THE EROSIONAL IMPACT OF GRAZING COMPARED TO SEVERE BLOWDOWN IN A COLORADO SUBALPINE WATERSHED AS RECORDED IN LAKE SEDIMENTS

Subalpine lakes in designated wilderness areas can be excellent archives of accelerated watershed erosion caused by human disturbances due to their often remote locations and the cessation of most human disturbances following wilderness designation. When logging, grazing and road-building cease, a natural disturbance regime, which includes fire, insect, disease and blowdown (or windthrow), is the dominant driver of accelerated erosion. To compare and quantify accelerated erosion due to human and natural disturbances, a sediment core from a subalpine lake in the Mt. Zirkel Wilderness Area, Colorado, USA was analyzed. The lake resides in a small watershed (0.23 km²) that was grazed since at least 1926 (first permit issued) and saw 50-89% of trees uprooted by the 1997 Routt-Divide Blowdown in approximately 25% of its watershed area. Based on watershed characteristics it is highly likely that most grazing occurred in the same area as the majority of the tree uprooting. The sediment core was dated using the ²¹⁰Pb method and sediment mass accumulation rates (MARs) were calculated.

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.