Results

In general, the debris cover thickens down-glacier from the uppermost stake, 12, and towards the sides of the glacier as one would expect from headwall sources and englacial transport. Mass wasting from the large “Little Ice Age” lateral moraines also contributes to the debris along the lateral margins (Lundstrom, 1992). Along the centerline, ablation rates decrease down-glacier and laterally from the centerline. At the clean ice the ablation rate was 3.81 m yr-1 and decreased to 1.23 m yr-1 at the (B) profile, 30 cm debris layer, to 0.31 m yr-1, 150 m down glacier from (B) where the debris is 47 cm thick.

Since initial measurements in 1940, the glacier has retreated and thinned. The lower (A) profile, which once spanned the glacier, now spans the valley floor (possibly stagnant ice) 350 m down-valley of the terminus (Figure 1). If stagnant ice exists it is covered by > 2 m of debris as we discovered when attempting to dig to the ice surface. Unfortunately, this area was not included in our GPR survey. At the (B) profile, we estimate the 1901 surface elevation from historic photographs (H.F. Reid) at about 2053 m suggesting local ice thickness was ~105 m (Figure 2). The current glacier elevation at (B) is ~2000 m and is remarkably close to that in 1940, suggesting a local ice thickness of 52 m. From 1982 to 2004 the glacier thinned 15-30 m (average rate = 1.0 m yr-1), returning to its 1940 elevation.

Figure 2

Figure 2. Elevation profiles on Eliot Glacier. Both plots represent no vertical exaggeration.

In general, surface velocities decrease down-glacier and laterally away from the centerline, as expected. Velocities were highest at the uppermost stake (Stake 12) with 1.17 ± 0.06 m of displacement over the six-week study period (2.8 ± 0.1 cm dy-1) and 7.52 ± 0.03 m over the 350-day study period (7.85 ± 0.03 m yr-1). Velocities decreased to near zero at the lowest boulders (boulders 1, 2, 1A, and 2A in Fig. 1).

In 1901 the area of Eliot Glacier was 2.03 ± 0.16 km2 and decreased to 1.60 ± 0.05 km2 by 2004 (-19%) resulting in a terminus retreat of 680 m (Figure 3). During this 103-year period, glacier area decreased until 1956 when it increased until the early 1970s and started to decrease again (Figure 4). The most pronounced shrinkage has occurred since 1995 with a loss of 0.14 km2 from 1995 to 2004. In comparison, the other six glaciers on Mount Hood exhibit similar patterns, retreating through the first half of the 1900s, advancing or at least slowing their retreat dramatically in the 1960s and 1970s, and then retreating again. Proportionally, Coe Glacier lost the least area, 15%, while White River Glacier lost the most, 61%. The ice-covered area of the seven glaciers lost 34% (Table 1).

Figure 3

Figure 3. Map of areal change of seven of Mount Hood's glaciers over the twentieth century.

Figure 4

Figure 4. a) Area change with time for individual glaciers; b) fractional area change.

Table 1

Table 1. Area and length change on the seven Mount Hood glaciers examined. *1901 for Eliot Glacier rather than 1907.