Scientists from the University of Oregon and their partners have mapped the amount of water stored beneath volcanic rocks at the crest of the central Oregon Cascades and found an aquifer many times larger than previously estimated — at least 81 cubic kilometers.
That’s almost three times the maximum capacity of Lake Mead, the currently overdrawn reservoir along the Colorado River that supplies water to California, Arizona and Nevada, and greater than half the volume of Lake Tahoe.
The finding has implications for the way scientists and policymakers think about water in the region — an increasingly urgent issue across the western United States as climate change reduces snowpack, intensifies drought and strains limited resources.
The western Cascades are characterized by steep slopes and deep valleys carved out by rivers. The high Cascades, meanwhile, are flatter, dotted with lakes and volcanic topography such as lava flows. The Cascade Range has been built up by volcanic activity over millions of years, making the exposed rocks in the high Cascades much younger than those in the western Cascades.
To better understand the flow of water through different volcanic zones, the team took advantage of projects begun in the 1980s and 90s. Past scientists had drilled deep into the ground and measured temperatures at different depths as part of the search for geothermal energy resources associated with the many hot springs that pepper the Cascades landscape.
Normally, rocks get hotter as you go deeper into the earth. But water percolating downward disrupts the temperature gradient, making rocks a kilometer deep the same temperature as rocks at the surface.
By analyzing where the temperature starts to pick up again in these deep drill holes, Leif Karlstrom, a UO earth scientist who led the study alongside collaborators from Oregon State University, Fort Lewis College, Duke University, the University of Wisconsin, the U.S. Forest Service, and the U.S. Geological Survey, could infer how deeply groundwater was infiltrating through cracks in the volcanic rocks. That allowed them to map the volume of the aquifer.
Previous estimates of water availability in the Cascades took the springs at face value, measuring river and stream discharge. Instead, Karlstrom and his colleagues went deeper — literally. But since those holes weren’t originally drilled with the intent of mapping groundwater, they don’t cover every area where one might like to collect such data. So, the new estimate of the size of the aquifer is a lower bound, and the actual volume might be even bigger still.
While it’s encouraging news that the aquifer is so much larger than previously believed, Karlstrom cautions that it’s still a limited resource that must be carefully stewarded and needs further study. Click here to read more.