THE EFFECT OF STRUCTURE ON GROUNDWATER AND SURFACE-WATER INTERACTIONS IN THE VOLCANIC AQUIFERS OF THE HAT CREEK VALLEY, CALIFORNIA, USA

Hydrogeologic systems in the southern Cascades Arc develop in volcanic rocks where volcanic morphology, stratigraphy, extensional structures and attendant basin geometry play a large role in groundwater-flow paths, groundwater/surface-water interactions, and spring discharge locations. High-volume springs (> 3 m³/s), flow from young (< 1 Ma) volcanic rocks in the Hat Creek and Fall River tributaries, contributing approximately half of the average annual flow of the Pit River, the largest tributary to Lake Shasta and the Sacramento River. We build a hydrogeologic conceptual framework for the Hat Creek Valley by combining new geologic mapping, water well lithologic logs, lidar mapping of faults and volcanic landforms, streamflow measurements, and an aerial stream-temperature survey (Thermal InfraRed; TIR). Data from geologic maps, well logs and lidar are used to define the geologic structure and the volcanic and volcaniclastic stratigraphy in the basin. Streamflow measurements and TIR estimates of stream-temperature allow for identification of locations of likely groundwater/surface-water interactions and focus attention on possible geologic controls. Two large streamflow gains suggest focused groundwater input to Hat Creek near Big Springs and north of Sugarloaf Peak. These large inflows likely result from geologic impediments (hereafter referred to as the Big Springs fault and the Sugarloaf barrier) that restrict groundwater flow and force water into the creek. These interpreted groundwater-flow barriers divide the aquifer system into at least three compartments (upstream, middle, and downstream), the farthest upstream gain spatially coincides with the Big Springs fault suggesting the fault may be the barrier. Immediately upstream of the Big Springs fault streamflow increases by >500% over <1 km of stream length. In the middle compartment, downstream of the Big Springs fault, Hat Creek loses streamflow to the aquifer system, until the Sugarloaf Barrier, likely a fault, forces groundwater into Hat Creek. In the downstream compartment, Hat Creek loses to the aquifer system downstream of the Sugarloaf barrier and gains from both dam operations and natural features (springs controlled by stream incision, structure and lava-flow morphology) as the creek nears the Pit River.