Muldrow Glacier Motion

Bradford Washburn

For A GREAT MANY years there have been all sorts of conjectures as to the rate of movement of Muldrow Glacier, which flows off the northeast flank of Mount McKinley. The rate at which any glacier moves is highly dependent on the spot where the measurements are made. Glaciers may also move faster in the summer than in the winter, so the date and place of observations of glacier-motion are very important.

While climbing McKinley in the spring of 1947, we measured the flow of Muldrow Glacier in the middle of the Great Icefall at an altitude of 8500 feet (2590 meters). On May 10, 1947 the rate was 15 inches per day. This is likely to represent rather close to the annual average (about 450 feet per year) at that spot.

While we were mapping the lower part of Muldrow in 1976 and 1977, we made a number of motion observations from our permanent survey station above McGonagall Pass. This station is about 300 yards east of the pass and 250 feet higher (5958 feet, 1819 meters) on the rounded and level shoulder leading eastward to Oastler Mountain. It is marked by Boston Museum of Science bronze disc No. 15 set in the top of a prominent rock on the crest of the ridge. This is an ideal location for measuring Muldrow’s motion, for, because the glacier veers slightly to the south at this spot, one can set up an instrument on one side of the glacier and yet look up it, with the ice mass moving almost directly toward the station.

We installed a large white canvas tarp about nine feet square nearly in the middle of the glacier, about two miles above the pass in August 1976. The tarp was weighted down by rocks placed all around its edges, with a number of smaller rocks scattered on its surface to prevent it from flapping in the wind. On August 10, 1976, using a Keuffel & Esser Laser Ranger loaned to us for the purpose, and a triprism set on the southeast corner of the tarp, we measured the distance from the pass as 13,126 feet.

On June 16, 1977 we set up again at the same place, relocated the target and remeasured the distance to it: 12,985 feet. These figures yielded a movement of 141 feet in 310 days—average rate 5.5 inches per day.

We also made another measurement on June 30, 1977 which resulted in a distance of 12,977 feet: 96 inches in 14 days. The rate of movement during this summer period was 6.86 inches per day. It had been suggested that we might encounter faster motion in the summer than the annual average and these figures seemed to suggest that this may be true.

The observations of 1976 and 1977 were made by our Boston Museum of Science party which was in the process of making a new large- scale map from Wonder Lake to McGonagall Pass—and also making a very precise new map of the entire lower valley of Muldrow Glacier from Gun sight Pass to the glacier’s terminus. The first of these two maps was published in the American Alpine Journal of 1980. The second is presently in the final stages of production and it is hoped that its five large sheets will be published by the Museum and the Swiss Foundation for Alpine Research of Zurich in 1983.

Last summer (1981) the National Outdoor Leadership School made another one of its many ascents of Mount McKinley via McGonagall Pass and Muldrow. We were delighted to find them willing to make a new distance-sight on our target in order to learn what had happened in the last four years. Mr. Herbert Ogden did this work for us on June 15, 1981—this time using a K & E Uniranger instrument, which is much lighter and more portable than the 36-lb. Ranger V used before. It weighed only 20 pounds, but used the same 12-volt DC power supply.

The distance this time was 12,223 feet. Compared with the 1977 distance, made 1445 days before, the movement of the glacier was 754 feet in 3.96 years: 180.4 feet per year and 6.24 inches per day. Two sights made 11 hours apart on June 15, 1981 revealed 0.34 feet (4.08 inches) of movement in slightly less than half a day. This movement translated into .371 inches per hour or 8.90 inches per day.

Although these observations were not at sufficiently frequent intervals to yield really interesting data about summer vs. winter ice flow or variations in flow between years, they do provide interesting basic factual data about Muldrow’s movement. We are very grateful for the help of NOLS and the personnel of our 1976 and 1977 parties who helped in making these distance sights, and we hope that a small “expedition” can be organized in the not-too-far future to take similar precise electronic distance-measuring equipment into the field with two objectives: first, to secure continuous hourly movement observations of this part of Muldrow (night and day) for at least two or three days; second, to move this equipment up-glacier four miles to the level shelf slightly south of the top of Gunsight Mountain where a comfortable camp can easily be set up—and similar observations made of the upper reaches of Muldrow to ascertain hourly and daily movement in the Great Icefall (8500 feet, 2591 meters) and the Lower Icefall (7300 feet, 2225 meters).

Muldrow Glacier is one of Alaska’s largest valley glaciers and its gathering-grounds lie far up on Mount McKinley’s northern slopes, so it is fed by a very substantial ice-reservoir. Its flow, however, may easily be complicated, as its course from Gunsight Pass to its big bend three miles west of Anderson Pass lies squarely atop a very active geologic fault, movement of which may easily accelerate ice flow from time to time. In fact, a major surge in the winter of 1957 resulted in incredible deformation of Muldrow’s surface for several years, a dramatic temporary advance of its terminus and a drop of almost 100 vertical meters in the surface-level of the glacier today, when compared with the conditions of 1956. Elevated lateral moraines and botanical evidence point to the occurrence of a surge of similar magnitude about 70 years ago. Muldrow is not only a famous glacier but one with extremely interesting behavior and further observations of it are certain to be of ever-increasing value as the years go by.