FIRE-INDUCED DEGRADATION OF LATE QUATERNARY MORAINES, PINEDALE, WY

The degradation of Quaternary moraines is often modeled using a diffusion process approximated by finite-difference methods. By assuming initial topographic conditions, slope dependent erosion rates in the model are set to produce the modern topography. However, field observations of many late Quaternary landforms indicates that modern transport rates are slower than models predict, implying that much of the total sediment transfer would have happened rapidly after moraine deposition and prior to any stabilizing force like vegetation cover. In this study, we provide estimates of modern moraine erosion rates observed after a sagebrush fire eliminated the majority of vegetation on several late Quaternary aged moraines near Pinedale, Wyoming in 2009. Freshly exposed rock surfaces at the base of boulders embedded in the moraines suggests removal of the vegetative cover reduced the slope stability and led to a rapid increase in downslope sediment transport. Vertical measurements of this fresh rock exposure at 20 locations along a 35 m transect of a moraine averaged 19 mm with a range of 9 to 51 mm and a general trend of increased sediment flux on steeper slopes. Within two years, surface vegetation restablized the moraine slope, with no visible sign of sediment transport. Our results suggest that the sagebrush fires provide a mechanism for episodic moraine degradation consistent with the erosion rates assumed in diffusion models. However, the historical frequency of such fires is likely controlled by long-term climate changes that may yield periods of substantially increased and/or reduced sediment transport over time. Such variability in erosion rates may impose errors when using models to predict the exposure ages of boulders on these moraines.