RESPONSE OF STEP-POOL STREAMS THREE YEARS AFTER THE WALDO CANYON FIRE IN COLORADO

Knowledge of the hydrological and geomorphological impacts of wildfire has advanced significantly in recent decades. Yet, predicting post-fire effects on a river network remains difficult. Challenges arise because of variations in fire and watershed characteristics, as well as post-fire rainfall events. These challenges are especially pronounced in deciphering the response of step-pool streams in steep mountain terrains. Because step-pool systems are stable features that respond to extreme events with recurrence intervals often exceeding 50 years, opportunities for documenting post-fire responses are rare. Temporal windows for observing post-fire effects are typically short, extending just a few years after fire. This study quantifies the response of step-pool streams within Pike National Forest on the Front Range of Colorado following the 2012 Waldo Canyon Fire. After the burn, before significant channel changes occurred, baseline data were collected in channel reaches representing various degrees of burn within the Williams Canyon and Camp Creek watersheds, as well as in neighboring reference streams. Terrestrial LiDAR scanning and field surveys provided longitudinal profiles and cross sections of stream channels and enabled quantification of erosion and deposition. Pebble counts and sediment samples enabled assessment of sediment caliber. Analysis of benthic macroinvertebrates further provided indication of ecological quality. Repeat measurements following major storm events over 2013 and 2014 showed the recovery of burned channels in relation to burn severity and the presence of the step-pool structure. Sites burned with high severity experienced greater channel changes including a reduction in the percentages of sensitive taxa. Channels able to retain the step-pool morphology facilitated recovery more readily, showing a trajectory toward reference values in both physical and biological characteristics. Step habitats maintained higher percentages of sensitive macroinvertebrate taxa through post-fire floods. These results suggest insights into the resilience of step-pool streams disturbed by wildfires. Nevertheless, longer-term data are needed to document the continuing responses more completely toward effective management of post-fire riverine ecosystems.