Introduction
This is the second of a three-part series of blog posts examining the impact of climate change on the evolution of Bridge Glacier. In part two, we take a closer look at what has been happening at the glacier over the last decade. In the first post, we established that since the end of the Little Ice Age , Bridge Glacier has been steadily retreating, and the rate of retreat has intensified exponentially over the last 70 years. The loss of so much ice mass has changed the morphology of the glacier and the forefield, and the magnitude of these changes will likely continue to increase for the foreseeable future. In order to better understand the current ramifications of this retreat on the evolution of downstream water resources and the calving dynamics of the glacier, here we zoom in and take closer look at what has happened at Bridge Glacier and its proglacial lake over a smaller timescale, the last decade.
Bridge the Calving Glacier
Bridge Glacier is a freshwater-terminating, lake-calving glacier. As previously mentioned, the reduced stress on the buoyant terminus floating in the lake generally leads to more rapid retreat, as compared to land-terminating glaciers undergoing the same change in climate (Benn et al., 2007). In some cases, even during positive accumulation years, it is still possible for these glaciers to lose significant volume due to calving (Chernos et al., 2016). Understanding the retreat rate due to calving is important to conceptualizing overall mass balance and freshwater contribution to downstream water bodies (Dyurgerov & Meier, 2005). Quantifying the rapid growth of proglacial lakes due to calving and retreat is essential, as a rapid increase in volume can heighten the risk of glacial lake outburst floods (GLOFs) hazards downstream (Kellerer-Pirklbauer et al., 2021).
Past studies of lake-calving glaciers suggest that the rate of retreat due to calving is contingent on the general flow speed of the ice, water depths in the lake, and width of the valley.These factors can vary dramatically over shortperiods so it is important to observe them over short time scales.Increased depth to lake bottom, more rapid flow of ice, and wider valley width all increase the rate of calving of a glacier and as these factors generally equilibrate following astep pattern themagnitude of change can be quite large over a short period of time. (Chernos et al., 2016; Motyka et al., 2003; Trüssel et al., 2013).
Landsat Imagery GIF of Bridge Glacier from 1984-2021
Chernos and co-authors focused on ablation from both calving and surface melt at Bridge Glacier, which were captured using time-lapse photography, ablation stakes and automated weather stations (AWS) during the summer of 2013 (Chernos et al., 2016). The meteorological data were fed into a Distributed Energy Balance Model (DEBM) to assess the melt rate by calculating the ingoing and going radiation at different elevation points along the glacier. The total ablation due to calving was quantified by measuring the retreat and flow speed at the terminus of the glacier where it entered the lake. The total volume of ice mass loss was based on the change in glacier length and the ice thickness, which they derived from the lake bathymetry and the height of the ice cliff at the terminus. This study indicated that calving contributed to about 23% of total ablation in 2013, but was not likely a significant source of ice mass loss until 1991, when retreat intensified in general after the separation of the terminus into the northern land-terminating section and the southern lake-terminating one (Chernos et. al 2016). Based on observations from other lake-calving glaciers, and the observed morphological changes in the proglacial lake, Chernos et al. (2016) concluded that Bridge Glacier will likely reach a point in the next few decades where the southern terminus will exit the lake and ablation from calving will no longer be a significant source of mass loss, potentially resulting in a stable terminus.
A study of lake-calving glaciers in Austria found that the eventual detachment of the glaciers from the proglacial lakes caused a decrease in backwasting (i.e. lateral retreat of ice or ice-cored slopes) (Haidong et al., 2010; Kellerer-Pirklbauer et al., 2021). As of the summer of 2021, Bridge Lake was over 7 km long, an expansion of approx. 2 km since 2013. This is fairly staggering as between 1985-2016 Bridge Glacier had retreated only 4.1 km , 3km of which was between 2004-2016 (Pelto, 2017). It is impossible to know without further bathymetric measurements if calving has reached a maxima, and the glacier terminus is grounded. Evidence from aerial imagery, however, shows a significant decrease in the number and size of icebergs in the lake over the last ten years, indicating that the mass loss from calving has likely been decreasing over this time.
Landsat Imagery showing decreasing number of icebergs
While there are several factors that control ablation at a calving terminus, at a grounded terminus ablation and glacier retreat is primarily controlled by the climate (i.e. surface melt due to increasing temperature or decreasing precipitation). This suggests that when Bridge Glacier becomes grounded and land terminating, any further retreat will become more directly predicted by climate change (Boyce et al. 2007; Chernos et al. 2016). The eventual detachment of Bridge Glacier from the proglacial lake will likely decrease the rate of overall mass loss, but the impact on the downstream hydrological systems , and up glacier valley slope stability has not been investigated and little is known on how this will impact the surrounding ecosystems and communities nearby. It will be critical to attempt and quantify how the transition from lake-terminating to land-terminating will spur on morphological changes at Bridge Glacier ( Kellerer-Pirklbauer et al., 2021).
Impacts of Bridge Glacier Retreat on Discharge into the Bridge River
It is generally accepted that with glacial retreat, runoff increases until the glacier has either completely wasted or reaches a new glacio-climatic equilibrium. A new equilibrium is reached when the glacier’s area decreases to a point where runoff generated decreases in proportion to the shrinking glacier area (i.e., a smaller glacier will generate less runoff than a larger one). This likely will decrease flow in subsidiary glacially-fed streams (Janson et al., 2003; Moore et al., 2009). This is particularly true in late summer, when the seasonal snowpack is no longer contributing to runoff (Moore et al., 2020; Braun et al., 2000).
Moyer and co-authors looked at downstream discharge variability from Bridge Glacier between 1979-2014, using historical hydrometric data from the Water Survey of Canada, weather data from Environment Canada stations nearby, and Landsat imagery of Bridge Glacier, to assess the change in discharge from the lake over time. They found that during winter months runoff from the lake downstream increased due ice and snow displacing water in the lake. In accordance with predicted trends discharge in the late summer decreased over time (not found for early summer). This was attributed purely to the change in the glacier area (i.e. the meltwater was proportional to the area of the glacier if it had not retreated) (Moyer et al., 2016). They predicted that as Bridge Glacier transitioned from a calving glacier to a land terminating glacier the rate at which summer discharge would decrease could be impacted (Moyer et al. 2016).
Winter discharge increased throughout the study due ice and snow displacing water in the lake, but since 2014 snow and ice present in the lake has decreased and the volume of the lake has increased, which could cause the winter discharge to decrease. That said there have been no studies of the winter discharge at Bridge glacier since 2014, so it is hard to make a definitive conclusion without further investigation.
Changes in discharge can have ramifications for downstream ecosystems, soil quality, biodiversity, community water supplies, and drinking water quality (Laurent et al., 2020). In order to assess these potential impacts it is crucial to get further predicted changes in discharge due to glacier retreat.
In conclusion, over the last ten years both the calving rate and calving contribution to total melt and discharge from the lake at Bridge Glacier have been altered by climate-induced retreat of the glacier. These changes all have the potential to cause significant harm to downstream communities and ecosystems and it will be critical to conduct further assessment of these ramifications. In our final post, we will look at the continuation of these trends into the 2021 field season, in the hopes of gaining new insights into how these changes are impacting the glacier forefield, as well as downstream communities and ecosystems.
