EVIDENCE FOR A CONNECTION BETWEEN WILDFIRES, EROSION, AND LANDSCAPE DEVELOPMENT OVER THE PAST 3600 YEARS IN SOUTHWESTERN COLORADO

Ancient alluvial deposits were exposed at several locations in channels incised by debris flows that formed after the 2002 Missionary Ridge fire. The alluvial stratigraphy in these channels consists of layers and lenses of silt and sand that contain pebbles and boulders of bedrock and glacial debris. This older alluvium also contains layers and pieces of charcoal. Charcoal layers generally bound distinct cycles of alluvium that are indicative of debris flow and sheet flow deposits.

A detailed investigation of the fluvial facies and stratigraphy of older deposits exposed in a wash west of Country Market at Vallecito Reservoir has established a cyclic pattern of debris-flow dominated, fire-related sedimentation. Combined with ¹⁴C ages of the charcoal layers these data give evidence that old debris flow deposits were triggered by major fires that occurred in the region over the past 3600 years, and that fire-related erosion and sedimentation constitutes an effective mechanism for landscape evolution in the study area.

Similar deposits of alluvium and charcoal exposed near Lemon Reservoir and Animas River valley suggests a regional pattern of fire, erosion, and landscape development. The cycles of major fires in the Durango area also correlate well with the sequence established in Yellowstone National Park, suggesting a climatic control for wildfires and debris flow events in the West. Results from this study provide a better understanding of the relationship of ancient wildfire events and Holocene geologic processes in southwestern Colorado.