RUNOFF AND SPRING DISCHARGE MEASUREMENTS RECORD WATERSHED RECOVERY FROM A FIVE-YEAR DROUGHT IN ICEHOUSE CANYON, SAN GABRIEL MOUNTAINS, CALIFORNIA

Icehouse Creek drains a high elevation (8985-4700 ft) alpine watershed that typically receives 250% of the rainfall observed in the adjacent Los Angeles basin. Stream flow is sustained by: (1) springs discharging from fault zones in crystalline bedrock, and (2) drainage from shallow unconfined aquifers in coarse landslide and talus deposits. We present a continuous record of stream runoff and spring discharge from multiple gauging sites measured between June 2014 and July 2017, in the context of precipitation events captured by a network of portable rain gauges. The time-series data set allows analysis of creek and spring response to specific recharge events and provides associated base flow recession curves. Our study period spans the last two years of a five-year drought during which minor rainfall events were broadly distributed over 8-10 months, and also captures the 2016-17 season when most of the above-average precipitation fell between mid December and late February, including five separate storms of 8.0, 3.08, 5.84, 6.93, and 5.43 inches. Over three years, flow gauges in Icehouse Creek consistently recorded gains within a few days of significant precipitation. Spring discharge remained relatively flat during the drought years, indicating minimal response to low-level precipitation. During 2016-17, two of four springs showed dramatic discharge spikes in mid February (2 months after the onset of heavy rains) while the other two springs recorded gradual increases between mid January and mid April. Six samples from fault-sourced springs yielded tritium ages of 24-32 years, suggesting significant contributions from old, deep-seated groundwater. Base flow recession constants of the springs and adjacent creek segments are low (a =0.0069 to 0.0086 days^-1) compared to those calculated along upstream and downstream creek segments that cross thick alluvium (a =0.0122 to 0.0264 days^-1). These relationships, coupled with hydrogeologic mapping, support a model of two integrated groundwater systems in Icehouse watershed. During rainy periods Icehouse Creek is fed by drainage from highly permeable unconfined aquifers, with lesser contributions from the springs. During droughts, creek flow is modulated by lower level spring discharge, and punctuated by short-lived unconfined aquifer drainage after minor storms.