Tag: Bridge glacier retreat

Bridge Glacier, Southeast Alaska Retreat & Lake Formation

On By mspeltoIn alaska glacier retreat

Bridge Glacier in Landsat image from 1984 and Sentinel image from 2017. The red arrow indicates the 1984 terminus where no lake exists, yellow arrow is 2017 terminus, orange arrows are selected tributaries and purple dots the snowline. 

Bridge Glacier drains the same icefield as the Wright and Speel Glacier 45 km southeast of Juneau, Alaska. Here we examine the changes in this glacier from 1984-2017 using Landsat and Sentinel imagery.

In 1984 the glacier ended on an outwash plain at the head of a branch of Speel River. The red arrow indicates the 1984 terminus for each image, the yellow arrow the 2017 terminus and the orange arrows three tributaries feeding the glacier.  The purple dots indicate the snowline at 1200 m. In 1984 all three tributary glaciers fed Bridge Glacier and the glacier has no proglacial lake at the terminus. In 1997 a lake basin is beginning to develop, though it is still largely filled by ice. The eastern tributary pink arrow, has lost all of its snowpack. The three tributaries at the orange arrows are connected to Bridge Glacier still and the snowline is at 1250 m. In 1999 the proglacial lake has formed and has length of 1 km, the lake has expanded south and north of the 1984 terminus position, and does not entirely represent glacier retreat.  In 2013 the glacier has retreated 1200 m from the 1984 position and the lake is still expanding. The orange arrows indicate that none of the three tributaries are still connected to the main glacier. The glacier in a sense is losing its income flow from these subsidiaries. The eastern tributary has retained some snowcover with six weeks left in the melt season in 2013, but this is mostly gone a month later, the snowline is at 1100 m. nbsp; Total retreat from 1984 to 2017 is 1900 m. In 2017 the snowline is at 1300 m, and the separation of the tributaries is by more than 500 m in each case. The snowline has been high by the end of each summer from 2014-2017 indicating retreat will continue. The retreat of this glacier is the same story as seen at nearby Patterson, Gilkey and Norris Glacier.

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Bridge Glacier in Landsat images from 1997 and 2013. The red arrow indicates the 1984 terminus , yellow arrow is 2017 terminus, orange arrows are selected tributaries and purple dots the snowline. 

Bridge Glacier in USGS map when it ended on the outwash plain in 1948.

 

Bridge Glacier Terminus Collapse, BC, 4 km retreat 1985-2016

On By mspeltoIn Canada glacier retreat1 Comment

Bridge Glacier comparison in 1985 and 2016 Landsat Images.  Red arrow is the 1985 terminus, yellow arrow the 2016 terminus and purple arrows indicate locations where tributaries have separated between the two dates. 

Bridge Glacier is an 17 km long outlet glacier of the Lillooet Icefield in British Columbia. The glacier ends in a rapidly expanding glacial lake and had an observed retreat rate of 30 m per year from 1981-2005 by Allen and Smith (2007). They examined the dendrolchronology of Holocene advances of the glacier and found up to 2005 a 3.3 kilometer advance from the primary terminal moraine band, with the most extensive advances being early in the Little Ice Age.  Chernos (2016)  indicates that the glacier in 2013 is approaching the upglacier end of the lake, which will lead to reduced retreat rates.  Here we compare Landsat imagery from 1985 to 2016 to determine response.

In 1985 the proglacial lake was 2.5 km long and 3.5 km upglacier of the terminus a major tributary joins.  The transient snow line is 2100 m. By 1993 the glacier has retreated 200-300 m and the snowline was at 2150 m.  By 2004 the terminus in a Google Earth image the terminus had retreated 1100 m since 1985.  By 2004 the tributary from the north has separated from the north side of the glacier.There are also some evident areas where the proglacial lake is visible up to 800 m upglacier of the terminus.  This suggests imminent collapse of this section of the terminus, which is afloat.  Matt Chernos researching this glacier documents this well with images.  Chernos (2016) observed that calving due to greater water depth and terminus buoyancy was key to retreat, but that most volume loss stemmed from melting.   In 2016 the terminus has retreated beyond the former junction of the Bridge Glacier and the northern tributary.  The glacier terminus is now within 500 m of a slope increase, likely marking the end of the developing lake basin. The total retreat in 31 years has been 4.1 km, this is a rate of 130 m/year, much faster than before. The 3 km retreat from 2004 to 2016 indicates a retreat of 250 m/year.  The separation of the three tributaries, purple arrows are not impacted by calving and indicate melting alone is sufficient to drive significant retreat.  The enhanced melt is also the cause of the high snowlines,, in 2016 the snowline is at 2150 m. The retreat is faster than nearby Klippi and Jacobsen Glacier, but both of those are also retreating fast.

This continued retreat and area loss will lead to glacier runoff decline in summer. This is crucial to the large Bridge River Hydro complex. This complex managed by BC Hydro can produce 490 MW of power, which is 6-8% of Province demand. Stahl et al (2008) note in their modeling study of the glacier that ,”The model results revealed that Bridge Glacier is significantly out of equilibrium with the current climate, and even when a continuation of current climate is assumed, the glacier decreases in area by 20% over the next 50 to100 years. This retreat is accompanied by a similar decreasein summer streamflow.”   Lillooet News (2016) notes that BC Hydro has commissioned research on the glacier to investigate impact on runoff tiiming. This parallels our findings on the Skykomish River in the North Cascades, Washington Pelto (2011).  The change in timing and the hydropower also impact salmon with late summer runs of chinook and fall coho runs.

Bridge Glacier comparison in 1993 Landsat Image.  Red arrow is the 1985 terminus, yellow arrow the 2016 terminus.

2005 Google Earth image of Bridge Glacier, note tributary separation from the north.

Closeup of terminus indicating exposures of proglacial lake upglacier of the terminus.Bridge Glacier Retreat Acceleration, BC, Canada