MORPHOLOGY, DEPOSITIONAL PACE, AND ICE VOLUME CONTRIBUTION OF ICY DEBRIS FANS TO MCCARTHY CREEK GLACIER, WRANGELL MOUNTAINS, ALASKA (2013-2015): AN INTEGRATED APPROACH USING FIELD OBSERVATIONS, TIME-LAPSE AND DRONE IMAGERY, GROUND PENETRATING RADAR, AND TERRESTRIAL LASER SCANNING

Integration of field observations, drone and time-lapse imagery, ground-penetrating radar (GPR) and terrestrial laser scanning (TLS) surveys over the past decade quantify the nature, pace, and volume of ice-dominated depositional processes on icy debris fans at McCarthy Creek Glacier in the Wrangell Mountains, Alaska. Icy debris fans (IDF) are supra-glacial landforms that form at the mouths of catchments along bedrock escarpments where valley glaciers have decoupled from icecaps during deglaciation. Time-lapse imagery recorded >400 depositional events on three IDFs between June 2013–June 2015; including ice avalanches (69%), slushflows (30%) and icy debris flows and rockfalls (<1%). Field observations noted hundreds of deposits trapped temporarily within catchments weekly. Only large events were recorded on fans by cameras. Dominant process varies between fans depending on catchment size, icecap terminus surface area, and connectivity between icecap and fan apex. Debris flows only occurred in larger catchments where previously deposited avalanche and rockfall material (visible in drone images) was remobilized by floods emanating from the icecap base.

Depositional events occurred throughout the year. Icy debris fans are exceptionally dynamic compared to alluvial fans, with resurfacing rates between 299–1,035% during the two year period. TLS-calibrated time-lapse imagery documented deposition of >2,000,000 m³ (mostly ice), ranging from ~ 3–34% of fan volumes. During this time, repeat TLS surveys showed variable response by the IDFs owing to catchment differences and icecap supply; East Fan grew by 3% while Middle and West Fans decreased by 22% and 25%, respectively. Although ice ablates from IDFs in summer months, significant ice is transferred to the valley glacier by surface and subsurface flow within IDFs. Estimates of glacier volume from integrated TLS-GPR data indicate that IDFs contribute 2.5%–5% of the volume of McCarthy Glacier annually. TLS surveys showed only minor thinning of McCarthy Glacier (< 1 m/yr) compared to significant thinning observed at three New Zealand IDF study sites (~5–10 m/yr). Supply of ice to valley glaciers via IDFs represents an important component of annual ice budgets for glaciers that are decoupled from high-level icecaps, slowing deglaciation of valley glaciers.