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:
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:
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:
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.
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 km2538 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
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.
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. 2432-12432-5).
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=32242-7) 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).
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.
Last Updated: 28-Mar-2006