ABSTRACT In portions of Washington, Oregon, and Idaho, the Columbia River Basalt Group (CRBG) hosts a regional aquifer system that is the primary, and in many cases the only, water supply for numerous communities, small water systems, individual homes, industry, and agriculture. In much of the semiarid Columbia Plateau, portions of the CRBG aquifer system have seen significant water-level declines and do not appear to receive significant, if any, natural recharge. Aquifer horizons within the Columbia River basalt generally are associated with intraflow structures at the top (e.g., vesicular flow-top breccias) and bottom (e.g., flow-foot breccias, pillow lava and hyaloclastite complexes) of sheet flows. The interiors of thick sheet flows (in their undisturbed state) have extremely limited permeability and act as aquitards, typically creating a series of stacked, confined aquifers within the Columbia River basalt aquifer system. The dominant groundwater flow follows horizontal to subhorizontal pathways along individual, laterally extensive, interflow zones. Vertical groundwater movement through undisturbed basalt flow interiors is greatly restricted except where basalt flow interiors are disturbed (such as by folds or faults), truncated (such as by flow pinchouts and erosional windows), or where they are cross-connected by wells.