GEOCHRONOLOGY OF DEBRIS FLOWS NEAR BIGHORN RANGER STATION, ROCKY MOUNTAIN NATIONAL PARK, COLORADO

More than 10 large debris flows occurred in and near Rocky Mountain National Park (RMNP) following the September 2013 rainstorm in the Colorado Front Range. These debris flows delivered sediment to valley bottoms and damaged park infrastructure. One 2013 debris flow that occurred near the historic Bighorn Ranger Station deposited sediment on a pre-existing debris fan. This site was selected for detailed geochronologic study to determine the ages of old debris deposits and evaluate debris flow frequency. Relative age analysis included stratigraphic analysis of sediments in the debris fan and mapping of old levee crests. Numeric dating techniques included radiocarbon analysis of organic material and ¹⁰Be radionuclide analysis of boulders collected from four debris flow levees.

Mapping of debris flow levees and ¹⁰Be ages confirm that at least 2-3 and possibly more large levee-forming debris flows have occurred at this site within the last 10²-10³ years. The recurrence of debris flows at this site indicates that it may experience similar mass movements in the future. Minimum ¹⁰Be exposure ages (i.e. maximum age) of three old debris flow levees are 30.3 ka, 7.97 ka, and 74.6 ka. Ranges of exposure ages for these old deposits are 54.2, 143, and 121 ka, respectively. A boulder sample collected from the 2013 debris flow levee returned an 18.1 ka ¹⁰Be exposure age. The falsely old age of the 2013 sample and the wide range of ages determined for each of the older landforms indicate that exposure histories of debris deposits are complicated by inherited atmospheric exposure prior to debris flows. Radiocarbon ages of material collected from this site are on the order of 10⁰-10² years old and do not cluster according to the landform sampled.

The scatter of ages established for old debris flow deposits at the Bighorn site suggests that debris deposits continue to be modified by secondary processes after the initial event. The downslope pathway of debris flow material reflects highly individual histories from source to temporary residence on the hillslope to deposition near the valley bottom. The numerous debris flows that initiated in the 2013 storm exemplify ongoing debris flow hazards in the Colorado Front Range and highlight the need for awareness of hillslope hazards in this region.