Geological Survey Professional Paper 705—A Inventory of Glaciers in the North Cascades, Washington

Measurements by K. B. Bengtson revealed that the terminus of the Coleman Glacier on Mount Baker began to advance in 1949 (Harrison, 1961). Prior to this, all North Cascades glaciers had been rapidly retreating for several decades, as is clearly demonstrated by extensive barren recently exposed ground near the lower margins of the glaciers. Hubley (1956) reported that 37 of 54 glaciers observed in the North Cascades between 1953 and 1955 were definitely advancing.

Since 1955 most of the glaciers that Hubley observed have maintained terminal positions fairly close to those of 1955; two, the Coleman and Roosevelt, have made significant further advances. The Deming, Boulder, Inspiration, and Boston Glaciers have made small gains, as have three others on Glacier Peak. Four less active glaciers (Lynch, White Chuck, South Cascade, and "Banded") have continued to retreat, and six stagnant, relict ice masses have undergone considerable losses. Aside from the last two groups, most of the glaciers of the North Cascades appear to have been in equilibrium with climatic conditions during the past decades.

Several glaciers demonstrate rather remarkable flow properties. Most unusual of these is "Spillway" Glacier (No. 2214-3). Every 2 to 4 years most of this small glacier becomes unstable and avalanches down a steep slope. These movements, however, are not regarded as true glacier surges.

Crevassed, steeply pitched hanging glaciers, common in the North Cascades, present unusual hazards as large avalanches may occur at any time. A spectacular ice avalanche occurs frequently on Johannesberg Mountain where the small hanging glacier (No. 2262-13) has spilled ice over an area as large as 0.5 km².

A more comprehensive analysis of recent glacier changes in the North Cascades will appear in another hydrologic report.

An important effect of glaciers on streamflow in the North Cascades can be shown by comparing run off from glacierized and nonglacierized drainage basins during years of contrasting climate (Tangborn, 1968). The greatest combined snow and ice melt in the mountains occurs during July and August (Meier, 1969) when precipitation is generally at a minimum. Because the annual snowpack has largely melted by the end of July and streamflow from this source is much reduced, the most critical months for Pacific Northwest water users are August and September. During these months the runoff from glaciers has the greatest significance.

Two contrasting years were 1964 and 1966. In 1964, the winter snowpack was above average, the summer was wet and cool, and ice ablation on glaciers was low. The glaciers that year gained in mass. In 1966, weather conditions were just the opposite, and nearly all North Cascades glaciers lost mass.

In table 3 the contribution of glaciers to runoff during August and September of 1964 and 1966 is given for a west-slope basin (Thunder Creek) and an east-slope basin (Stehekin River). A nonglacierized basin (South Fork Nooksack River) on the west slope is shown for comparison.

TABLE 3.—Volumes of August and September runoff, in million cubic meters, at selected river basins in the North Cascades, 1964 and 1966

[One million cubic meters equals 811 acre-feet]

In 1964, runoff from glaciers was low for two reasons: the cool wet weather reduced the heat input by solar radiation and eddy convection, and the persisting reflective cover of the previous winter's snow on glaciers further reduced the efficiency of melting by solar radiation. The melting of glacier ice contributed only 13 percent of the August and September flow of Thunder Creek and 5 percent of the Stehekin River discharge. The source of most of the runoff was from precipitation and the melting of the abnormally heavy snowpack.

During the autumn of 1966 the seasonal snowpack was nearly depleted, and what little snow remained was restricted to small high-altitude patches and to the snow remaining in the accumulation areas of glaciers. Late-summer base flow and rainfall runoff in nonglacierized basins was very low—only one quarter of the 1964 amount in the South Fork Nooksack River basin. However, the melting of glacier ice was very effective in sustaining the flow of both Thunder Creek and the Stehekin River. For Thunder Creek basin, total runoff during August and September was actually greater in 1966 than in 1964. Glacier melt contributed an estimated 34 percent of Thunder Creek discharge and 27 percent of the flow in the Stehekin River.

The effect of all glaciers on streamflow in the North Cascades can be estimated. Snow- and ice-melt data obtained during more than a decade of research at South Cascade Glacier show that the average yearly ablation is about 3.5 m of water. An average figure for all the glaciers is somewhat less than this because South Cascade Glacier lies at a lower altitude than most of the North Cascade glaciers. Assuming an average annual ablation of 3 m, the glaciers in the North Cascades, which cover an area of 267 km², contribute annually about 800 million cubic meters (650,000 acre-feet) of water to streamflow in Washington. When it is considered that nearly two-thirds of this water is released during the warmest part of summer and that the greatest ice melt occurs during years that are abnormally dry, the importance of glaciers in the North Cascades is evident. A more detailed analysis of the contribution of glaciers to streamflow in the North Cascades will appear in another hydrologic report.