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

This report has been compiled by utilizing aerial photographs and large-scale topographic maps recently published by the Geological Survey. In areas for which these are not available, planimetric maps compiled by the U.S. Forest Service were used. The glacier boundaries were determined from vertical and oblique aerial photographs taken when little snow remained from the previous winter. Information obtained from the photographs was augmented by personal observations. The outlines of the glaciers were traced on the maps and their geographical coordinates determined (pl. 2). Each glacier was classified as to type, form, source, surface, nature of terminus, and activity, using a somewhat modified form of a standard glacier inventory guide recommended by the International Commission of Snow and Ice (UNESCO/IASH, 1970). These data were placed on computer punchcards. The tabulation scheme and computer compilation program were designed for all types of glaciers occurring in the United States; thus, not all these categories have been used in this particular study.

LIST OF GLACIERS

The resulting glacier tabulation is shown as table 1, the headings for which are explained below:

BASN gives the location in four digits, each denoting a subdivision, as follows from left to right:

First digit. The number 2 signifies the State of Washington.

Second digit. The major river basins are delineated as follows:

1	Fraser and Nooksack Rivers
2	Skagit River
3	Stillaguamish and Snohomish Rivers
4	Columbia River

Third digit. Indicates a secondary river basin.

Fourth digit. Indicates a tertiary drainage basin of one or more smaller streams.

GL refers to individual glaciers, numbered in a clock wise direction, in each subbasin.

LAT and LONG refer to the latitude and longitude of the glacier, in degrees and minutes. Where several very small glaciers are close enough to fall under the same coordinates, the order in which the numbers appear on plate 2 aids in identification.

AREA indicates the area of the glacier, in square kilometers.

A gives the probable accuracy of the area determination in three categories:

1	Excellent (95 percent). Derived from recent topographic maps of 1:62,500 or larger or from U.S. Forest Service planimetric maps of scale 1:31,680.
2	Good (not used in this report).
3	Fair (5 percent). Where derivation was from modern maps that do not show the glaciers or topography in detail or where aerial photography of good quality was not available.

0 indicates the orientation of the glaciers based on an 8-point compass rose, with 1 as north, 2 northeast, and so on. The orientation represents an average where varying directions of flow are present.

LNTH gives the length of the glaciers, in kilometers.

CLASS indicates classification of the glaciers by a series of five digits, reading as follows from left to right:

First digit. Glacier form:

0	Insufficient information to classify
1	Outlet glacier from a large ice sheet (not used in this report)
2	Valley glacier, simple
3	Valley glacier, branched
4	Branch of another glacier (not used in this report)
5	Icecap on summit or ridge crest
6	Cirque or niche
7	Slope or irregular topography
8	Small ice or snow patch
9	Cirque glacier associated with active rock glacier below (not used in this report)

Second digit. Snow or ice accumulation source:

0	Insufficient information to classify
1	Direct snowfall and minor drift snow
2	Primarily drift snow
3	Avalanche snow

Third digit. Surface conditions:

0	Insufficient information to classify
1	Not appreciably crevassed
2	Moderately crevassed
3	Severely crevassed
4	Moderately crevassed in upper part of glacier but inactive in terminal area

Fourth digit. Type of terminus:

0	Insufficient information to classify
1	Expanded foot, piedmont glaciers (not used in this report)
2	Calving in salt or fresh water
3	Coalescing, noncontributing tributary to a large glacier (not used in this report)
4	Hanging, little reconstructed ice in valley below
5	Hanging, perennial reconstructed ice in valley below
6	Ends on moderate to gentle slopes

Fifth digit. Terminal activity:

0	Insufficient information to classify
1	Rapid retreat
2	Slight retreat
3	Stationary
4	Slight advance
5	Rapid advance
6	Possible surge (not used in this report)
7	Known surge (not used in this report)
8	Small-scale avalanching
9	Infrequent large-scale avalanching; may be periodic

TOP lists the altitude of the highest point of the glacier, not including snow chimneys or ice patches of little area.²

BOT lists the bottom altitude, generally the terminus.² Where active glaciers discharge over cliffs to perennial avalanche ice in the valley below, the lowest altitudes of the avalanche ice is indicated.

ACC lists the mean altitude of the snow accumulation area.²

ABL lists the mean altitude of the ablation area.²

FRN shows the mean firn-line altitude (average lowest altitude of the seasonal snow remaining at the end of the summer melt season).² The data shown were derived from averaging 3 or more years' information.

