AN INTEGRATED STUDY OF POST-FIRE WIND AND WATER EROSION IN WESTERN RANGELANDS

The Soda Fire burned over 280,000 acres in southwest Idaho and southeastern Oregon in August 2015. The fire burned >25% of the Reynolds Creek Critical Zone Observatory (RCCZO), creating an opportunity to investigate post-fire erosional response in a sagebrush-steppe dominated watershed with complex topography. While many post-fire erosion studies focus on water-driven processes (e.g. floods and debris flows), observations of post-fire erosion from this and other rangeland systems suggests wind transport of sediment and charred material may be an important driver of erosion. Field observations of ‘dust-drifts’ pre-loading channels below leeward slopes immediately following wildfire indicate a strong interaction between complex terrain, heterogeneity of vegetation types, and dynamic atmospheric conditions. We measure the spatial variability, magnitude, and nature of wind and water erosion through the use of 1) passive dust/colluvium traps and soil sampling 2) repeat terrestrial/areal LIDAR scans, and 3) analysis of meteorological and hydrological data. Preliminary results indicate that over winter months, elevations with aspect-dependent snow cover develop snowdrifts on leeward NE aspects and trap dust and saltating debris within the burnt area. In-channel, upper slope colluvium traps within the fire perimeter show a peak in sediment flux during winter months with fluctuating snow cover. Passive dust traps within and proximal to the fire boundary contain significantly more dust and burnt organic material than more distal traps down-wind. High burn severity is tightly linked to pre-fire vegetation density; riparian corridors and slopes with dense sagebrush and thick soils were severely burned as compared to less densely vegetated, generally rockier, south-facing slopes. Preliminary measurements indicate greater volumes of material eroded from north-facing vs. south-facing hillslopes. The Murphy Creek channel shows evidence of a small post-fire debris flow, and new development or migration of channel knickpoints. A better understanding of the interplay between wind and water erosion over complex terrain in a small watershed can lead to better hazard assessment and erosion models.