Introduction
This is the third of a three-part blog series examining the impact of climate change on the evolution of Bridge Glacier. 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. In the second, we took a closer look at what has been happening at Bridge Glacier over the last ten years, learning that 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. In part three we will be looking at current activities at Bridge Glacier and their utility in better understanding the hydrological, and morphological changes at the glacier site as well as the future of Bridge glacier and its downstream communities.
Summary of the 2021 Field Season
The Climate and Cryosphere Lab’s 2021 field season consisted of two trips, the first on July 10- 15 and the second on September 11-15. The July trip consisted of placing two time-lapse cameras, one with full view of the calving front, and the other with a view of the stream flow into the lake on the land terminating side in order to observe changes in streamflow over the summer. In addition, six ‘RaspberryShake’ seismometers were placed at various points along the glacier as an experimental technique used to assess the sediment movement from the sub-surface of the glacier along with taking multiple acoustic hydrophone recordings of the water flowing from the glacier. Three dilution gauging tests were completed at the stream at the edge of the grounded terminus of the glacier to assess the flows. The September trip consisted of collecting the time-lapse cameras and seismometers as well as completing a Ground Penetrating Radar (GPR) of a transect of Bridge Glacier in order to assess the current thickness. Two additional stream dilution tests were also completed in order to gain insight into the seasonal streamflow changes.
Seismometer Installation
It has been established that as Bridge Glacier and numerous other glaciers in its vicinity retreat there will be hydrological consequences. But what hasn’t been discussed as much is understanding the changes in sediment output from the glacier and its impact on downstream communities. The mechanisms governing how sediment is released from retreating glaciers, for the most part, are poorly understood (Beaud et al., 2014 ). Seismometers were installed as a potential way to quantify this sediment transport. Seismic waves are propagated through Earth’s material due to generated elastic energy due to an applied force (Oldenburg et al., 2017). In this case, the sediment movement below the glacier was hypothesized to create the described energy impulse.
The technique consisted of placing seismometers at various distances from the glacier in order to profile how sediment is moving. This is currently the only way to quantitatively hypothesize how sediment is being transported below the glacier (Beaud et al. 2014; Beaud et al., 2018a; Beaud et al., 2018b; Delaney et al., 2017; Glasser & Bennett, 2004). The quantity, size, and speed of the sediment can be correlated with the seismic signal (Bartholomaus et al., 2015; Gimbert et al., 2016; Vore et al., 2019). Seismic data was successfully collected from five of the six seismometers and is currently being processed and analyzed to better understand how the glacier’s retreat is directly related to changes in sediment flux.
Time-Lapse Cameras
The time-lapse cameras were placed with a view of the calving front and the second placed in front of the glacial stream. This was done in order to assess how both the calving front and stream changed from July-September. The consistent loss of ice from the calving front can be observed throughout the summer along with the drop in the streamflow. The temperatures during this July time period were unseasonably warm which likely contributed to the high stream flow.
Dilution Gauging
Salt dilution gauging was completed during both trips in order to assess flow in the glacial stream and how this progressively changed seasonally and daily. This was accomplished using a salt injection method. 2kg of salt was injected upstream and two “Quik Quak conductivity probes” were placed in the stream to measure the change in conductivity as the salt was transported downstream. The depth of the stream was estimated using a water level logger. With this information, the change in conductivity over time and the profile of the stream bed could be integrated to calculate the flow. This was done at 8:00, 12:00, and 16:00 during a single day on the July trip and around 10:00 and 12:00 on two separate days of the September trip. The data from the dilution gauging is still being processed in order to better conceptualized flow throughout the summer.
Acoustic Recording
Recordings using a hydrophone were taken while installing the seismometers. These recordings were recently used as part of Michelle Koppes’ and Susie Ibara’s exhibitions at the TED Countdown conference in Edinburgh in October 2021. Recordings will also likely be paired with the seismic data during the same time frame to better conceptualize how sediment flux is changing with flows.
Ground Penetrating Radar
Ground Penetrating Radar (GPR) was used to access the thickness of a transect of Bridge Glacier. This was done during the September trip using a ‘PulseEkko GPR’ mounted on skis so it could be dragged along a transect of the glacier. GPR uses an oscillating radio wave pulse that penetrates through the ice but is reflected off of the bedrock surface. This occurs because ice has a relatively low dielectric permittivity (ε-the polarization of material due to an applied electric field). This means that the radio wave is more easily transmitted compared to the rocky surface where the relatively higher dielectric permittivity causes the wave to be reflected back up to the surface (Oldenburg et al., 2017). Based on the return of the reflected waves to the surface the thickness of the ice can be estimated.
Implications
The 2021 field season at Bridge Glacier was an overall success. The majority of the data is still being processed, so there are no tangible conclusions to be drawn from it as of now (stay up to date with publications here). That being said, there is valuable insight to gain from observations during the field work. Based on discussions with local pilots and previous experience of lab members at Bridge Glacier, the marketed retreat of the glacier over a very short period of time was validated. The July trip was just after a large heat wave throughout the lower mainland, and evidence of this could be observed in the high streamflows that did seem to taper a bit throughout the trip. It also should be noted that both trips had to be navigated around British Columbia’s wildfire season which could impact further research at Bridge Glacier. It will be critical to apply knowledge gained through the 2021 field season at Bridge Glacier in order to better tailor methods for future projects and to understand implications of changes at glaciers for downstream areas.