REGIONAL PHYSICAL AND CLIMATIC SETTING
The glaciers in western North America are in a great chain of highlandsthe North American Cordillera. This chain comprises two vast mountain systems that roughly parallel the Pacific coast from California and Colorado through Canada and Alaska; these systems are separated by a complex of basins, plateaus, and isolated mountain ranges.
On the west is the Pacific Mountain System, comprising principally the Aleutian and Alaska Ranges and Kenai, Chugach, Wrangell, and St. Elias Mountains in Alaska; the Coast Mountains in southeastern Alaska and western British Columbia; the Cascade Range in Washington and Oregon; and the Sierra Nevada in California (pl. 1). The Pacific Mountain System consists mainly of eugeosynclinal sediments and volcanic rocks which have been strongly and almost continuously deformed from the Mesozoic to the present time and into which enormous granitic batholiths were intruded, especially during the late Mesozoic. Vulcanism, warping, and faulting continue to the present day.
Farther inland is the Rocky Mountain or Eastern System. This includes the Brooks Range in northern Alaska, the Mackenzie and Selwyn Mountains near the Yukon-Northwest Territories border, and the Rocky Mountains along the Alberta-British Columbia border and south through Montana, Wyoming, and Colorado to New Mexico. In general, these mountains consist of miogeosynclinal sediments that comprise both carbonate and clastic rocks and range in age from Paleozoic to Mesozoic. Most of the mountain-producing deformation, folding, and thrust faulting occurred in the Late Cretaceous. From southern Montana to New Mexico, uplift followed by erosion has exposed Precambrian rocks. Thick sequences of volcanic rocks occur in northwestern Wyoming and southern Colorado.
Between the Pacific and Rocky Mountain or Eastern Systems only a few mountain ranges are sufficiently high and continuous to contain large numbers of glaciers. These include the Wood River Mountains in western Alaska, mountainous areas in north-central British Columbia, and the Selkirk, Monashee, and Purcell Mountains in southeastern British Columbia. These mountains are of varied geology; deformed sedimentary and volcanic rocks predominate except for large areas in southern British Columbia which are dominated by granitic intrusions.
The region shown on plate 1 extends from lat 34° N. to 72° N. and from the sea coast to points 1,500 kilometers inland; its altitude ranges from 84 meters below to 6,194 m above sea level. The Pacific Ocean adjacent to the Alaskan, Canadian, and Washington-Oregon coasts is relatively stable in temperature all year. Most of the precipitation that forms glaciers in this region is derived from the North Pacific. In winter, moisture-laden storms spawned from the semipermanent Aleutian low-pressure area move inland along the Pacific coast. Very heavy precipitation occurs when these storms impinge on and are lifted by the Pacific Mountain System; annual values of more than 5,000 millimeters have been measured near the coast in Washington, British Columbia, and Alaska. Annual runoff as high as 10,000 mm averaged over river basins on Baranof Island, Alaska, indicates that some coastal mountains receive much more precipitation than is recorded at present precipitation-gage locations. Strong precipitation-shadow effects occur where these storms move over high ridges.
The rather complex precipitation pattern shown near the Pacific coast on plate 1 is a generalization; most individual mountain ridges or mountainous islands show a strongly negative windward-leeward precipitation gradient. The storms from the Aleutian low also bring moisture to the ranges farther inland, but much less total moisture is available to those ranges. Movement of warm moist air into the interior is frequently blocked in winter by a persisting high-pressure area north and east of the Rocky Mountains as well as by the mountains themselves. Consequently, winter precipitation decreases markedly from the coast inland. In summer the Aleutian low shifts northward and a North Pacific high-pressure system develops. This brings fog but little precipitation to the coast of Oregon and California. Storms moving north of the high-pressure area bring to the Alaskan coast abundant summer precipitation, and precipitation rates in late summer and fall may be the highest of the year. Western Washington experiences a mixture of weather types but is generally dry in July. Farther inland, moisture is precipitated from cyclonic or convective storms moving eastward toward low-pressure areas east of Hudson Bay. In the inland mountains of Alaska and western Canada and in the Rocky Mountains, precipitation may be greater in summer than in winter. Even so, the amount of summer snowfall is low, and the annual precipitation is much less than along the coast. North and east of the coastal mountains, annual-precipitation values less than 500 mm are common.
Temperatures are moderate along the Pacific coast; Cordova, Alaska (lat 61° N.), and Eureka, Calif. (lat 41° N.), have approximately the same average yearly maximum temperature (30°C). Winter temperatures are colder along the north coast but not extremely so; the average January temperature of Cordova is -5°C. Inland, on the other hand, the range of temperatures is much more extreme; average yearly high and low temperatures at Snag, Yukon Territory, at lat 62° N. near the Alaska-Yukon border, are about +30°C and -50°C.
