In early-September a story broke regarding a so-called disappearance of a glacial river in northern British Columbia.
The purpose of this post is to show a bit of visual evidence of, and context for, what happened and why.
First, a few bits of information regarding Llewellyn Glacier, British Columbia:
Llewellyn Glacier is one of the largest glaciers in British Columbia at 458 km2 (as of 2011-08-03). It is an outlet glacier of the Juneau Icefield, which sits along the northern end of the Coast Mountains of Alaska and British Columbia (Fig. 1).
Figure 1: This 2001-08-15 Landsat ETM+ satellite image (panchromatic band, 15 m resolution) shows the Juneau Icefield with Llewellyn Glacier outlined in blue. Each gray grid cell is 10 km by 10 km. Downtown Juneau, Alaska is located at approximately 535000 E., 6462000 N. (information on the UTM geographic coordinate system here).
Llewellyn Glacier has been receding rapidly, particularly during the past two decades. Of the 53 outlet glaciers of the Juneau Icefield, Llewellyn Glacier has receded the most – in terms of surface area – in recent years (Beedle and Raup, 2008). From 1991 to 2011 Llewellyn Glacier shrunk from 471 km2 to 458 km2, a loss of 13 km2, or nearly 3% of the 1991 surface area (Fig. 2).
Figure 2: This 2011-08-03 Landsat TM satellite image (false-color composite of bands 4, 3, and 2 – vegetation appears red) shows the terminus of Llewellyn Glacier with outlines from 1991, 2001, and 2011 showing recent recession. Gray grid cells in this image are 1 km by 1 km.
Prior to late-summer 2011 two rivers drained from two lakes at the terminus of Llewellyn Glacier to Atlin Lake. One drained to the north into Llewellyn Inlet, and the other drained northeast into Sloko Inlet. Both inlets are portions of much larger Atlin Lake (Fig. 3).
Figure 3: This 2011-08-03 Landsat TM satellite image (false-color composite of bands 4, 3, and 2 – vegetation appears red) shows the terminus of Llewellyn Glacier, the two terminal lakes, the two rivers – one draining into Llewellyn Inlet and the other draining into Sloko Inlet, and a portion of Atlin Lake. Gray grid cells in this image are 1 km by 1 km.
On August 11th, at the end of their 2011 field season, Juneau Icefield Research Program (JIRP) students and scientists hiked down Llewellyn Glacier to Llewellyn Inlet. On a portion of the usually high-and-dry trail they found themselves wading through what was at times chest-deep water. In late-August, Atlin, British Columbia locals Diana Thayer, followed by John Lyons, noted the disappearance of the river that had previously drained into Sloko Inlet. The assessment of the cause of this rerouting of a significant amount of water by John Lyons after visiting the site and flying over the glacier (see John’s comments on the CBC story here), is the same that we show here (Fig. 4).
Figure 4: This image shows a Landsat TM satellite image from 2011-08-03 (left) and a Landsat ETM+ satellite image from 2011-09-12 (right), both freely available from the USGS Global Visualization Viewer. The black ‘striping’ in the ETM+ image is due to a malfunction that occurred on board the satellite in May 2003, see here for more information. The gray grid cells in this image are 1 km by 1 km. Numbers correspond to descriptions below. Names in italics refer to place names that do not have official names (to the best of our knowledge), and are used here to describe the events that occurred.
Sometime in early August the receding Llewellyn Glacier rerouted water that once flowed into Sloko Inlet. Refer to the image above for the following descriptions:
1. After August 3rd (Fig. 4, left image), but before August 11th, when JIRP students and scientists encountered abnormal water levels, the receding Llewellyn Glacier separated from a bedrock ridge, allowing water from “Lake 1” to flow into “Lake 2” through a previously-ice-dammed channel.
2. Subsequently, the water level of “Lake 1” dropped below the outlet into “Sloko Inlet River”, separating the river’s bed from its source. Note the large area that was once covered by waters of “Lake 1”, just below (south) of the number ‘2.’ in the image on the right.
3. “Sloko Inlet River” ‘disappears’.
4. “Llewellyn Inlet River”, now with the extra water diverted from “Lake 1” to “Lake 2”, swells and – in some areas – changes course. Note the area just above (north of) the number ‘4.’, where the river swelled significantly between the two images. This area is pictured in figures 5 and 6, below. Both images are taken facing south (roughly) – the terminus of Llewellyn Glacier visible in the background. Thanks to JIRP student/scientist Kent Walters for these images.
Figure 5: Juneau Icefield Research Program students and scientists wade along the flooded ‘trail’ between Llewellyn Glacier and Llewellyn Inlet. “As we hiked out there were plenty of trees which were flattened in the direction of water flow and also uprooted trees floating around in Llewellyn Inlet with roots completely in tact.” Photo and quote courtesy of Kent Walters.
Figure 6: Looking towards “Lake 2” and Llewellyn Glacier from the trail leading to Llewellyn Inlet. The flooded area in the foreground was previously dry. The primary “Llewellyn Inlet River” channel is on the left of this image. Photo courtesy of Kent Walters.
The rerouting of water away from “Sloko Inlet River” to “Llewellyn Inlet River” was the result of the recession of Llewellyn Glacier away from a bedrock ridge. The glacier previously served as a dam between “Lake 1” and “Lake 2”. Further recession may uncover lower-elevation bedrock, possibly resulting in a similar failing of the ice ‘dam’ and a sudden pulse of water into “Lake 2” and “Llewellyn River Inlet”. However, this will be determined by sub-glacial bedrock topography, behavior of Llewellyn Glacier, and the difference in elevation between the two lakes – if a difference remains after the flooding event of 2011. A reemergence of “Sloko Inlet River” is unlikely without an advance of Llewellyn Glacier, and a damming of the channel between “Lake 1” and “Lake 2”.
Links to additional information:
Beedle, M. J. and Raup, B., 2008, A GLIMS inventory of the Juneau Icefield, Alaska, IGS International Workshop on World Glacier Inventory, Lanzhou, China, Sept. 20-24, 2008.
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