THE EFFECTS OF CATCHMENT MORPHOLOGY ON THE TRANSFER OF ICE AND SEDIMENT FROM ICE CAPS TO ICY DEBRIS FANS AND VALLEY GLACIERS IN DEGLACIATING ALPINE ENVIRONMENTS

Decoupling of icecaps from valley glaciers exposes bedrock catchments that convey ice and lithic debris to icy debris fans. This project characterizes the influence of catchment morphology on the nature/pace of ice/sediment delivery to icy debris fans in the Wrangell Mountains of Alaska (McCarthy Creek Glacier) and the Southern Alps of New Zealand (Douglas, La Perouse, Mueller Glaciers). Repeat Terrestrial Laser Scanning surveys between 2013 and 2015 characterize the morphology of the catchments, fans, and glaciers, enabling quantification of changes in surface area and volume. Integration of these data with time-lapse imagery documents the pace and volume of avalanches, slushflows, icy debris flows, and rockfalls.

At the McCarthy Creek Glacier, small icy debris fans occur below catchments with significant morphologic complexity (large space for temporary ice/sediment storage) and little icecap surface area contributing to the catchment. A larger fan has less complex catchment morphology (little space for temporary ice-sediment storage) and more icecap surface area. For example, an ~888-m-long catchment with abundant storage space and ~40,000 m² of icecap supply face has a fan volume of ~3,125,000 m³, whereas an ~686-m-long catchment with less storage space and ~95,000 m² of icecap supply face has a fan volume of ~24,660,000 m³. Between 2013 and 2015, smaller fans decreased in volume by up to 1,200,000m³, whereas the large fan increased in volume by 700,000 m³. These changes reflect the smaller volume of ice/sediment supplied to the smaller fans compared with the larger volume of ice/sediment deposited on the larger fan, documented by time-lapse imagery of depositional events.

Fan volumes are also affected by deflation of valley glaciers. Icy debris fans along the lateral margin of LaPerouse Glacier, formed below long catchments (> 900 m) with little storage space, volume increases of ~191,000 to 397,000 m³, coeval with an average 5 m deflation of the valley glacier. As the glacier deflated, fans lengthened and fan volume increased at a rate compatible with the volume of ice/sediment added to fans, documented by time-lapse imagery.

All valley glaciers studied are deflating while associated icy debris fans exhibit a range of dynamic behavior linked to variations in icecap supply and catchment morphology.