UNRAVELING HYDROGEOLOGY IN PUMICE MANTLED LANDSCAPES OF UPPER KLAMATH BASIN, OREGON

Pumice deposits from the cataclysmic eruption of Mount Mazama blanket the Williamson River basin, the northernmost basin of the Upper Klamath Basin. Jack Creek, a small tributary of the Williamson River, drains volcanic uplands where pumice deposits range from 2 to 3 m thick. Distribution and interactions between surface and ground water are sensitive to distribution of Miocene, Pliocene, and Pleistocene volcanic bedrock units and geomorphic evolution during these epochs. Precipitation, as snow, accumulates during winter and provides a pulse of water during spring melt. Summer and autumn are dry. Relative humidity (r.h.) and temperature measured within pumice at 0.6 m and 1.2 m of the surface record wetting during spring snow melt and drying during the rest of the year. Patterns relate to the volume of water contributed to the pumice profile during melting of winter snow pack. During 12 months (August 2008 to 2009) at 1.2 m, r.h. = 100% until 1 February, slowly declined to 94% by 15 February before abruptly (2 hrs) dropping to 44%. On 30 March r.h. started to rise to 95% (8 April) before declining to 81% and slowly rising to 91% by August.

Hydrogeology is divided into 1) low-relief uplands where groundwater is perched in meadows distributed relative to morphology of lava flows and 2) valley of Jack Creek which is cut into low permeability basalt hydrovolcanic and poorly sorted sedimentary rocks.

In low-relief upland meadows, pumice overlies the pre-eruption paleosol, summer water table is within 0.3 m of the ground surface, and plant communities are diverse. Water drains from meadows during the summer through low discharge streams (<0.1 ft³s^-1); some maintain year round flow to pumice-filled meadows in Jack Creek valley, but most sink below the surface.

Geomorphology of Jack Creek valley developed by stream erosion during the Pliocene, but was modified by Pleistocene basalt flows that segment the valley into alternating pumice-filled meadows and bedrock-lined channels. Flow ceases when water levels drop below the elevation of the bedrock-lined channels. Within pumice-filled meadows the water table remains approximately 1 m below surface and stagnant water may persist through the summer in channels. Year round flow occurs only in the upper reaches; flow in middle and lower reaches occurs infrequently during snow melt (2000, 2006).