Landslide Frequencies and Logging on Vancouver Island: An Analog Showing Varied yet Signifi cant Changes

Despite a long history of landslide research in British Columbia, there remain few data on the actual change in rates of landslides following harvesting activities other than from the Queen Charlotte Islands and the Clayoquot Sound region of Vancouver Island. The application of these data to other areas in British Columbia is problematic. This paper discusses implications of the results of a study of three watersheds on Vancouver Island: Macktush Creek, Artlish River, and Nahwitti River watersheds. Some 363 landslides, from 0.02 to > 1 ha, were identifi ed in three watersheds from air photographs, beginning at a date that essentially preceded logging up to the present. Landslide frequencies increased in Macktush Creek, Artlish River, and Nahwitti River by approximately 11, 3, and 16 times, respectively. Two to 13 times more landslides reached streams following logging; most of these were between 0.2 and 1 ha. Landslide density analyses produced variable results, ranging from 2.4 to 24 times increases in number of landslides. Road landslide frequencies increased by 27, 12, and 94 times for Macktush, Artlish, and Nahwitti, respectively. Landslide frequencies need to be determined for many more watersheds to provide better information on the effects of logging activities on both Vancouver Island and the Interior of British Columbia. It is against this baseline that geoscientists and geotechnical engineers practising in the forest sector can measure their successes at reducing the impact of landslides in British Columbia.