Holocene Fire-related Alluvial Chronology and Geomorphic Implications in the Jemez Mountains, NM

Abstract

Following a fire, extensive erosion may occur on hillslopes due to reduced infiltration and increased runoff as well as a decrease in vegetative anchoring and surface roughness. This increased erosion and subsequent sedimentation on alluvial fans at the base of the hillslope may be the primary process of geomorphic change in fire-prone mountains in the Western US. Insolation differences on north and south facing slopes may also be another potential influence on geomorphic change due to soil moisture and vegetation differences, which may affect the spatial distribution of erosion as well as sediment transport processes. Due to the long recovery period of forest stands in fire-prone areas, it is important to understand the natural variability of erosion for the purposes of forest and river ecology and management as well as 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. In order to determine the importance of fire and aspect on erosion and sedimentation, over 15 sections within alluvial fans with both north and south aspect were studied. Debris flow, hyperconcentrated flow and stream flow make up the majority of sediment transport processes in this area. Therefore, deposits formed by these processes were described, and evidence for fire-related sedimentation was assessed. Additionally, the relative importance 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, the importance of fire and aspect in influencing erosion and sediment transport was