Hydrogeologic systems in the southern Cascade Range develop in volcanic rocks where volcanic morphology, stratigraphy, extensional structures and attendant basin geometry play a central 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, contribute 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 basin that combines 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 integrate 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. 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 groundwater-flow impediments that restrict lateral groundwater-flow and force water into the creek. The inferred groundwater-flow barriers divide the aquifer system into at least three compartments. The two downstream compartments lose streamflow in the upstream sections (immediately downstream of the groundwater-flow impediments) and gain in downstream sections
