THE EROSIONAL IMPACT OF GRAZING COMPARED TO SEVERE BLOWDOWN IN A COLORADO SUBALPINE WATERSHED AS RECORDED IN LAKE SEDIMENTS
Between approximately 1920 and 1930 MARs increase more than 40%. For the next 40 years the average MAR decreases slightly until the mid-1960s when MARs drastically decrease until the 1980s when they appear to gradually approach a background rate through present time, a decrease of greater than 65%. These results suggest that grazing resulted in accelerated erosion while mass tree uprooting resulted in little to no accelerated erosion. The decrease in MARs that begins in the mid-1960s is approximately concurrent with the designation of the Mt. Zirkel Wilderness Area. There were not roads in the watershed and no evidence of logging at that time so it may represent a decrease or cessation of grazing, though grazing was still allowed following wilderness designation. If there is another factor contributing to or causing the decrease in MARs it may have subdued or erased any accelerated erosion that resulted from the 1997 blowdown. Analysis of historical grazing records and lake sediment organic and inorganic matter concentrations will be used to further refine and describe the influence of human and natural disturbances on the watershed erosion record.