Mass balance of adjacent debris-covered and clean glacier ice in the North Cascades, Washington 

Mauri S. Pelto, Nichols College, Dudley MA 01571peltoms@nichols.edu

Columbia Glacier Terminus in 1995, debris cover on right

Abstract:  As part of the ongoing annual mass balance measurements on Lyman Glacier and Columbia Glacier, North Cascades, Washington, measurements were completed from 1986-1998 on adjacent clean and debris-covered sections of each glacier.  On Columbia Glacier annual ice ablation is 3.3 m w.e (water equivalent) for clean glacier ice and 2.3 m w.e. for debris-covered areas.  On Lyman Glacier from 1986 to 1999, average annual ablation on the clean glacier ice is 3.4 m and under the debris cover average annual ablation is 2.6 m w.e.  On both glaciers annual ablation is significantly reduced (25-30%) under the debris cover.  Late-summer ablation, after both the clean glacier ice and debris-covered ice have lost all snowcover, is reduced 30-40% under the debris cover.

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INTRODUCTION                                                                                                 Mazama Glacier Debris covered terminus

From 1984-1999 the North Cascade Glacier Climate Project has measured the annual mass balance of nine North Cascade glaciers (Pelto, 1996).  None of these glaciers is primarily debris-covered; however, the Lyman Glacier and Columbia Glacier both have small, but significant debris-covered areas near their termini.  To accurately determine mass balance ablation stakes were placed in both the debris covered and adjacent clean ice areas of the glacier.  Previous researchers have noted the ablation rate of adjacent clean and debris covered areas of a glacier (Nakawo and Young, 1981; Sturm et al., 1986; Rogerson et al., 1986); however, this study provides a long-term record of comparison of the influence of two different types of debris cover on ablation. 

COLUMBIA GLACIER

Both glaciers have retreated continuously during this century (Pelto, 1993).  Columiba Glacier retreated 70 m from 1975-1998.  On both glaciers the thinning rate has been prominent in the terminus region since 1984.   the debris-covered areas has developed notable increased vertical relief from the general glacier surface, since measurements began in 1984 on Columbia Glacier. 

Table 1 provides the basic geographic characteristics of each glacier.  Columbia Glacier is a south facing cirque glacier with a comparatively low slope for a North Cascade Glacier.  The glacier has the lowest mean elevation (1600 m) of any glacier over 0.5km² in the North Cascades.  This low mean elevation despite its southern exposure is due to the tremendous avalanching off of the 800 m high cirque walls on the east and west sides of the glacier and the radiational shading the cirque walls provide.  The avalanches yield a surprisingly limited amount of debris for the glacier surface.  Seldom is more than 10 % of the surface of the glacier in the ablation zone adjacent to two of the large avalanche fans, covered by debris.  Debris on the surface snowpack each year is limited to typically only a few rocks in each avalanche fan. 

Debris Cover Description

The debris cover of interest on Columbia Glacier is in the terminus region and contrasts sharply with the large angular fragments of the avalanche debris.  The debris cover is a fine-grained clay-sand mixture and covers the underlying glacier ice completely.  The extent of the debris cover is 250 m².  The average thickness of the debris cover has been measured in 1985, 1990 and 1998.  In 1985 and 1990 the mean thickness was 0.20-0.25 m.  By 1998 the mean thickness had increased slightly to 0.22-0.27 m.  More than 80% of the debris cover has a grain size diameter between 1 and 6 phi.

In 1985 and 1996 deposition of some of this debris was observed from englacial and/or subglacial streams that emerged at the glacier surface near the terminus.  The stream at that time yielded a sand-clay slurry that quickly covered the glacier surface to a depth of 5-15 cm over an area of 50 m².   The origin of the larger existing debris covered area is postulated to be from similar supraglacial discharges, since the grain size composition is the same.  When first observed in 1984 the debris-covered areas of the glacier had a 1-2 m of vertical relief from the adjacent clean glacier ice (Fig. 1).  Glacier ice was observed with active crevassing on both sides of the debris cover.  Since 1984, the debris cover has become largely detached from the glacier with rapid terminus and marginal retreat.  The debris-covered ice in 1998 is separating from the main glacier due to downwasting and retreat and has a relief of 10-12m from the adjacent clean glacier ice (Fig. 2).    The debris-covered area at first glance today looks like a well-developed ice cored terminal moraine; however, it was observed to have developed on the glacier itself not at the margin of the glacier.

Table 1.  Geographic Characteristics of Columbia Glacier and Lyman Glacier.  

LYMAN GLACIER

            Lyman Glacier has retreated continuously during this century.  From 1950-1998 Lyman Glacier retreated 512 m.  In the 1930's, a large rockfall fell from Chiwawa Peak onto the upper portion of Lyman Glacier.  This rockfall is evident in photographs of the glacier in 1940 by J.B. Richardson and in 1944 by W.A. Long both of the U.S. Forest Service.  The rockfall progression down glacier is visible in photographs of the glacier from 1958-1999 taken by the USGS.  In 1986 the debris-covered area of the glacier began to reach the glacier front (Fig. 3).  In 1999 the debris-covered area still dominates the southern half of the terminus, and has noticeably reduced the retreat rate on this side of the glacier (Fig. 4).  The rockfall traveled a distance of 430 m from the 1930’s to 1986, an average velocity of 6-7 m/a.  In 1986 the rockfall debris covered an area of 1200 m².  By 1999, the debris-covered area had been reduced, by calving into a lake at the terminus, to 700 m².

