WHERE, WHEN, AND WHY DO SPRINGS OCCUR IN VOLCANIC TERRANES? A REGIONAL ANALYSIS: WESTERN OREGON AND WASHINGTON, NORTHEASTERN CALIFORNIA, AND SOUTHERN IDAHO, USA

The ~500,000 km² Northwest Volcanic Province (NVP) contains over half of Meinzer’s high-volume springs of the U.S. (Meinzer, 1927). The USGS-delineated regional aquifers, the Columbia Plateau Regional Aquifer System (CPRAS) and the Eastern Snake River Plain (ESRP), and a new USGS study area, the Northwest Volcanic Aquifer Study Area (NVASA), comprise most of the NVP. Findings from CPRAS, ESRP, and several sub-regional studies are compared and contrasted with recent findings from the NVASA study, identifying the geologic controls on groundwater flow and discharge in volcanic terranes. A new thematic interpretation of existing state-level geologic maps covering the NVASA provides an updated and refined distribution of the composition and age of geologic units for the purposes of assessing hydrologic resources of the region. Comparison of the new geologic categories with available hydrologic data and regional soil-water-balance modeling shows that younger volcanogenic terranes tend to have higher primary permeability than older terranes. Decrease in primary permeability with age is attributable to weathering, hydrothermal alteration, and secondary mineralization (e.g., deposition of zeolites). These processes accelerate at temperatures >30 °C. The bottom of regional aquifer systems in the NVP tend to be low-permeability pre-middle Miocene terranes or to occur at depths where temperatures favor hydrothermal alteration of volcanic rocks. Considerable secondary permeability may occur where Quaternary faults have larger dilational tendency. Spring density as a function of geology and precipitation can be used to infer groundwater flow path length within regional aquifers. NVASA findings are compared and contrasted with findings from the CPRAS and ESRP studies, with particular emphasis on explaining the general patterns of spring location, size, and seasonality in volcanic terranes.

Meinzer, O.E. (1927), Large Springs in the United States: U.S. Geological Survey, Water-Supply Paper 557, 94 p.