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Streamflow response during the rapid retreat of a lake-calving mountain glacier Moyer, Alexis


There has been increasing attention over the last decade to the potential effects of glacier retreat on downstream discharge and aquatic habitat. Of particular interest is the timing of "peak water," when the reduction in ice area associated with retreat begins to offset the increased rate of climatic warming-induced melt. This study examines streamflow variability downstream of Bridge Glacier, in the southern Coast Mountains of BC. The glacier currently calves into a proglacial lake, and has been retreating rapidly since 1991, when the terminus retreated into an over-deepened basin. Despite the glacier's areal decrease from 92 km² in the early 1990s to 81 km² in 2014, interannual climatic variability has obscured any resulting reductions in late-summer streamflow. The objective of this study was to diagnose trends in streamflow, as associated with the accelerating retreat of a lake-calving glacier, examining the role of calving and retreat on the magnitude and timing of summer streamflow, as well as the persistence of icebergs in the basin. Snow melt, ice melt, and rainfall-runoff were estimated using a semi-distributed hydrological model, with glacier area determined from Landsat imagery. Two seasonal discharge trends were observed, an increase in winter flow attributed to ice discharge into the lake, and a decrease in late-summer flow attributed to decreasing glacier area behind the grounding line. Decreasing streamflow trends suggested that the glacier has passed peak water, and that streamflow will continue declining with reducing glacier area. Surface and subaqueous iceberg melt were computed using an energy-balance approach, assuming that net radiation received by the ice-proximal basin was consumed by subaqueous melt and heating of 0⁰C melt water. Fractional iceberg cover in the proximal basin was determined by spectral unmixing of Landsat imagery. Estimated melt volumes suggested that in the absence of large calving events, icebergs persist for roughly a year, with high fractional iceberg cover allowing for persistence into a second year. 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.

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