rainbow glacier ablation – From a Glaciers Perspective

Sholes Glacier snowcover extent change from 8-8-2022 to 10-17-2022. Snowcover declined from 98% of glacier to 10% of glacier during this period. Black dots are measurement sites, yellow dots the transient snowline, purple contour= 1.5 m, green contour= 2 m, brown contour= 2.5 m, and orange contour= 3 m snow depths on 8-8-2022.

The 2022 melt season for Moutn Baker glaciers was one for the record books, with a slow start and a prolonged intense melt lasting into Late October. Peak snowpack was not reached until May 20, 2022 at the Lyman Lake (1515 m) and Middle Fork Nooksack (1825 m) Snotel sites, with limited melt before June 1. These two sites have the highest correlation with our glacier mass balance observations (Pelto, 2018). Peak snowpack at Paradise, Mount Rainier (1565 m) was reached on May 26. The snowpack on June 1 at LL and MFN was 1.45 m Snow Water Equivalent (SWE) and 1.40 m SWE respectively. Snowpack at LL melted completely on July 14 and at MFN on July 11, with an average daily loss of 3.2 cm/day SWE. From June 1-Oct. 19 when the melt season ended, observed melt exceeded the previous highest years, we have observed during the 1984-2022 period. In 2023 peak snowpack was reached in mid-April at both LL and MFN, with rapid melt reducing snowpack during the first half of May.

Lyman Lake and Midde Fork Nooksack Snotel site snowpack depth in cm SWE observations beginning April 1 in 2022 and 2023. In 2022 May was a period of snowpack increase in 2022, while the first half of May 2023 has resulted in rapid snowpack depletion.

Heather Meadows snowpack depth (inches) at 1300 m, with 2022 rising above average during late April, while 2023 dips to average by the start of May.

On Sholes Glacier on August 6th-8th, 2022 we observed snow depths at 110 locations with an average snow depth of 2.25 m, 1.35 m SWE. We also checked two ablation stakes emplaced on June 1 indicating 3.55 m of snow melt, 2.1 m SWE. Sentinel images from Aug.8, Aug. 30, Sept. 9, Sept 27 and Oct. 17 reveal the recession of the snowline through the observation network allowing identification of snow ablation during these intervals. On Aug. 8, 98% of the glacier was snowcovered. On Aug 30, this had declined to 55%, with the snowline intersecting regions of the glacier that had 1.1 m SWE of snow cover on Aug. 8. By Sept. 9, the glacier was 40% snowcovered. On Sept. 27 the glacier was 25% snowcovered, with the snowline interseting sites that had 1.9 m SWE on Aug. 8. This is usually approximately the end of significant melt. However, in 2022 summer conditions continued through Oct. 19. The glacier was 10% snowcovered on Oct. 17, with the snowline intersecting sites that had 2.7 m SWE on Aug. 8.

The total observed snow melt for the June 1-Oct. 17 period was 4.8 m SWE on Sholes Glacier, eclipsing the previous June-end of melt season highs in 2015 of 4.0 m and in 2021 of 4.4 m. In both of those years the melt season did not extend into October, though May had significant melt. The Sholes Glacier did not suffer as much mass loss, because the initial snowpack was significantly greater in 2022. To have an equilibrium mass balance a glacier typically requires 55-65% of its area be snowcovered at the end of the melt season. A 10% snowcover indicates substantial mass losses.

On Rainbow Glacier on Mount Baker observations of snow depth on Aug 5-6, 2022 identified snow depths across the glacier. By Oct. 17th the areas of the glacier with 3.8 m or less of snowpack in early August had lost snowcover, indicating ablation of 2.4 m SWE of snowpack after early August. There was an area of exceptional snow algae at ~2100 m downwind of Dorr Steamfield on Rainbow Glacier, that Alia Khan’s Western Washington University research group examined. We led them through the Rainbow Icefall to this location.

On Easton Glacier, Mount Baker at 2500 m on Aug. 10th there was 5.25 m of snow remaining, compared to 2.75 m on September 27. At 2100 m there was 2.6 m of snowpack on August 10th with this snowpack melting completely between Sept,. 22 and Sept. 27. Indicating 1.6 m SWE of ablation during this period.

What 2022 illustrated is that a good winter season of accumulation, followed by a delayed melt season start, still cannot offset the persistent extended heat the region has experienced the last two summers. With the melt season off to a faster start in 2023 the outlook for Mount Baker glaciers is for another significant mass loss.

Snow depths on Rainbow Glacier on Aug 5-6, 2022.

Snow algae on Rainbow Glacier at 2100 m on Aug. 5th. Alia Khan’s WWU collecting samples.

Contrasting snow depth in crevasse in mid-August of 2020 and 2022 at 2500 m on Easton Glacier. Snow depths remaining on August 10, 2022 was ~5.25 m in 2022.

Snow depth at 2100 m on Aug 10th, 2022 on Easton Glacier

From 1984-2014 we have monitored the mass balance of the Rainbow Glacier on Mount Baker, North Cascade Range, Washington. This entails detailed monitoring of snowpack depth in July and August, and subsequent ablation to the end of the melt season. On July 13th the glacier was completely snowcovered. By August 10th the snowline had risen to 1575 m, with 1.4 m of ablation since July 13th. We measured the snowdepth at 85 locations on the glacier, with only 6 measurements exceeding 3.5 m on Aug. 10th.

Ben Pelto and Ashley Edwards examining crevasse stratigraphy both the 2013 and 2014 layers evident.

Limited snowpack below the main icefall at 1750 m on Aug. 10.

Minimal 2014 snowpack in crevasses at 1650 m on Aug. 10th 1.25 m. — Limited snowpack below the main icefall at 1750 m on Aug. 10.

By September 27th the snowline had risen to 1975 m with a few pockets of snow retained where the snowdepth had exceeded 3.2 m on Aug. 10th. Snowdepth at the Sept. 27th snowline had been 3.0 to 3.2 m on Aug. 10th. This indicated ablation of approximately 3 m of snowpack, 1.8 m of water equivalent from Aug. 10th to Sept. 27th. Ablation from July 13th to Sept. 27th was 5.3 m of snowpack and 3.2 m w.e. This was 5-10% more ablation than any other year since 1984. A comparison of images from the ground on Aug. 10th indicates the snowpack on the glacier from 1550-1950 m, the blue arrows indicate locations where a patch of 2014 snow remained on Sept. 27th. The Sept. 27th image was taken by Tom Hammond from Rainbow Ridge the only location where the whole glacier can be viewed. The firn is simply old snow that survived a summer and could be from 2012 or 2013.

Tom Hammond Image adusted to show firn, ice and retained snow. Sept. 27th

Winter snowpack was normal on Mount Baker, the record ablation then led to a large negative mass balance for the glacier of -1.8 m, but not a record loss. The ablation did lead to many significant surface streams on the glacier shown below, which drain into the glacier at moulins.

The terminus of glacier retreated rapidly from 1984 to 1998, but has slowed as it currently is in an avalanche runout zone. We visited the terminus in 1984 and all retreat is measured from that point when the glacier was in contact with an advance moraine from the 1955-1980 period of advance. A comparison of 1993 and 2006 Google Earth images indicates the retreat, red outline 2006 margin and black outline 1993. The 2014 image taken by Tom Hammond indicates that the terminus did get exposed in 2014 which will lead to additional retreat when we measure the terminus position next summer. Total retreat from 1984 to 2014 is 490 m.