Geomorphic characteristics influencing post-fire river response in mountain streams

Abstract

As wildfires increase in frequency and severity, there is a growing interest in understanding river response to the wildfire disturbance cascade. Numerous headwater mountain catchments within the Cache la Poudre (Poudre) River basin in the Colorado Front Range, USA burned severely and extensively during the 2020 Cameron Peak fire. Debris flows and flash floods occurred in many of these catchments triggered by convective storms after the fire. The downstream effects of the floods and sediment varied along a continuum from attenuated and largely contained within the catchment, to releasing substantial volumes of water and sediment to the Poudre River. We conceptualize these catchments as exhibiting decreasing absorbance of post-fire disturbance along the continuum described above based on the geomorphic evidence of relative sediment export. We conceptualize characteristics on different spatial scales as driving or resisting response to disturbance and therefore impacting the absorbing capacity (ability to attenuate post-fire fluxes) of the catchment. As the magnitude of resisting characteristics increases at the catchment, inter- and intra- reach scales, we hypothesize that a catchment will increasingly absorb the impacts of the wildfire disturbance cascade. We conducted longitudinally continuous surveys to measure reach-scale characteristics within each study catchment. We focus on the catchment- and reach-scale geomorphic, vegetation, and burn characteristics. The floods observed at the study catchments illustrate fire influencing the elevation above which rainfall-induced flooding occurs due to the efficient conveyance of water from hillslopes to channels after wildfire. Results suggest that inter- and intra-reach spatial heterogeneity are better aligned with absorbance capacity than catchment-scale characteristics: greater absorbing capacity is associated with greater longitudinal variations in floodplain/channel width and more reaches with wide floodplains, low channel gradients, beaver-modified topography, and multi-stem deciduous vegetation. We connect the capacity to absorb the impacts of disturbance as informing the catchment response to wildfire disturbance.