HYDROGEOLOGIC INVESTIGATION OF THE WILLIAMSON RIVER BASIN, KLAMATH COUNTY, OREGON

The Williamson River basin is the northernmost tributary of the Klamath River. Differences in the style of volcanism and sedimentation during the Pliocene and Pleistocene influence the hydrogeologic properties of bedrock. Relatively low permeability lava and pyroclastic flows and basalt hydrovolcanic deposits of Pliocene age lie to the southeast of an irregular northeast-trending boundary while highly porous and permeable basalt lava flows and interbedded medium-to coarse-grained sedimentary rocks of Plio-Pleistocene age lie to the northwest. This boundary is cut by NNW-striking normal faults, the easternmost of which is the Wildhorse Ridge-Military Crossing (WRMC) fault zone. The WRMC fault zone is active and cuts pyroclastic-fall deposits from the Holocene eruptions of Mount Mazama. The zone is the eastern border of the Cascades regional neotectonic zone. Ponding of the Williamson River on the down-thrown block produced the Klamath Marsh, a 350 sq km wetland complex. Pyroclastic-flow and -fall deposits from Mounta Mazama are present throughout the Williamson River basin. The porous pumice in pyroclastic-fall deposits holds water and reduces discharge to streams. Pyroclastic-flow deposits have relatively low permeability and serve as a confining layer over pyroclastic-fall deposits, and Plio-Pleistocene poorly consolidated sedimentary rocks and thin basalt flows. Ground water flows east and southeast from the Cascade Range beneath the pyroclastic-flow deposits and discharges in Klamath Marsh. During summer months, evapotranspiration exceeds precipitation, discharge of the Williamson River, and ground water discharge to the marsh. Water levels drop and flow in the Williamson River stops downstream from the marsh.