WHERE POST-FIRE DEBRIS FLOWS STOP

Wildfires burn steep mountain slopes in a variety of plant communities ranging from desert scrub to chaparral to tree-dominated forests, creating debris-flow hazards. However, there have been few studies that explore how debris flow erosion and deposition patterns differ as a function of the pre-fire vegetation. Here we present new work on debris-flow mobility in two small watersheds within contrasting plant communities: 1) Sonoran Desert scrub burned at low to moderate severity by the 2019 Woodbury Fire, Tonto National Forest, AZ (0.3 km²) and 2) Ponderosa Pine woodland burned ~50% low severity and ~50% moderate and high severity by the 2020 Tadpole Fire, Gila National Forest, NM (0.7 km²). Debris flows were triggered by high intensity rainstorms several months after a wildfire at both sites (peak 15-minute rainfall of 111 mm/hr at the Woodbury site and a peak 15-minute rainfall of 34 mm/hr at the Tadpole site). We found that debris flows in the Sonoran Desert Scrub study area resulted in channel scour to bedrock and a distinct transition from erosion to deposition. At the point of deposition, 1,200 m³ of sediment infilled the channel with an average depth of 0.29 m. The sediment transport patterns were primarily determined by the local channel slope and geomorphic connectivity. In contrast, erosion and deposition by debris flows in the forested study area promoted in-channel deposition behind large woody debris (LWD). We found that the total channel storage of sediment behind LWD (150 m³) was larger than the amount of sediment deposited at the base of the watershed on a road (100 m²). This work suggests that predictions of post-fire depositional patterns may ultimately depend on the pre-fire vegetation regime, in addition to the local geomorphology. Results have implications for assessing post-fire impacts on riparian habitat, sedimentation, and debris-flow runout.