Figure 1 shows the variation of net (or annual) balance as a function of altitude for certain glaciers. For each glacier the equilibrium-line altitude, or ELA, is given; this is the altitude, in meters above sea level, at which the balance equals zero. The whole altitude range of the ablation area is shown for each glacier, but for Nisqually, Saskatchewan, Seward-Malaspina, and Taku Glaciers, the whole altitude range of the accumulation area is not shown. The gradient of net balance in the vicinity of the equilibrium-line altitude is termed the activity index1 (Meier, 1962), or A1, and is a measure of the rate at which mass is transferred from higher to lower altitudes; this is also shown for each glacier. In the region studied, equilibrium-line altitudes tend to increase with continentality and (or) a decrease in precipitation totals and tend to increase with decreasing latitude. In general, high activity indices tend to indicate a maritime high-precipitation environment and low activity indices a dry continental environment. However, the activity index is to a slight extent also a function of glacier size (small glaciers tend to show higher activity indices).
Data for the following glaciers were taken from the indicated sources: Dinwoody (Meier, 1951), Maclure (Scully, written commun., 1969), Grasshopper (Alford and Clark, 1968), Saskatchewan (Meier, 1960), Peyto, Place, and Woolsey (Ostrem, 1966), Gulkana (Mayo, this rept.), Blue (LaChapelle, written commun., 1969), Berendon (Mathews, written commun., 1969), Taku (Nielsen, 1957), Lemon Creek (LaChapelle, written commun., 1961), Nisqually (Hodge, written commun., 1970), South Cascade (Tangborn, written commun., 1970), McCall (Keeler, 1958), and Wolverine (Mayo, this rept.).
Most of the glacial ice and the largest glaciers in western North America occur in the Pacific Mountair System. A wide variety of glacier types and glacier environments occurs in this mountain system, as can be noted from the large range in equilibrium-line altitudes and activity indices of the maritime glaciers shown in figure 1. Glaciers occupy about 73,000 km2 in Alaska alone. The coastal glaciers are probably temperate except for those parts of glaciers that are at very high altitudes. Glaciers that are on the inland side of the St. Elias and Chugach Mountains may, in part, be colder than 0°C. Completely polar glaciers probably do not occur in western North America.
The coastal Kenai Mountains (fig. 2) are fairly low, but snowfall is so heavy that large icefields occur and equilibrium-line altitudes are the lowest of the major glacierized areas in western North America. Chenega Glacier (fig. 2), a major outlet of the Sargent Icefield, has an ELA of only 550 m. Glaciers northwest of coastal icefields have less snowfall and higher equilibrium-line altitudes. For example, the ELA of Wolverine Glacier is about 1,200 m and its A1 (fig. 1) is about 9 mm/m. Most of the glaciers in this area are stable or are retreating slightly.
Northeast of the Kenai Mountains the Chugach Mountains are covered with large icefields and hundreds of mountain glaciers. Many large valley glaciers debouch into the sea; an example is the Columbia Glacier, which is 64 km long and has an area of 1,070 km2 and an ELA of about 850 m. Some of these glaciers are retreating, but most are nearly stable; a few, such as the Harvard and Meares, are advancing slightly. No surging glaciers have been identified in the Kenai Mountains and only two in the Chugach Mountains west of the Copper River.
North of the Kenai and Chugach Mountains and extending well into interior Alaska is the long arc of the Alaska Range. The central part of the range contains the highest peak in North America, Mount McKinley (alt 6,194 m), and is the center of an extensive icefield covering about 4,300 km2 and containing the largest glaciers of the Alaska RangeKahiltna Glacier being 75 m long and covering approximately 475 km2. East of Mount McKinley are two other mountainous areas in which ice covers 2,000 km2 and 1,400 km2. A continuous structural valley traversing the northern part of the range (the Denali fault zone) is the locus of a large number of surging glaciers, such as the Muldrow, Susitna, Black Rapids, and Gakona. The Gulkana is a moderate-size valley glacier in the eastern part of the Alaska Range (fig. 3). The rather continental climate of the Gulkana area is indicated by a higher ELA, 1,800 m, and a low A1, 6 mm/m (fig. 1). Except for glaciers that periodically surge, most glaciers in the Alaska Range are retreating markedly. Extensive stream icings (aufeis deposits) form in winter in the valleys below most Alaska Range glaciers.
