UNDERSTANDING THE ROLE OF FIRE AND SLOPE ASPECT IN HILLSLOPE PROCESSES IN THE JEMEZ MOUNTAINS, NEW MEXICO, USA

In fire-prone mountains in the Western US, extensive erosion may occur after fire due to reduced infiltration and increased runoff, and loss of vegetative anchoring and surface roughness. Increased runoff after fire may be the primary contributor to erosion on hillslopes and sedimentation on alluvial fans in these areas. Additionally, insolation differences on north and south facing slopes also influences soil moisture and vegetation, which may affect the spatial distribution of erosion as well as sediment transport processes. Since forest stands in fire prone areas can have long periods of recovery after a fire, it is important to understand the natural variability of erosion for the purposes of forest and river ecology and management and mass movement-flooding hazard. The 2002 Lakes Fire area in the Jemez Mountains, NM, provides a natural study area with incision of alluvial fans after the Lakes Fire exposing the internal structure of these fans. The study area displays steeper, drier ponderosa pine dominated south-facing slopes and less steep, moister Douglas-fir dominated north-facing slopes, which suggests that slope aspect may influence fire regime and post-fire erosion in the Jemez Mountains. To assess the importance of fire and aspect on erosion and sedimentation, over 15 sections in alluvial fans were studied, including both north and south aspects. Sediment transport in this area is mostly by debris flow, hyperconcentrated flow, or stream flow. Deposits formed by these processes were described, and evidence for fire-related sedimentation was assessed. Additionally, the prevalence of sediment transport types in relation to north versus south slope aspects was examined. Finally, charcoal fragments within deposits from north and south aspects were analyzed in terms of their abundance and angularity in order to aid in estimating the severity of the fire event associated with the deposit. In this way, fire and slope aspect were assessed for their importance in influencing erosion and sediment transport in the study area.