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²Altitude is given in meters. Where 0 appears, insufficient information was available to obtain a meaningful figure.

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E indicates the probable accuracy of the altitude figures in columns ACC, ABL, and FRN. Number 1, indicating areas measured by planimeter and computed, is not used in this report. Number 2 indicates that glacier outlines on maps of a scale 1:62,500 or larger and a contour interval of 30 m or less were checked or adjusted by plotting from recent aerial photographs. On these maps the accumulation and ablation areas were then outlined, and the mean altitude of each was estimated by observing which contour line most nearly divided the area in half. Where 0 appears, usable data were not available.

SUMMARY OF DATA

The present inventory lists 756 glaciers that cover 267 km²—538 of these (220 km²) are west of the Cascade divide and 218 (47 km²) are east of the divide.3

Within the boundaries of North Cascades National Park, 318 glaciers cover an area of 117 km²; this may be compared with 87 km² of glaciers in Mount Rainier National Park and still smaller ice-covered areas in all other national parks except Mount McKinley National Park, Alaska.

Glacier areas and volumes are tabulated by size categories in table 2. Using data for South Cascade and Blue Glaciers and other inventories, values of mean thickness were assumed for glaciers in each size class. Thickness data are available for only one glacier in the North Cascades, South Cascade Glacier, which has a mean thickness of 83 m for the main trunk glacier and is 2.6 km² in area (Meier and Tangborn, 1965, p. 564). The Blue Glacier, in the Olympic Mountains of Washington, has an area of 4.3 km² and a mean thickness of 133 m (LaChapelle, 1965, p. 613). Thickness values in table 2 are based on these data and a compromise between the assumed values for small glaciers given in Canadian (Ommanney and others, 1969) and Russian (Avsiuk and Kotlyakov, 1967) inventories. Most (83 percent) of the glaciers in the North Cascades are small, less than 0.5 km³, but these glaciers contribute only 29 percent of the total area and only 10 percent of the total volume. It is interesting to note that each size category (arranged in a geometric progression), except the largest, contributes a roughly equal share to the total glacier area.

TABLE 2.—Glacier areas and volumes by size categories

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³The glaciers Nos. 2455-3—2455-9 and 2463-1—2463-7, shown on plates 1 and 2, were discovered too late to be included in table 1 and the data analysis. The combined area of these small glaciers is about 1 km².

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Of all the glaciers in this inventory, those classified as ice or snow patches make up 47 percent; those on a slope or irregular topography, 30 percent; and those occupying cirques or niches, 19 percent. The largest glaciers in the North Cascades are classified as valley glaciers. Although in numbers these represent only 2 percent of the inventory, they account for 17 percent of the total glacierized area.

Eighty-one percent of the glaciers are nourished directly by snowfall and minor amounts of drift snow. Fifteen percent are small deposits of ice and snow at the base of steep gullies where most of the snow accumulates as a result of avalanches. Two percent are fed primarily by drift snow. Insufficient information was available for classifying 2 percent of the glaciers either by type or by source of snow.

MEAN ALTITUDE AND ORIENTATION OF GLACIERS

The western slopes and crests of the Cascade Range are subject to heavy precipitation in winter as moisture-bearing storms sweep in from the North Pacific Ocean. As these storms rise and pass over the mountains, most of their moisture is released as rain or snow (fig. 1). Temperatures are usually moderate at all times of the year. At higher altitudes snow flurries may accompany summer storms and, in winter, rain may occur occasionally, even on the higher peaks. Under normal conditions fall, winter, and spring temperatures are cool enough to permit extremely heavy snow accumulation at altitudes above 1,000 m. At Mount Baker Lodge, where annual precipitation averages 2,790 mm (millimeters), snow on the ground has been measured to a depth of 7.6 m. At South Cascade Glacier, 3,800 mm average annual precipitation has been measured, and snow depths may exceed 10 m in April and May. East of the divide, temperatures are more extreme, and precipitation decreases sharply. At Stehekin the average annual precipitation is 864 mm, while at Chelan (at the eastern edge of the mountains) it is only 277 mm.

