An Assessment of Rockfall Activity using Dendrogeomorphology: The Amphitheater, Ouray, Colorado, USA
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A U.S. Forest Service campground and adjacent hiking trails are subjected to rock-fall activity from a nearby bedrock escarpment. The campground is situated in a glacial-carved valley of the Ouray, Colorado, Amphitheater in the San Juan Mountains. A dendrogeomorphic assessment of the trees and steep slopes surrounding campground investigate the past and present stability conditions to answer the question: What are the spatial and temporal characteristics of rock fall? More specifically, for this area, where spatially is the slope subjected to rockfall activity and what are the magnitude and frequency of events? Also, will reconstructing a rockfall chronology using a dendrogeomorphic methodology of Abies concolor (White fir) help identify potential triggering mechanisms? To answer the research questions, the following two objectives were established for this study: (i) delineate the spatial patterns of rockfall activity, and (ii) establish a temporal chronology of rockfall activity. The spatial extent of rock-fall activity is assessed by mapping areas of talus accumulations and trees exhibiting visual evidence of rock-fall activity. Visible rock-fall induced tree disturbances observed in the study area include: impact scars, tree decapitation, curved or leaning trees, and elimination of neighboring trees or ‘broken crown’. The area with the greatest hazard to rockfall activity is the area located below the gullies of the overlying cliffs and steep slopes, the scree slopes below the base of the cliff, and for ~70 m into the forest from the upper protective forest boundary and ~100 m from the base of the cliff. ‘Centurion’ trees are the most likely to record a growth defect as result of rockfall activity. The sampling of selected trees with an increment borer allowed for microscopic analysis of the variations of tree-ring width measurements as a trees response to rock-fall induced growth defects (GD) (reaction wood, suppression and release, and cellular scaring/callus tissue). The initiation, duration, and termination of the growth defects (GD) allowed for the reconstruction of a rock-fall chronology. This reconstruction alluded to the observation of rockfall activity being dominated by low magnitude, high frequency events, and interpreted as being attributed to annual freeze and thaw activity. For the time period of investigation (1910-2015), no large mass movement event occurred within the study area. Although there were no observations of a large-scale mass movement event, several years recorded abnormally high rockfall rates, as determined from the reconstructed rockfall chronology. Years with abnormal rockfall rates are a result of some additional external factor ‘promoting’ and/or ‘triggering’ the rockfall event.
Reed, John Christopher (2016). An Assessment of Rockfall Activity using Dendrogeomorphology: The Amphitheater, Ouray, Colorado, USA. Master's thesis, Texas A & M University. Available electronically from