A RIVER RUNS UNDERNEATH IT: NEW INSIGHTS INTO HYDROGEOLOGY OF THE OREGON CASCADES AND THE ROLE OF NATIONAL FOREST LANDS

New research into the sources of major rivers draining the Cascade Mountains in Oregon reveals that large groundwater reservoirs located on National Forest land near the crest of the range provide a disproportionate contribution of late summer cold streamflow to both western and central Oregon. The Cascade Mountains are comprised of an older Miocene volcanic center (Western Cascades) adjoining an active Plio-Pleistocene volcanic arc. These two regions support rivers with distinctly different flow, temperature, and sediment regimes. High Cascade streams are characterized by stable, uniform flows, with muted winter peaks and sustained high base flows. Many of the major rivers from this region are fed by voluminous springs located at contacts between massive lava flows, and water temperatures are remarkably constant throughout the year. Temperature and isotopic analyses indicate that High Cascade streams are fed by an extensive groundwater reservoir that underlies the young permeable lava flows along the ridge crest of the range.

In contrast, Western Cascade streams exhibit high, flashy winter peaks in response to rain and rain-on-snow events, and very low base flows. Temperature regimes are strongly influenced by thermal warming. Large rivers, such as the Willamette, that drain both the High and Western Cascades are dominated by a Western Cascade flow regime during the winter and a High Cascade regime during the summer.

This work emphasizes the importance of National Forest lands in providing water for much of the population of Oregon for the full range of water uses. The adequacy of existing management plans for protecting source areas and springs are considered in light of these new findings.