FIGURE 1.—Southwest-northeast profile across the North Cascades showing topography, precipitation, and gradient of mean glacier altitudes. Short, heavier segments on topographic profile indicate glaciers. Precipitation is indicated by circles where measured and by dashed line where inferred from streamflow records. (click on image for an enlargement in a new window)

Glaciers in the contiguous Western States are related to precipitation and latitude on a regional basis. Glacier altitudes are lower toward the north and toward the Pacific coast (Meier, 1961). In limited areas such as the North Cascades, which present a wide variety of topographic forms, these relationships become obscured by local effects of glacier orientation and exposure. Between lat 47° and 49°N., north- and northeast-facing glaciers of the North Cascades receive far less solar radiation than glaciers flowing south and southwest. Glaciers in deep north- and northeast-facing cirques are further protected; snow is also swept into these glacier basins from slopes exposed to the prevailing south and southwest winds. Thus, under otherwise similar topographic conditions, the largest percentage of glaciers lies on north- and northeast-facing slopes (fig. 2). Glaciers draining south are generally much smaller (fig. 2). Other local anomalous situations occur. On the exposed slopes of Mount Baker and Glacier Peak, the mean altitudes of glaciers are exceptionally high relative to those of other glaciers in the vicinity. Some unusually low mean altitudes of glaciers are also found, generally where snow and ice masses are sheltered by high protective cliffs, such as at the Entiat glaciers (Nos. 2432-1—2432-5).

FIGURE 2.—North-south profiles between (A) Marble Creek and McAllister Creek and (B) Basin Creek and Skagit Queen Creek, showing the effect of orientation on glacier size. (click on image for an enlargement in a new window)

The heavy snow accumulation west of the Cascade divide has resulted in a few glaciers being situated below the regional timberline. South Cascade Glacier, which has a mean altitude of 1,875 m, is the lowest large glacier in the North Cascades. Trees grow on slopes above the accumulation area of this glacier. The other extreme is found well east of the Cascade divide at the southeast extremity of the Stuart Range, where the Snow Creek glaciers (Nos. 2242=3—2242-7) have a mean altitude of 2,493 m, the highest mean altitude of any glaciers in the North Cascades.

The mean altitude of selected glaciers and the annual precipitation on a southwest-northeast profile across the range are shown in figure 1. The distribution of glacier mean altitudes is shown in figure 3A. Glacier orientation and the configuration of cirque walls, the occurrence of snow avalanches, and the efficiency of a basin to trap wind-drifted snow are not uniform throughout the area so it is virtually impossible to construct a consistent map of glacier altitudes. The altitudes of selected glaciers of relatively uniform characteristics are shown in figure 3B; note how low these altitudes are in comparison with those of the relatively unprotected glaciers on the volcanic cones of Glacier Peak (GP) and Mount Baker (MB).

FIGURE 3.—Mean altitudes of glaciers. The dotted line is the Cascade divide. A, Distribution of mean glacier altitudes. All mean glacier altitudes within each secondary drainage basin have been averaged and weighted by area; plotting points are the centroids of glacier area in each basin. Although smooth contours can be drawn, a large amount of scatter exists. B, Mean altitudes of glaciers in selected glacierized areas. Solid dots represent north- and east-facing glaciers in cirques where there is no appreciable avalanching of snow or ice at the base. Glaciers on the volcanic cones (solid triangles) show mean altitudes about 300 m higher. Areas are identified as follows: B, Bonanza Peak; C, Mount Challenger; CM, Clark Mountain; CP, Cardinal Peak; CR, Chimney Rock; D, Mount Daniel (two points); E, Eldorado Peak; GP, Glacier Peak; J, Jack Mountain; L, Mount Logan; MB, Mount Baker; MS, Mount Shuksan; R, Mount Redoubt; RR, Ragged Ridge; SC, Snow Creek; SK, Snow King Mountain; SP, Sloan Peak; SS, Silver Star Mountain; T, Tommyhoi Mountain; TS, Twin Sisters; WC, White Chuck Mountain; WH, Whitehorse Mountain. (click on image for an enlargement in a new window)

Orientations of the glaciers, by drainage basin, are shown in figure 4. North or northeast orientations are favored in most areas. Some drainage basins show very different orientation patterns (for example, basin 213). These orientation anomalies result from high-altitude topography which favors glacier development in more exposed quadrants.

FIGURE 4.—Orientation of glaciers, by drainage basin. Length of bar indicates percentage of glaciers having that orientation. Circles represent 20 and 40 percent. Basin numbers are the first three digits of the identification numbers in table 1 and on plate 2. (click on image for an enlargement in a new window)