Research

Postfire basin response and the history of wildfires and debris flows near Vallecito Reservoir, Colorado

Ancient alluvial deposits were exposed at several locations in channels incised by debris flows that occurred 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 till. This older alluvium also contains layers and pieces of charcoal. These accumulations of charcoal are generally associated with deposits that are indicative of debris flows and sediment laden floods.

A detailed investigation of the fluvial facies and stratigraphy of old deposits exposed in a wash west of Lake Vallecito Country Market at Vallecito Reservoir has established a pattern of debris-flow dominated, fire-related sedimentation. Combined with ¹⁴C ages of the charcoal layers these data provide evidence that old debris flow deposits were triggered by wildfires that occurred in the region over the past 3600 years. They also indicate that fire-related erosion and sedimentation constitutes an effective mechanism for landscape evolution in the study area. This investigation also indicates that over that time period the basin's hydrologic response to wildfire has gradually changed to produce increasingly fine-grained deposits. This change in response is best explained by a decrease in the amount of coarse material available for erosion within the channel.

Charcoal samples were also collected from a similar deposit near Lemon Reservoir. The ages of these samples combined with the ages of samples from the Country Market basin provide a record of fires and debris flows that extends back to 4300 BP. The cycles of fire-related sedimentation in the Durango area correlate with periods of drought in the nearby La Plata Mountains and the sequence of fire-related sedimentation established in Yellowstone National Park. This suggests a climatic control for wildfires and debris flow events in the western United States. Results from this study provide a better understanding of the relationship between ancient wildfire events and Holocene geologic processes in the region.

The complete paper can be downloaded as a PDF here.

Frechette, J. D., Gonzales, D. A., Kenny, R., Thompson, J. R., 2003, Evidence for a connection between wildfires, erosion, and landscape development over the past 3600 years in southwestern Colorado: Geological Society of America Abstracts with Programs, v. 35, no. 5, p. 36.

Gonzales, D. A., Starr, D., Frechette, J., Dorin, F., Costello, K., Cullicott, C., Kolody, R., Remley, K., Graham, K., 2004, Geologic map of Electra Lake 7.5-minute quadrangle, La Plata County, Colorado: Colorado Geological Survey OF03-21, scale 1:24,000, 1 sheet, 29p. text.

Gonzales, D. A., Matthews, V. III, and Frechette, J. D., 2003, Insight into a record of fire and related debris flows in southwestern Colorado: Using geologic mapping and field studies to understand the past and to predict future events: Geological Society of America Abstracts with Programs, v. 35, no. 6 (in press).