RELATIONSHIPS BETWEEN WILDFIRES AND ALLUVIAL FAN FORMATION IN THE UINTA BASIN, NORTHEASTERN UTAH

Streams on the southern margin of the Uinta Basin in northeastern Utah incise into the lacustrine Eocene Uinta Formation. In places where the bedrock is resistant to erosion, steep tributary valleys have formed, and are currently depositing alluvial fans onto the floodplain surface of the mainstem valleys. On the South Unit of the Ashley National Forest, we identified 36 fans in Indian Canyon and Sowers Canyon whose distal margins have been incised by the mainstem streams. Description of the sediments exposed in three of the fans indicates that debris flow and hyperconcentrated flow deposits are the dominant fan-forming process in this setting.

Commonly, debris flow deposits are found immediately overlying burn horizons and wildfire-related charcoal fragments, suggesting a strong causal relationship between fire frequency and fan formation. Radiocarbon dating of charcoal deposits from three fans provides chronologic control on the timing and rates of sediment deposition. Radiocarbon dates of 1930 ± 40 ¹⁴C yr BP (BETA-232604) and 460 ± 40 ¹⁴C yr BP (BETA-232603) retrieved from charcoal-stained alluvial sediment in fan LIC-4 suggests that over 5.5 m of debris flow sediment has accumulated on the fan in the past ~2000 calendar years, and as much as 4 m of sediment accumulated in the past ~500 calendar years. Similar radiocarbon dates of 1570 ± 40 ¹⁴C yr BP (BETA-232607) from RIC-1 and 280 ± 40 ¹⁴C yr BP (BETA-232609) from RIC-2 indicate that as much as 2.7 m of sediment has accumulated on RIC-1 in the past ~1440 calendar years and as much as 2.9 m of sediment has accumulated on RIC-2 in the past 380 calendar years. These data are significant for land-management in this setting, considering that the current development plan for natural gas resources on National Forest land is preferentially locating drilling platforms on alluvial fan surfaces. A total of 20 radiocarbon dates have been accumulated from five fans; over half of these dates cluster into a time period from 1000 to 300 ¹⁴C yr BP. This suggests that climate variability exerts control on fire regime, which is then reflected in debris flow frequency. The period of frequent fires documented by these data corresponds to a period of smaller than modern bankfull floods previously documented in the adjacent Uinta Mountains (Carson et al., 2007).