The Nines Creek Ice and Rock Avalanche: an Example of the Impact of Climate Change on Catastrophic Geomorphic Processes in the Kluane Ranges, Yukon Territory, Canada

Detailed field investigations of an exceptionally large ice and rock avalanche deposit were undertaken to characterise the cause, behaviour, and impacts of the failure. Distinguishing features of the deposit include a general paucity of matrix material, a high concentration of large boulders along the periphery and at the toe of the deposit, and virtually ubiquitous perched clasts and boulders. The failure had an exceptionally long runout distance (H/L= 0.31), transporting several boulders larger than 50 m3 a distance of up to 1.8 km. The largest intact boulder at the toe of the fan was 250 m3. Plough marks made by boulders and large ice blocks that have since melted are traceable for nearly 40 m in aerial photographs. Arcuate push ridges, developed in the terminal zone, show evidence of both shearing and folding of the pre-existing soil. Calving of a hanging glacier initiated the avalanche. A portion of the glacier detached and dropped down a 200 m high near-vertical basalt cliff, entraining massive blocks of bedrock in the process. Below the cliff, the ice fragments and rock debris travelled 1100 m over a valley glacier to a 165 m wide constriction in the valley bottom. The mass then travelled up to 600 m further, spreading out over the low-angled glacier fore-field with an average slope of 10 degrees. The deposit covers an area totalling 0.4 km2. The event was likely seismically triggered by a M 5.2 earthquake that occurred in June 1995, during the same week as the failure. Repeated aerial photography of the site, however, indicates that glacial recession and thinning had taken place for at least five decades prior to the failure and is likely an important pre-condition for the initial ice collapse. The results of this study distinguish the deposit from other types of debris accumulations, and highlight a rarely documented climate change related glacier hazard with potential to damage infrastructure in the northern Cordillera.