- The basaltic landscapes of the Oregon High Cascades form a natural laboratory for examining how geologic setting and history influence groundwater flowpaths, streamflow sensitivity to climate, and landscape evolution. In the High Cascades, highly permeable young basaltic lavas form extensive aquifers. These aquifers are the dominant sources of summer streamflow for the Willamette River and its tributary, the McKenzie River, whose watershed forms the study area. Groundwater is discharged at large volume, cold springs, and discharge, temperature, and isotopic measurements at seven springs were used to constrain groundwater patterns. Lava flow geometries have a significant influence on groundwater recharge areas and flowpaths, sometimes superseding topographic controls. Transit times to springs are ~3-14 years, and flowpaths are generally shallow and have limited contact with deeper geothermal systems, except at fault zones. Time-series analyses of historical discharge, precipitation, snow, and temperature records reveal that the distribution of permeable rocks and resultant groundwater systems strongly influence streamflow response to climatic forcing. The annual hydrograph is shaped by groundwater storage and release, but inter-annual streamflow variability is largely the result of climatic forcing. Over the past 60 years, warmer winters and earlier snowmelt have lengthened the summer recession period and decreased autumn minimum discharges in a groundwater-dominated watershed. A chronosequence of dated rock units, combined with field
observations and measurements of slopes, contributing areas, and drainage densities, shows that the geologic history of a watershed controls the stage of drainage development. Groundwater flowpaths remain the dominant drainage mechanism for up to one million years, but as chemical weathering, glaciation, and other processes reduce the land surface permeability, channel networks grow up-slope of the springs. Within three million years, water is carried via a fully developed runoff-dominated stream network, groundwater discharge is insubstantial, and the landscape is shaped by fluvial and mass-wasting processes. The geology of a landscape is important for understanding hydrological processes at time scales ranging from a single season to several million years.