The eastern Chugach Mountains and the Wrangell Mountains (just south of the eastern extremity of the Alaska Range) merge into the St. Elias Mountains north of Icy Bay. Here an area of more than 50,000 km2 is covered with ice; these are the most extensive ice fields on the North American continent. Noteworthy glaciers include the Bering, the largest glacier in North America (200 km long and 5,800 km2 in area); the Seward-Malaspina, the type example piedmont glacier (5,200 km2 in area); and the Hubbard, a valley glacier 140 km long. On the seaward side of the range, equilibrium-line altitudes are very low and activity indices are rather high. For instance, the Seward-Malaspina Glacier has an ELA of 1,000 m and, in spite of its large size, has an A1 of about 10 mm/m (fig. 1). On the inland side of the range, the equilibrium-line altitudes are considerably higher and the activity indices are presumably lower, although none has been measured. On the extreme northeastern fringe of the range, subpolar ice (having temperatures considerably below the freezing point) is known to occur in the lower reaches of the glaciers. Many surging glaciers occur in the St. Elias Mountains; most other glaciers there are probably stable or are retreating slightly, although some tidal glaciers (such as the Hubbard) are advancing. A few other tidewater glaciers are receding exceptionally fast. A vast system of icefields and isolated glaciers extends along the crest of the Coast Mountains of southeastern Alaska and British Columbia from the Yukon border south to the State of Washington. The Juneau Icefield, about 5,100 km2 in area, is best known. Glaciers associated with it include the strongly advancing large Taku outlet glacier (ELA 1,000 m, A1 about 13 mm/m; fig. 1) and the much smaller retreating Lemon Creek Glacier (ELA 1,000 m, A1 about 18 mm/m). Another large icefield is situated north of the Stikine River. Farther south are many large valley glaciers and innumerable mountain glaciers. The Berendon Glacier near Stewart, British Columbia, has an ELA of about 1,400 m and an A1 of about 7 mm/m (fig. 1). In southern British Columbia, the Place Glacier has a much higher ELA (2,200 m) but about the same activity index. Many of the smaller glaciers of the Coast Mountains were advancing in the early 1960's, but only a few had continued to do so by 1969; most large glaciers (with the notable exception of the Taku) have been retreating.
The Pacific Mountain System continues southward into the United States as the Cascade Range and smaller outliers, the Olympic Mountains and the Coast Range, to the west. Blue Glacier in the Olympics has an ELA of 1,700 m and an anomalously low A1 of 5 mm/m (fig. 1). South Cascade Glacier in the North Cascades of Washington has an ELA of 1,900 m and a more typical A1 of 17 mm/m. The North Cascades are particularly rugged (fig. 4); many glaciers in this range advanced in the period 1950-58. From southern Washington to northern California the glaciers occur only on high volcanoes. Nisqually Glacier on Mount Rainier, Washington, advancing in response to increased snowfall since 1946, reflects the maritime climate in this area in its A1 of 20 mm/m (fig. 1). Farther south warmer summers and lower precipitation cause the equilibrium-line altitude to rise abruptly. Several small cirque glaciers occur in the Sierra Nevada (fig. 5); these show both maritime and some continental characteristics. Maclure Glacier has an ELA of 3,600 m and an A1 of 23 mm/m (fig. 1). Most glaciers in the southern Cascades and in the Sierra Nevada are stable or are retreating slowly.
The Rocky Mountain System contains thousands of mountain glaciers but no major icefields. Many tiny cirque glaciers occur in the Brooks Range, but moderate-size valley glaciers occur only in the Romanzof Mountains near the eastern end of the Brooks Range. In this high-latitude environment the glaciers are subpolar and small stream icings form below the termini of the large valley glaciers. McCall Glacier has the lowest A1 (2 mm/m; fig. 1) of any glacier measured in western North America. Small mountain glaciers in the Mackenzie and Selwyn Mountains of Canada are undoubtedly subpolar but have not been studied in detail.
Considerable information is available on glaciers in the Canadian Rockies along the Alberta-British Columbia border. Small icefields covering up to 280 km2 occur near the headwaters of the Columbia, Athabasca, and Saskatchewan Rivers. Saskatchewan Glacier (21 km long) has an ELA of 2,500 m and an A1 of 9 mm/m (fig. 1); the nearby Peyto Glacier has an ELA of 2,600 m and the same activity index. In the Rocky Mountains of the United States, only small cirque glaciers occur except in the Wind River Mountains of Wyoming. The glaciers in Montana, Wyoming, and Colorado are found at very high altitudes (approaching 4,000 m in Colorado). However, in some areas such as the Wind River Range, the local mesoclimate results in large amounts of orographic and convective precipitation and abundant wind drifting so that considerable snow accumulation is possible; the high-altitude (ELA 3,700 m) Dinwoody Glacier in the Wind River Mountains has an A1 of 13 mm/m, and the slightly lower altitude Grasshopper Glacier in the Beartooth Mountains has an A1 of about 22 mm/m (fig. 1). Almost all the glaciers in the Rocky Mountains of Canada and the United States are retreating markedly.
In the mountain ranges between the Rocky Mountain System and the Pacific Mountain System, many scattered small mountain glaciers exist. The only area where glaciers have been studied is in the Selkirk and Monashee Mountains. Woolsey Glacier in the Selkirk Mountains has typically intermediate values of ELA, 2,300 m, and A1, 9 mm/m (fig. 1). Tiny vestigial glaciers occur in the intermontane ranges of the United States, such as the Wallowa, Salmon River, and Wasatch Mountains in northeastern Oregon, Idaho, central Utah, and eastern Nevada. Whether some of these are true glaciers or not is a matter of definition. In northwestern Alaska, small glaciers are present only in the Wood River and Kigluaik Mountains.
Last Updated: 28-Mar-2006