Vegetation and channel recovery ten years following the Waldo Canyon Fire of Colorado

Abstract

ABSTRACT Although wildfire alters the hydrogeomorphic functioning of river systems through removal of vegetation and changes in soil properties, knowledge of postfire recovery has come primarily from studies in the initial years following fire. This paper reports biogeomorphic responses to the 2012 Waldo Canyon Fire of Colorado, USA, over a longer time frame, ten years after the fire. Extending an early study of three initial postfire years through 2014, this project tracked changes in channel morphology and grain size through 2021 by resurveying three channel reaches in Williams Canyon affected by moderate- and severe-intensity burn. Analysis of patterns of postfire rainfall and changes in vegetation cover since the burn, which was proxied by the satellite-based normalized difference vegetation index (NDVI), also enabled inferences about the processes of biogeomorphic recovery over this intermediate time frame. Results showed trajectories of channel recovery toward redevelopment of step-pool bedforms—by aggradation where the initial response was incision, and removal of postfire fine sediment where it may have buried bedforms. An overall coarsening of bed material accompanied these channel adjustments. These changes occurred coincident with initial vegetation regrowth, with changes in the NDVI exhibiting variability. The emergence of large wood in the study channels after the initial years—from felled trees and cut logs contributed by management activity—influenced channel adjustment and introduced further spatial and temporal variability in channel morphology and grain-size distributions. The stability, movement, and influence of the burned wood and cut logs, therefore, become important variables in the long-term adjustment of the fluvial system after wildfire. Greater understanding of the recovery of burned fluvial systems in the intermediate time frame is increasingly important as wildfires grow in frequency and magnitude under warming climate regimes.