IMPLICATIONS OF STRUCTURAL GEOLOGY AND VOLCANISM FOR THE REGIONAL HYDROLOGY IN THE PIT RIVER DRAINAGE BASIN, NORTHERN CALIFORNIA, USA

The Pit River, in northeastern California, is the largest average-annual inflow to Shasta Reservoir, which is the largest reservoir in California. More than half of the flow of the Pit River comes from large-volume springs that discharge from extensive young (<1 Ma) lava flows in areas with no well-developed stream networks. The geologic and structural setting of the region is complex and controls on groundwater-flow paths and spring discharge locations are poorly understood. This project integrates structural, stratigraphic, and volcanological data with geo-statistical methods to constrain the hydrology of the Pit River basin in northeastern California. The Pit River study area lies at the boundary between the Basin and Range extensional province, the Walker Lane province, and the Cascades Volcanic Arc. Geologic heterogeneity, age, and structural fabric control the surface water and groundwater flow paths in the region. In the study area, major constructional volcanic centers Medicine Lake Volcano and Lassen Peak form the highest topography. Lava flows from topographic highs fill lowlands, including the fault-controlled Hat Creek Valley. These Cascade arc volcanic flows partially mute the topography created by N- and NW-trending normal faults. For example, Medicine Lake Volcano, a Cascade-arc off-axis basaltic shield volcano erupted the Giant Crater Lava Flow 12,500 years ago, partially covering the range-bounding fault responsible for uplift of the Big Valley Mountains to the east of Hat Creek Valley. Lassen Peak, a Cascade-arc composite volcano, erupted extensive lavas 24,000 years ago that flowed down the Hat Creek graben, partially filling some of the Hat Creek Valley. Goals of this project include development of a 3-D subsurface geologic framework that will enable the exploration of linkages between groundwater flow, heat flow, deformation in this structurally complex region, and volcanic and geomorphic history.