RECHARGE AREAS AND DISCHARGE OF GROUNDWATER IN A YOUNG VOLCANIC LANDSCAPE, MCKENZIE RIVER, OREGON

In young volcanic terranes, actual groundwater recharge areas may not conform to topographically-delineated watershed boundaries. In the central High Cascades of Oregon, many watersheds are covered by Quaternary lava flows with extremely high permeability. This results in infiltration over most of the land surface, minimal surface runoff, and groundwater discharge concentrated at discrete large springs. Reconnaissance studies in the 1920s identified the groundwater-dominated hydrology of the area, but most High Cascades streams are ungaged and there is little information about their discharge patterns or recharge areas and timescales. This information is critical to our understanding the long-term behavior and dynamics of flow regimes from this hydrologically important area.

We used a mass balance approach to constrain the recharge area for springs at Clear Lake, which are the designated headwaters of the McKenzie River in Oregon. The site has been continuously gauged since 1947, allowing for accurate estimation of discharge. Average daily discharge in the wettest month is 2.7 times that of the driest month, and September daily unit discharge is 200 times greater than an adjacent surface runoff dominated watershed. Autocorrelation analysis indicates that the system has a long memory with damped peaks and slow recession rates.

The long gauging record makes Clear Lake an excellent prototype for testing a mass balance approach. A spatially distributed rainfall model (PRISM) combined with geologic maps of the area allows calculation of recharge amounts for given watershed areas, while d¹⁸O constrains mean recharge elevation. Watershed areas are iterated until mean annual recharge balances discharge and estimated evapotranspiration loss. A similar approach is used to identify recharge areas for springs with newly established gauges and unknown watershed areas.