Forest Disturbance and the Impacts on Maritime Snow in the Oregon Cascades

The maritime mountain snowpack in Oregon’s Cascades is a critically important source of flows for the Willamette River Basin. Recent warm winters have reduced snowpack due to precipitation falling as rain rather than snow. Furthermore, landscape-altering forest fires have altered the energy balance, affecting snowpack retention. This research investigates changes in snow water equivalent (SWE) due to warmer climate and fires in the McKenzie River Basin. The McKenzie River supplies water and hydropower to the city of Eugene Oregon and is a key tributary of the Willamette River. In 2003 the B&B Complex Fire (367 km2) burned through Santiam Pass, at the headwaters of the MRB, severely burning the forest surrounding the Hogg Pass SNOTEL site. For this study, data were acquired from multiple SNOTEL and meteorological sites for the 20 years before and 15 years after the fire. We examined pre- and post-fire differences in SWE at the Hogg Pass SNOTEL site, in comparison with the nearby and unburned McKenzie SNOTEL site. We also modeled the changes in SWE, calibrating the model to the unchanged McKenzie SNOTEL and rerunning the model with a clear-cut forest cover to simulate disturbance. Next, we tested this model at the Hogg Pass SNOTEL for the years of pre- and post-fire. Results show that the measured SWE at the Hogg Pass SNOTEL it noticeably less in the years following the fire. Results from the modeled disturbance overestimate SWE at all times during the post-fire 2004 water year. From this, we conclude that the model’s landcover doesn’t fully represent the impacts on the snowpack following a forest fire. Further model development is needed to include these processes. This study is particularly relevant to water resource and forest managers who seek to understand how the declining seasonal snowpack will affect water availability and could increase the likelihood of more forest fires during the dry months.