A comparison of post-wildfire geomorphic response over annual and millennial time scales

Abstract:

Wildfires have profound, highly variable impacts on erosion, sediment transport, and stream channel morphology. Climate change and fuel management actions have altered the current fire regime relative to the historic fire regime. Many researchers have quantified post-fire geomorphic response immediately following events and also over millennial timescales using geochronologic techniques and field study. While these studies have informed our understanding of the post-fire geomorphic response during the dry and wet periods of the Holocene, there are still some fundamental questions about long-term landscape erosion that we have yet to answer, particularly in fire-dominated landscapes.

The Hayman fire burned 55,700 hectares in Pikes Peak National Forest, Colorado in 2002. Hillslope- and small watershed-scale sediment yield data were previously collected for 5 to 7 years in areas burned by high severity immediately after the fire. Plot data from 33 hillslope-scale plots indicate an average of 0.8 mm per year of erosion. Wildfires are common to this area and there is observable evidence of extreme geomorphic response following historic fires, similar to what has been monitored after the Hayman fire. In this study, we collected samples of channel alluvium deposited since the Hayman fire and sediment from alluvial fans thought to have been deposited in association with wildfires that may have occurred pre-European settlement. Samples were used to measure in-situ ¹⁰Be to estimate millennial-scale, catchment-averaged denudation rates that were compared to the erosion rates measured from the hillslope and watershed-scale plots to determine the proportion of erosion that is generated post-wildfire relative to undisturbed periods. Material from older alluvial fans was collected to try to evaluate whether denudation rates have changed and the ages of the older alluvial fans were determined using radiocarbon or optically-stimulated luminescence.