ESTIMATING KARST CONDUIT LENGTH USING CONDUCTIVITY AND DISCHARGE MEASUREMENTS IN LILBURN CAVE, KINGS CANYON NATIONAL PARK, CALIFORNIA

In the Redwood Creek basin, a tributary to the Kaweah River in Kings Canyon National Park, fluvial waters sink into and traverse a karst system and emerge at Big Spring, a single-orifice Vauclusian resurgence. The lower 20% of this karst contains Lilburn Cave, at 21.08 miles (33.92 km) the longest in California. Lilburn Cave affords access to the upper and lower sections of the phreatic conduit system, each of which has been dived to a distance greater than 300 meters. The description of the diver (Bill Farr) is of single tubular passage approximately 3 m in diameter at either end, with no obvious large tributary passages connecting to these conduits. In 2006, a snowmelt flood pulse was recorded in Redwood Creek upstream of the karst and also at Big Spring, permitting an estimate of the volume of water contained within the karst conduits below base level. From this volume, the potential conduit length and associated cave passage length were also estimated. At both sites the time delay between the initial decrease in conductivity and minimum value recorded for conductivity was approximately one day. Lateral sources of input were assumed to be negligible due to 1) little or no delay between the increase in discharge and decrease in conductivity at Big Spring, 2) the similar time scales of decreased conductivity at both Big Spring and above the karst in Redwood Creek, 3) the observed decrease in discharge between the Redwood Creek site and Big Spring (as some water bypassed the subterranean conduits by flowing on the surface), and 4) the description of the dived conduits. The flow at Big Spring is thus dominated by the input from the sinking of Redwood Creek. The time delay between the onset of the discharge event above the karst and at Big Spring was approximately 1 day. The discharge curve at Big Spring was integrated from the onset of conductivity decrease above the karst until the conductivity decrease signal was expressed at Big Spring. Thus the total volume of water expelled from the conduits connecting the sink at Redwood Creek to Big Spring was determined to be 48,700 cubic meters. Assuming a single cylindrical conduit of 3 m diameter, we determine a minimum conduit length of approximately 6.9 km. Based on the current known ratio of surveyed cave passage length to subterranean main stream passage length of 20:1, we estimate a minimum potential cave length of 138 km.