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.

ABSTRACT The Miocene Columbia River Flood-Basalt Province is one of the youngest and perhaps the best studied flood-basalt province on Earth. This field guide describes a three-day field trip through the central, eastern, and western portions of the Columbia Plateau region of this province, visiting field localities that have been key to understanding the geologic and structural history of this province. The guide provides a brief summary of our current understanding of the geologic and tectonic evolution of this flood-basalt province. Recent refinements in Columbia River basalt stratigraphy have confirmed the huge size of many of the Columbia River basalt flows (1000– 5000 km3 in volume) and a wide range of emplacement rates. The emplacement rate estimates range from as low as one to two months to as high as three to four years. Many aspects of Columbia River basalt volcanism appear to be associated with regional-scale deformation (e.g., regional-scale subsidence, folding, and faulting).