|Abstract or Summary
- Climate change will continue to profoundly affect water supply and aquatic ecosystems in the Pacific Northwest. Changes such as warmer air temperatures, increases in the proportion of winter rain versus snow, reduced spring snowpack, and earlier snowmelt all affect streamflow. The response to these climate impacts includes earlier runoff peaks, decreased baseflows, increased summer water temperatures, and increased winter flooding from rain on snow events. Developing effective adaptation strategies to address these impacts requires knowledge on the climate vulnerability of stream ecosystems.
Not all streams in the region respond similarly to the same climate signal. Two important landscape factors influencing the streamflow response to climate are elevation and geology. Elevation and its effects on air temperature and the form of winter precipitation are widely recognized. It is generally thought that basins at intermediate elevations on the cusp of rain-snow transitions will be most susceptible to warmer winter temperatures and reduced snowpack. Less attention has been given to geology and the partitioning of hydrologic flowpaths between surface and sub-surface flow. This partitioning affects summer/fall baseflow volumes, the timing and attenuation of the snowmelt peak, water temperatures, flooding and geomorphic characteristics, and ultimately, the streamflow response to climate.
This study explores some of the streamflow characteristics and responses of groundwater-dominated versus surface-dominated streams to climate and climate change. Baseflows are much greater in groundwater streams and are seasonally very important for sustaining downstream mainstem flows during summer. But these same flows may also be more susceptible to reduced snowpack and earlier snowmelt. Summer water temperatures in groundwater streams are generally cooler and may be less sensitive to warming air temperatures. Flooding risks from rain-on-snow events may be lower as well, since streamflows in these systems are generally more stable and less flashy than in surface-dominated streams. The unique characteristics and responses of groundwater streams demonstrate the importance of considering geology as well as elevation when evaluating streamflow response to climate change, both past and future.