The Lyman Glacier debris-covered area developed no distinctive profile from the general glacier surface through 1967, during this interval the rockfall was in the accumulation zone.  From 1967-1986 the debris covered area moved into the ablation zone and developed 8-12 m of surface prominence from the clean glacier ice.

The debris-covered averages 0.15-0.25 m in thickness, but varies widely.  The debris consists mainly of coarse gravel and small iron-stained gneissic boulders.   The debris cover is 85% complete with clean glacier ice exposed in particular around the larger boulders. 

ABLATION MEASUREMENT

On each glacier several ablation stakes have been emplaced in adjacent clean glacier ice and debris-covered glacier ice for 15 years  (1984-1998) on Columbia Glacier and 13 years (1986-1998) on Lyman Glacier.   Ablation is measured on each glacier in early summer, mid-summer and again at the end of the ablation season.  Ablation stakes are typically redrilled during each field visits.  Table 2 documents the observed annual ablation at stakes in adjacent clean glacier ice and debris-covered glacier ice.  On Columbia glacier there are two stakes in the clean glacier ice and two in the debris covered area forming a 15 m wide square.  On Lyman Glacier there are two stakes in the clean glacier ice and two stakes in the debris-covered ice again forming a 15 m wide square. 

Surface relief of the debris-covered area on Columbia Glacier has increased from 1-2 m in 1984 to 10-12 m by 1998.  This indicates that during the intervening 14 years cumulative ablation totaled approximately 10 m more for the clean glacier ice area.  

From 1967-1986 the debris-covered area of Lyman Glacier 8-12 m of surface prominence from the clean glacier ice, indicating less 0.5m+ of reduced ablation annually under the debris cover.

Average winter accumulation is considerably less on the debris-covered area because of their vertical prominence.  On Columbia Glacier the debris-covered area is typically exposed by early July, and clean glacier ice not until middle to late July.  On Lyman Glacier, the debris-covered area is exposed by mid-July, and the clean glacier ice by late July. 

Ablation measurements from the initial visit in late July to the end of the summer indicate that the debris cover reduces ablation by 30-40% on both glaciers (Table 3).   The annual ablation reduction is less significant because of the longer period of exposure at the surface.  

The observed reduction in ablation is less than that observed by Rogerson et al (1986) in the Torngat Mountains, Labrador Canada ablation under the debris cover was reduced 65% compared to clean ice ablation.   The thickness of the debris cover on these two North Cascade glaciers is also moderate.  On the Piedmont terminus section of the Drift Glacier, Alaska an observed 1m+ fine grained debris cover eliminated ablation in an area that had previously experienced 5m/a of ablation (Sturm et al., 1986).

Columbia Glacier           Lyman Glacier

Table 2. Observed annual ablation in m w.e., on Columbia Glacier and Lyman Glacier, for adjacent clean glacier ice and debris covered glacier ice.  

IMPACT ON TERMINUS BEHAVIOR

The impact on terminus behavior is limited by the small areal extent of the debris cover on Lyman and Columbia Glacier.  In both cases the debris cover is limited to one side of the terminus.  In neither case has that side of the glacier had a substantially different rate of retreat, despite the reduced rate of thinning.  There are other glaciers in the region where extensive debris cover has noticeably influenced terminus behavior.  Mazama Glacier (Fig. 5) has a complete debris cover at the terminus.  This has prevented complete melting of the stagnant ice beyond the active terminus, remaining from the more advanced position of the Little Ice Age.  All of the other glaciers on Mt. Baker have lost this stagnant ice zone that was observed on each glacier by Austin Post in the 1940’s.  Total retreat from the Little Ice Age maximum is less on Mazama Glacier than any other Mt. Baker glacier.

Carbon Glacier on Mt. Rainier, Washington retreated 900 m in the last century compared to more than 1600 m for all other Mt. Rainier glaciers (Driedger, 1986).  Carbon Glacier was also the last glacier on Mt. Rainier to begin to recede after the 1950-1980 period of glacier advance (Driedger, 1986).  The comparatively slow and smaller retreat, is in part due to the extensive debris-cover on the terminus of the Carbon Glacier.  The headwall of the Carbon Glacier experienced a major rockfall in 1916, which is the source of much of the surface rock cover on the glacier (Driedger, 1986).

CONCLUSIONS

In the North Cascades it is evident that a complete debris cover in excess of 0.20 m in thickness mid-late summer ablation rates by 30-40%, however, such regions have a longer ablation season and overall annual ablation is reduced 25-30%.  Based on our observations the finer-grained debris cover on Columbia Glacier has a greater insulating capacity for the underlying ice.  On both glaciers the debris cover is small enough so that it cannot play a substantial role in altering glacier behavior. 

Long-term measurement of ablation on adjacent clean and debris-covered glacier ice in the North Cascades indicates a reduction in observed annual ablation of  0.8-1.0 m/a of water equivalent per year.  The mass balance impact of debris-cover is substantial and quantifies the potential impact on glaciers in the area that have a more substantial debris-cover.  This is the case on the Mazama Glacier, Mt. Baker and Carbon Glacier, Mt. Rainier. 

Year	Clean	Debris	Clean	Debris
1986	-2.9	-1.6	-3.0	-2.2
1988	-3.1	-1.7	-3.1	-1.7
1990	-2.8	-1.9	-2.6	-2.4
1992	-3.0	-1.9	-3.3	-2.0
1994	-2.8	-1.6	-2.8	-1.8
1998	-3.2	-2.1	-3.0	-2.3
Mean	-3.0	-1.8	-3.0	-2.1

 Table 3.  Late July-Late September ablation on Columbia and Lyman Glacier In m w.e., under a debris cover and for adjacent clean glacier ice.