USING DENDROGEOMORPHOLOGY TO STUDY THE DONALD DUCK CURVE MASS MOVEMENT COMPLEX ON GRAND MESA, CO: A PRELIMINARY ASSESSMENT

Grand Mesa exhibits a dynamic environment with various features from mass movement along its flanks that create complex problems for people. The area is underlain by rocks that are moderately to highly susceptible to slope failure or have moderate to high incidences of landslides. Rockfall, slump, Earth-flow deposits, and landslides occur beneath cliffs of basalt that overlay weak Tertiary rocks. Although these features can be observed on aerial photographs or in the field, we lack a time frame for these movements. Concepts of magnitude and frequency require for these processes require dating these events. This study investigates the timing of movement on one landslide on the south flank of Grand Mesa. This movement is the most important one because it is the main pathway for a huge volume of sediment that has been mobilized. Trees on the movement surface were studied using dendrogeomorphological methods. The direction and degree of tilting of trees as well as the deformation of trunks was compared to trees growing on adjacent stable surfaces. To determine when landslide moved, tree-ring cores from tilted and damaged trees on the landslide were examined. Signs of disturbance in the tree-ring cores indicate when landslide movement occurred. Disturbances are displayed as marked reduction in the widths of annual rings and/or the formation of reaction wood. The tree-ring cores suggest multiple periods of landslide movement during the past 100 years. The temporal occurrences of the reactivations of mass movement were estimated using a visual analysis of growth anomalies on trees growing on the landslide. Specific time periods are associated with intense past sliding as a result of increased rainfall that subsequently increased soil moisture/saturation.