RECONSTRUCTION OF THE FIRE HISTORY OF THE WESTERN SAN JUAN MOUNTAINS, COLORADO: PRELIMINARY RESULTS FROM UPLAND CHANNEL AND ALLUVIAL FAN STRATIGRAPHY

In 2002, the Missionary Ridge Fire burned several low-order tributary watersheds along three main valleys of the western San Juan Mountains near Durango, Colorado. Post-fire debris flows and flooding incised tributary channels and alluvial fan deposits, thus creating exposures of alluvial stratigraphy. Alluvial sediments have been sampled from several exposures in upland tributary channels and alluvial fans. Stratigraphic units range from well-sorted and fine-grained deposits, to cobble-sized fluvial deposits and poorly-sorted debris flow deposits. When stratigraphic units contain charcoal in an appropriate context, they are used to interpret past fire events in the contributing basin. Low-severity fire events are typically represented by well-sorted fine-grained deposits, while high-severity fires are represented by charcoal-rich debris flow deposits. In this study area, charcoal is preserved in lenses of fine-grained sediment within debris flow or coarse fluvial deposits. It has also been observed at the boundaries of overlying debris flow deposits, possibly representing burned soil surfaces. Dates from possible fire-related sedimentation events preserved in terraces within upland tributary channels range between 800 – 1,100 ¹⁴C years before present. In one case, a preserved terrace dates to ~ 2,200 ¹⁴C years before present. In the alluvial fan setting, individual deposits range between 300 – 2,500 ¹⁴C years before present, though ages are typically less than 1,000 ¹⁴C years before present. The range of stratigraphic units in each exposure, as well as among exposures, suggests a post-fire geomorphic response associated with both low and high severity fires, though the dominance of one type of historical fire regime is not yet clear. Tree-ring records from three watersheds have been collected, and will be analyzed for fire regime characteristics over the past 300 years.