Interacting geomorphic and ecological response of step-pool streams after wildfire

Abstract

Abstract Detailed study of the response of step-pool mountain channels to wildfire is rare despite increasing vulnerability of these systems. This paper reports the initial biogeomorphic response of step-pool streams to storms following the 2012 Waldo Canyon Fire, Colorado, USA. Field surveys, benthic macroinvertebrate sampling, and light detection and ranging (LiDAR) terrestrial laser scanning through 2014 generated a substantial data set comparing changes within seven channels burned by a range of severity with three unburned reference reaches. Results showed that wildfire increased the susceptibility of step-pool channels to destabilize according to the severity of burn. Whereas the step-pool morphology (step height H, step length L, and H/L) remained relatively unchanged in unburned and low-severity burned channels—even through an extreme rainfall event with recurrence intervals up to 500–1000 years—study sites affected by high burn severity altered significantly following the first comparatively minor storms. Study channels burned by moderate severity exhibited variable responses ranging from minor to substantial morphological changes, suggesting that additional factors, including slope, local topography, and proximity to other highly disturbed sites may play explanatory roles. The ecological response corroborated the geomorphic findings, with data for benthic macroinvertebrates varying according to burn severity along with the changing post-fire step-pool morphology. Ordination analysis enabled data synthesis and showed that precipitation intensity (r = 0.59) and the severity of burn (r = 0.60) together influenced the interacting morphologic and ecological responses after the Waldo Canyon Fire. Variations in H (r = 0.72), L (r = 0.56), and thalweg elevation (r = 0.75) correlated the most with the types of benthic macroinvertebrates present. Following the Waldo Canyon Fire, the highest rainfall intensities coincidentally occurred over study sites in high-severity burn areas. Although disentangling the effects of rainfall intensity and burn severity proved difficult given this scenario, the interacting and complementary roles of burn severity and rainfall intensity likely led to the greatest morphologic and ecological impacts at those sites. Findings offer insights for the post-fire management of step-pool streams that characterize frequently burned mountain areas. As wildfires grow in frequency and magnitude under warming regimes, management becomes increasingly important for anticipating future changes in burned river landscapes, mitigating potential floods and hazards, and promoting sustainable river ecosystems.