Analysis/Discussion

The spatial and morphological changes of Eliot Glacier over the past 104 years are a reflection of the climate of Mount Hood. Average summer temperatures (five-year running averages) increased on Mount Hood from 5.6 °C in 1902 to 8.8 °C in 2002 (Daly and others, 1997) whereas no overall trend in winter precipitation is observed. From 1900 through 1940, summer temperatures warmed and winter precipitation was low generally resulting in glacier recession and thinning (Figure 4). From the 1950’s to 1970’s temperatures cooled and precipitation increased resulting in glacier advance or slowing of retreat. Since the middle 1970’s air temperature increased and precipitation decreased resulting in further recession and thinning.

Ablation rates have changed on the glacier. At the (B) profile the ablation rate was 1.95 m yr-1 in the early-1940s (Matthes and Phillips, 1943), dropping to 1.08 m yr-1 between 1940 and 1956 (Handewith, 1959), and increasing to 1.23 m yr-1 currently. Daily values of summer ablation near the (B) profile in 1988-89 were about 0.24 cm dy-1 (Lundstrom, 1992) while our data show about 0.33 cm dy-1. Average thinning rate from 1984-1989 over the debris-covered portion of the glacier was 0.8 m yr-1 (Lundstrom and others, 1993) whereas the current rate from 1989 to 2004 is 1 m yr-1. Mean monthly temperatures on Mount Hood show mean summer (July-September) temperatures during Lundstrom’s study were 9.6 °C while during ours were 11.5 °C. Clearly the summer air temperature has increased by almost 2 °C since the Lundstrom study yet the ablation rate increased by only about 0.1 cm dy-1. We hypothesize that as the debris cover thickens the insulating effects are increased, partly offsetting the ablation effects of atmospheric warming. A statistical examination of ablation rates with debris thickness and local temperature demonstrates that debris cover has a greater effect on ablation than does adiabatically-dependent summer temperature, with debris thickness explaining 64% of the variance in ablation rates. Additionally, regression analyses show significant effects on ablation by debris cover (R2=0.40, p=0.01), whereas effects by temperature are not significant (R2=0.23, p =0.09).

To estimate the rate of debris thickening over time we apply a one-dimensional continuity equation (Equation 2; Lundstrom and others, 1993). We assume no loss of debris and no direct contribution of debris through rock avalanches local to the ablation zone because no evidence for them exists and aeolian input is insignificant (Lundstrom, 1992). Results show that the strain thickening of the debris increases down-glacier and the rate of debris melt-out decreases. Together, these two processes compensate resulting in a spatially constant debris supply over the ablation zone of 5 mm yr-1. At the uppermost stake segments, where debris is ~6 cm thick, strain thickening accounts for roughly 7% of the thickening and melt-out accounts for the remaining 93%. At the lowermost stake segments, where the debris is ~70 cm thick, strain accounts for 82% of the debris thickening and melt-out contributes only 18%. Predictive estimates of debris thickness along the glacier’s centerline suggest 30 cm of debris at the B-Profile, close to the actual value of 32 cm. Overall, a correlation of 0.93 exists between field data and model results. Over the ~15 year interval between Lundstrom’s study and ours, we estimate the debris layer thickened by ~7.5 cm, which dramatically slows ablation rates for originally thin (~3 cm) debris covers and significantly slows thicker covers (~40 cm). Therefore we regard the thickening of debris to be an important factor in buffering the glacier mass balance response to climate warming.

Eliot Glacier continues to thin, however, and the increasing debris thickness only partly buffers the effects of climate warming. Consequently, we infer that the thinning rate would be greater without the debris cover. Because of the insulating effects of the debris cover we expect the glacier to respond more to changes in mass input to the glacier rather than to changes in mass loss through melting. We have seen evidence of a sensitive response to a period of positive mass balance that resulted in the initiation of a kinematic wave and thickening of the glacier. That the current surface elevation of the (B) profile is only now at the elevation of the pre-wave elevation in 1940 points to the reduced effect of ablation caused by the presence of the debris cover. However, the rate of debris thickening is not keeping pace with the rate of climate warming and the glacier is accelerating its retreat. It would be tempting to explain the relatively small shrinkage of Eliot and Coe glaciers compared to other glaciers on Mount Hood solely in terms of a thickening of the debris layer (Figure 4). However, other mitigating factors exist. Both Eliot and Coe have the highest accumulation zones which head near the peak of Mount Hood (3425 m). Therefore, rising freezing levels and snow lines have not affected these glaciers as much as the other glaciers, which have a smaller elevation range. Aspect is likely another factor, as Eliot and Coe are the most northerly-flowing glaciers on the mountain. These factors have also been documented on Mount Rainier (Nylen, 2004). It is worth mentioning Ladd Glacier’s large retreat despite its high debris cover and northwest aspect. A low accumulation area and a unique low slope near its terminus may have accelerated glacial loss. Overall, the changes exhibited on Mount Hood since 1901 are similar to glacier variations elsewhere in the American West (e.g. Marston and others, 1991; Key and others, 2002; Nylen, 2004; Granshaw and Fountain, 2006; Hoffman and others, 2006).