Development of a Generalized Three-Dimensional Geologic Model for Simulation of Regional Groundwater Flow in the Upper Klamath Basin, Oregon and California

The development of geologically based permeability-distribution models for simulating groundwater flow is a challenge in complicated, heterogeneous terrains. The task is made more difficult in many areas, including the upper Klamath Basin, by deficiencies in geologic mapping and paucity of subsurface control. A geologic model for regional groundwater flow simulation in the upper Klamath Basin has been developed using hydrologic observations to guide generalization of approximately 250 geologic map units into 9 regional hydrogeologic units, and to augment incomplete geologic information in areas only mapped at regional scales. Hydrologic information also provided guidance in understanding the role of geologic structure, including the prominent northwest-trending normal faults prevalent in the region. The types of hydrologic observations that were found useful included hydraulic head distribution, locations of major springs, distribution of groundwater discharge to streams, and productivity of wells in specific units. The principal hydrogeologic units include: Quaternary sediments (primarily younger stream deposits and basin-filling sediments, broken into fine and coarse facies), late Tertiary sediments (primarily fine-grained lacustrine deposits), Quaternary pyroclastic deposits (primarily ash fall and ash flow from Mt. Mazama), Quaternary volcanics (primarily lava flows and vent deposits, generally near or at the surface), late Tertiary volcanics (also primarily lava flows and vent deposits, but often at depth), Tertiary hydrovolcanic deposits (mostly palagonitic tuffs), and early to middle Tertiary volcanics (mixed volcanic deposits, generally low permeability). Hydrologic information indicates that Basin and Range faults in the region mainly affect groundwater where units of contrasting permeability are juxtaposed, and that faults may or may not be barriers to flow. The above approach enabled development of a geologically-based, regional-scale permeability-distribution model that is reasonably simple but is still consistent with hydrologic observations and geologic information. It is anticipated that refinements will occur during flow-model calibration.