USING LICHENOMETRY TO EVALUATE ROCKFALL RECURRENCE INTERVAL AND TALUS FAN BEHAVIOR, GLENWOOD CANYON, COLORADO

Lichenometry, a Quaternary age dating technique that uses lichen sizes to estimate surface age, is used to evaluate rockfall recurrence interval in Glenwood Canyon, CO. Glenwood Canyon hosts a stretch of Interstate-70, which has a history of severe rockfall problems. Previous rockfall incidents in Glenwood Canyon have resulted in injuries, fatalities, and lengthy road closures for making repairs. Rockfall recurrence interval has the potential to aid in prediction of future events, allowing mitigation to be completed proactively, rather than reactively. This research seeks to estimate recurrence intervals for individual talus fans, to evaluate talus accretionary behavior, and to compare talus behavior across multiple fans. Calibration curves were locally prepared for two lichen species, Lecidea Atrobrunnea and Lecanora Novomexicana. In total, 654 L. Atrobrunnea and 595 L. Novomexicana lichens were measured on boulders larger than 0.5m on the long axis from six talus fans: three in Glenwood Canyon and three in a connected canyon along Grizzly Creek. Only largest lichens were measured. GPS coordinates, boulder size, and lithology were also collected for each boulder on which lichens were measured. Lichen sizes were converted to dates using the calibration curves, and the dates were used to estimate recurrence intervals for each fan. Lichen dates were also plotted spatially to observe accretionary trends. Observations during data collection and processing suggest that the accuracy of lichenometry used to evaluate rockfall recurrence is limited by unpredictable lichen colonization and inheritance of lichens that began growing on boulders before they fell from the source zone. Calibration is also difficult in the vicinity of Glenwood Canyon because of the small number of lichens available for calibration on local surfaces of known age, the relatively short history of settled human habitation, and the limited number of previous geochronology studies in the region. These problems must be resolved for effective application of lichenometry to rockfall recurrence studies. Therefore, several potential solutions are considered, including calibration curve matching, modeling of growth and mortality in whole lichen communities, and evaluation of lichen areal coverage.