Geological Survey Professional Paper 715—A Combined Ice and Water Balances of Gulkana and Wolverine Glaciers, Alaska, and South Cascade Glacier, Washington, 1965 and 1966 Hydrologic Years

ICE AND WATER BALANCES AT SELECTED GLACIERS IN THE UNITED STATES

COMBINED ICE AND WATER BALANCES OF GULKANA AND WOLVERINE GLACIERS, ALASKA AND SOUTH CASCADE GLACIER, WASHINGTON, 1965 AND 1966 HYDROLOGIC YEARS

By MARK F. MEIER, WENDELL V. TANGBORN, LAWRENCE R. MAYO, and AUSTIN POST

Glaciers occur in northwestern North America between lat 37° and 69° N. in two major mountain systems. The Pacific Mountain System, near the west coast, receives large amounts of precipitation, has very mild temperatures, and contains perhaps 90 percent of the glacier ice. The Rocky Mountain or Eastern System, on the other hand, receives nearly an order of magnitude less precipitation, has temperatures that range from subpolar to subtropic, and contains glaciers that are much smaller in both size and total area.

As a contribution to the International Hydrological Decade program on combined balances at selected glaciers, the U.S. Geological Survey is conducting studies of ice and water balance on four glaciers in the Pacific Mountain System: Wolverine and Gulkana Glaciers in Alaska, South Cascade Glacier in Washington, and Maclure Glacier in California. Similar data are being collected by other organizations at five glaciers in western Canada, including two in the Rocky Mountain System, and at one glacier in the Rocky Mountain System in northern Alaska.

Gulkana, Wolverine, South Cascade, and Maclure Glaciers have dissimilar mass balances, and each is fairly representative of the glaciers for its particular region. Gulkana Glacier (lat 63°15' N., Alaska Range, Alaska) normally has an equilibrium line at an altitude of 1,800 m (meters), an activity index of about 6 mm/m (millimeters per meter), a winter balance of about 1.0 m, and an annual exchange of about 2.2 m. (Balance values are given in terms of water-equivalent measure; the winter balance of 1 m, for example, indicates a volume of ice equal in mass to a volume of water 1 m in depth covering the area of the glacier.) The normal approximate parameters for the other glaciers studied are as follows: Wolverine Glacier (lat 60°24' N., Kenai Mountains, Alaska)—equilibrium-line altitude 1,200 m, activity index 9 mm/m, winter balance 2.5 m, and annual exchange 5.5 m; South Cascade Glacier (lat 48°22' N., North Cascades, Wash.)—equilibrium-line altitude 1,900 m, activity index 17 mm/m, winter balance 3.1 m, and annual exchange 6.6 m; and Maclure Glacier (lat 37°45' N., Sierra Nevada, Calif.)—equilibrium-line altitude 3,600 m, activity index 23 mm/m, winter balance 2.3 m, and annual exchange 4.6 m.

Mass balances of these four glaciers and their drainage basins are measured annually by standard glaciological techniques. In addition, the hydrologic balance is calculated using streamflow and precipitation measurements. Combining these independent measurements results in fairly well defined values of water and ice balance for the glaciers and drainage basins. A revision of the standard International Hydrological Decade mass-balance system permits combination of annual and stratigraphic terms.

The annual balance of South Cascade Glacier at the end of the 1965 hydrologic year was slightly positive (+0.07 m averaged over the glacier), but continued ablation and deficient accumulation in October 1965 resulted in slightly negative net balances for both the glacier and the drainage basin. Factors tending to produce this near-zero balance were the above-average late-winter balance (3.48 m) and the numerous summer snowfalls. Ice ablation averaged about 39 mm of water per day during the main melt season. Runoff during the summer ablation season was lower than the 1958-64 average.

The South Cascade Glacier annual balance in 1966 (-0.94 m) was considerably more negative mainly owing to the deficient winter snowpack (the late-winter balance was only 2.52 m) and the warm dry summer. Ice ablation averaged about 44 mm of water per day during the melt season. The loss in storage of this and other glaciers in the North Cascades increased the runoff of many valley streams by approximately 50 percent during August and September.

The 1966 Gulkana Glacier annual balance was slightly positive (+0.06 m); on the basis of past observations and the rapid terminus retreat of this glacier, this value is considered unusual. Accumulation (late-winter balance) was probably near the average of recent years. The ablation season was quite short, ending over most of the glacier on August 15; this factor alone probably accounts for the slightly positive balance. Ice ablation averaged approximately 26 mm of water per day during the melt season. Runoff from this glacier was not measured in 1966 but is estimated on the basis of ice-balance and precipitation data to be about 2 m, which is 5-10 times the amount measured in nearby lower altitude basins.

In the Kenai Mountains of Alaska, the 1966 Wolverine Glacier annual balance was negative (-0.26 m); the winter balance (1.83 m) was based on precipitation records at nearby Seward and was probably deficient. Ice ablation was heavy during the summer (about 50 mm of water per day average).

Collection of data on ice and water balance for Maclure Glacier began in late 1966 and will be given in subsequent reports.