Streamflow Response During Rapid Retreat of a Lake-Calving Mountain Glacier

English Abstract:

There has been increasing attention over the last decade to the potential effects of recent and projected future glacier retreat on downstream discharge and aquatic habitat. Of particular interest is the timing of "peak water," the time at which the reduction in ice area associated with retreat begins to offset the increased rate of ice melt associated with climatic warming. This study examines the streamflow variability downstream of Bridge Glacier, located in the southern Coast Mountains in British Columbia. The glacier currently terminates in and calves icebergs into a proglacial lake, and has been retreating rapidly since 1991, when the terminus retreated into an overdeepened basin. The glacier's area has decreased from 92 km² in the early 1990s to 81 km²in 2013. Despite this decrease in area, there has not been any clear reduction in late-summer streamflow. The objective of this study was to diagnose the relative contributions of snowmelt, ice melt, and off-glacier runoff to streamflow. Snow and ice melt were estimated using a semi-distributed melt model, supported by snow-covered area determined from MODIS imagery and the area of the glacier as determined from Landsat imagery. Surface melting from icebergs was computed using a melt model and the area of the lake surface occupied by icebergs, as determined by linear spectral unmixing of Landsat and MODIS imagery. Sub-aqueous iceberg melt was estimated by assuming that net radiation received by the portion of the lake occupied by icebergs was consumed by sub-aqueous melt rather than heating of the water column, based on results of fieldwork conducted in 2013. Rainfall-runoff from both glacierized and non-glacial portions of the catchment were estimated using a semi-distributed hydrologic model. The results from this study contribute to our understanding of streamflow response for retreating valley glaciers, many of which will likely experience a transient lake-terminating phase as the terminus retreats into over-deepened basins.