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

The Missionary Ridge Fire started on June 9, 2002 approximately 10 miles northeast of Durango, Colorado. Over the next 40 days the fire consumed about 80,000 acres and destroyed nearly 100 homes and buildings. Debris flows triggered by thunderstorms after the fire caused damage to homes, blocked transportation routes, and contaminated community water sources.

Geologic mapping that was done in part of the burn area prior to the Missionary Ridge Fire identified Quaternary debris fans and zones of mass movement in the Hermosa and Vallecito Reservoir 7.5-minute quadrangles. Mapping also constrained the distribution and extent of various bedrock types and surficial deposits in relation to topography, thus identifying zones that were most prone to disaster from debris flows and surface failure. Many of the ancient debris fans identified by mapping were the sites of the most extensive post-fire flooding and debris flows in the summer of 2002.

Field studies and mapping done after the Missionary Ridge Fire led to the discovery of several ancient debris fans in the vicinity of Vallecito Reservoir that were incised by the recent flows and floods. Charcoal layers and sediment layers exposed in these channels revealed a regional history of fire-induced debris flows that extends back to about 4000 years with a frequency of 300 to 500 years. These results provide a better understanding of the relationship of ancient wildfire events and Holocene geologic processes in southwestern Colorado.

Geologic mapping and related field studies in southwestern Colorado have provided connections between past and present geologic events and natural disasters, and given a community insight into how geology influences and controls their lives. This information is beneficial to homeowners, developers, and county planners in identifying hazardous areas that can be avoided or mitigated before disaster occurs. Geologic mapping has given our local community a tool to understand these processes and to predict future events.