumulative 15.1 Mileage interval 7.1 Exit right onto Baseline Road. Get in the left-turn lane. Basement exposures of the eastern San Gabriel Mountains are straight ahead to the north. Alluvial fan deposits derived from San Antonio Canyon are exposed in excavations in the foreground to the northeast. Turn left onto Baseline Road (west-bound) and immediately get into the right lane. Mileage cumulative 15.6 Mileage interval 0.5 Turn right onto Padua Avenue and proceed north Mileage cumulative 17.4 Mileage interval 1.8 Reset you mileage to 0 at this four-way intersection with stop light. Turn right onto the Mt. Baldy Road. We have now entered the Mt. Baldy Quadrangle (Dibblee, 2002). Drive east along the approximate trace of the Sierra Madre-Cucamonga reverse fault (Crook et al., 1987; Morton and Miller, 2003). The Mt. Baldy Road begins to climb steeply. Mileage cumulative 1.3 Mileage interval 1.3 Turn right onto a large dirt parking area. Park near sign describing San Antonio Dam. Stop #1: San Antonio Dam Walk through a gap in the chain-link fence, then follow the paved access road south, stopping briefly to view the concrete spillway (elevation 2238 ft). Continue east up another road to the crest of San Antonio Dam (elevation 2260 ft) for a regional overview. San Antonio Dam (Figure 3) was built by the Army Corps of Engineers in 1956 to prevent a repeat of the high waters that inundated Upland, Claremont, and La Verne during March of 1938. It played a crucial role in reducing damage during the 1969 flood. In addition to flood control, the dam also serves the purpose of groundwater recharge. Its porous and permeable substrate supplies renewable well water to the City of Upland. San Antonio reservoir has never exceeded the height of the spillway, but its water elevation reached 2193 ft and 2226 ft during January of 1969 and February of 1980, respectively (Figure 4). The earth-fill design of San Antonio Dam (Figure 3) is prudent given its location near the intersection of three active faults that conceivably might rupture a concrete structure. The dam is positioned across an apparent left-lateral deflection or tear in the Sierra Madre-Cucamonga thrust. Coinciding with this deflection is a prominent left-step between the San Antonio Canyon 2 fault and the inferred northeast projection of the San Jose fault (Herber, 2001). Given that both of these latter structures record left-lateral displacements, the fault geometry suggests that the thick alluvium beneath San Antonio reservoir occupies a pull-apart basin. The epicenter of the 1990 magnitude 5.4 Upland earthquake was directly south of San Antonio Dam (Hauksson and Jones, 1991). This earthquake and a similar event in 1988 (M=4.5) are believed by these authors to have ruptured the San Jose fault. Detailed analysis of the 1990 Upland event and its aftershocks indicates left-lateral oblique reverse displacement on a fault with an orientation of N26E/77NW (Dreger and Helmberger, 1990). Rupture initiated at a depth of 6 km. The Sierra Madre-Cucamonga fault has not ruptured in the immediate vicinity during historical time. However, the entire frontal fault system is classified as active and capable of generating a magnitude 7 earthquake (Dolan et al., 2005). Its youthfulness is implied by prominent escarpments along the range front, and evidence for accelerated erosion that we will view later today. Also, both the east and western abutments of San Antonio Dam preserve dramatic examples of north-dipping thrust faults that break crystalline rocks. These faults, synthetic to the modern-day Sierra Madre-Cucamonga thrust, have facilitated uplift of the eastern San Gabriel Mountains. Road log to Stop #2: Continue north along Mt. Baldy Road. To the left are outcrops of quartzite and pelitic gneiss overlain by Quaternary terrace gravels and debris flows. These rock units reside in the hanging wall of the Sierra Madre fault. Look for an intersection with Shinn Road (marked with a brown sign to Upland and Ontario). Mileage cumulative 2.5 Mileage interval 1.2 Turn right and follow Shinn Road down hill to a bridge crossing of San Antonio Creek. Park on the right just past the bridge. Stop #2: Lower San Antonio Canyon--Erosional Remnants of the 1969 Flood and Pleistocene Deluges The hairpin turn we are situated upon is a rebuilt segment of the road that was washed out during the flood of January 25-26, 1969 (and almost taken out again in January of 2005—notice recent erosion of the road base directly north of the parking area). As shown in Figure 5, the entire flood plain in front of us was inundated by San Antonio Creek in 1969. Boulders the size of small cars were transported during this event. Peak flow at the now defunct USGS gauge 3 mi upstream was 16,400 cfs on January 25, 1969. Water impounded behind San Antonio Dam rose to within 45 ft of the spillway. It was necessary to let 7830 cfs through the dam outlets to keep the water below this level (Herber, 2001). The 1969 flood was induced by a series of warm tropical storms in middle to late January that followed a December 25-26 storm that dropped 18 in of snow at Mt. Baldy Notch (elevation 7900 ft). Daily precipitation records indicate that 51.25 in of rain fell at Mt. Baldy Notch between January 18 and 28, while 39.82 in fell at Sierra Powerhouse, located at 3110 ft elevation 3 on lower San Antonio Creek. Of these totals, 32.3 in were recorded between January 24 and 26 at the Notch, while 22.5 in fell at the Sierra station. Thunder Mountain (elevation 8600 ft, located at the top of the Mt. Baldy Ski Lifts) recorded 62.76 in of precipitation during the month of January, and 120 in during the whole season. For comparison, the famous flood of March 2, 1938 produced a peak stream flow of 21,400 cfs on Lower San Antonio Creek. That flood was partly due to heavy snow pack melting during a four-day deluge that dumped 32.2 in of rain at Kelly’s Camp, elevation 8300ft. Prior to 1969, a paved road (Mountain Avenue) diverged from this site and followed the approximate axis of San Antonio Canyon northeastward to Mt. Baldy Village (Figure 6). This road crossed San Antonio Creek in several places. Today, only isolated segments of the road remain, nicely exposed by the recent wildfire of October, 2003. Detailed mapping by students in my GIS course (Wicks et al., 2008) documented the presence of two generations of the Old Mt. Baldy Road in Lower San Antonio Canyon. An old, thin asphalt road was mostly washed away by the flood of 1938, then repaved and locally rerouted, only to be destroyed again in 1969. Five bridges were washed out during the 1969 flood. The remains of one bridge are preserved about 250 m upstream from the parking site of Stop #2. Carefully make your way down into the wash, crossing the reinforced concrete road base (watch out for loose gravel and broken glass!).