HYDROGEOLOGIC CHARACTERISTICS OF HOLOCENE PYROCLASTIC DEPOSITS OF THE WILLIAMSON RIVER BASIN, KLAMATH COUNTY OREGON

Pyroclastic-fall and –flow deposits from the mid-Holocene eruptions of Mount Mazama underlie the Williamson River basin. The hydrogeologic characteristics of these deposits substantially influence precipitation recharge of the groundwater. The correlation coefficient for pumice lapilli from the pyroclastic deposits and water content ranges 0.84 to 0.99. Coarse ash has a correlation coefficient ranging –0.62 to –0.99. The infiltration rate into the pyroclastic flow deposits ranges from 10^-5 to 10^-6 m s^-1 east of Crater Lake and ranges from 10^-3 to 10^-4 m s^-1 northeast of Crater Lake near Miller Creek. Winter maximum precipitation as snow in the rainshadow of the Cascades Range (33 ± 16 cm) recharges groundwater only in areas where pyroclastic deposits are thin or where elevation is high enough to enhance precipitation (Sugarpine and Yamsay Mountain) and exceed the holding capacity of pumice-rich deposits. Klamath Marsh (346 km²) is the most prominent surface water feature in the Williamson River basin. Streams were gaged from March 29, 2002 – April 13, 2003 and evapotranspiration was estimated from previous studies. These data allow an estimate of the annual discharge into Klamath Marsh from the Cascade Range of 3.08 x 10⁸ m³.