Intermountain West - Environment
Eastern Americans are accustomed to an undulating terrain where variations in elevation are seldom dramatic. Where mountains occur, most do not contain an elevation change from the mountain's base to its top that exceeds 1,000 meters. By comparison, dramatic changes of 1,000 meters or more are common in the interior West.
A second element of the region's physical geography is its ruggedness. Most of the mountains of the eastern United States appear rounded and molded; the ranges of the West present abrupt, almost vertical slopes, and the peaks frequently appear as jagged edges pointing skyward. This difference is due partly to age. Most of the western mountains, although by no means all of them, are substantially younger than the eastern ranges. Thus erosion, which results in an eventual smoothing of the land surface, has been active for a much shorter time.
During the most recent period of geologic history, the Pleistocene, the carving done by mountain glaciers did much to form the topography of the interior West, and remnants of the glaciers can still be found in parts of the region. Most widespread in the Pacific mountains of southern Alaska, smaller glaciers are found as far south as the central Rocky Mountains in Colorado and the Sierra Nevada of California.
Alpine glaciers form in higher elevations and gradually flow downhill as the volume of ice increases. The moving ice is a powerful agent for erosion. Where this erosion pattern continues for a sufficiently long time, a deep U-shaped valley is created with almost vertical sides and a relatively flat bottom. If two glaciers flow side by side, a narrow ridge line is formed, characterized by jagged small peaks called aretes. Yosemite Valley in the Sierra Nevada, an almost classic glacially carved valley nearly 2 kilometers deep, is perhaps the region's most photographed example of alpine glaciation.
Most of the Intermountain West is occupied by plateaus rather than mountains. Probably the most scenically dramatic portion of this section is the Colorado Plateau along the middle Colorado River in Utah and Arizona. Although there are some large structural changes in relief, most of the area is underlain by gently dipping sedimentary rocks. The major landscape features are a result of erosion by exotic streams (so-called because they carry water, something otherwise unknown - or exotic - into this arid environment) that cross the plateau, most notably the Colorado River and its tributaries. In this environment, streams are easily the predominant erosive influence.
Thus, when accompanied by recent substantial geologic uplift over much of the plateau, great downward erosion has resulted, primarily in the immediate vicinity of the streams. The canyonlands that have been produced are some of the best known examples of America's natural scenic resources. In fact, the Grand Canyon of the Colorado River in Arizona is one of the country's most widely recognized natural scenic attractions. In Grand Canyon National Park, a canyon system has been created that is at places more than 16 kilometers wide.
In addition, the variable resistance of strong and weak rocks in these sedimentary formations has created an angular pattern of scarps and benches that is especially characteristic of the area.
Filling the country from the Colorado Plateau to the south across southern New Mexico and Arizona, west into Death Valley and the Mojave Desert in California, and as far north as Oregon and Idaho, is the basin and range region. This wide area is composed of a series of more than 200 north-south trending linear mountain ranges that are usually no more than 120 kilometers long and typically rise 1,000 to 1,600 meters from their base within a collection of some 80 broad, flat basins. North and west of the Colorado River basin, most of the area has interior drainage; that is, streams begin and end within the region, with no outlet to the sea. One result is that much of this area has received vast quantities of alluvia eroded from the surrounding mountains.
During the Pleistocene, substantial parts of the region were covered by lakes that resulted from a wetter climate and the melt of alpine glaciers. The largest, Lake Bonneville, covered 25,000 square kilometers in northern Utah. Most of these lakes are gone or greatly diminished in size because stream flow now depends on a lower annual precipitation, and many of the lakes that remain, such as Pyramid Lake in Nevada or Utah's Great Salt Lake, are heavily saline. Flowing water always picks up small quantities of dissolvable salts, which normally make a minor contribution to the salinity of the world's oceans. But because they lack an outlet to the ocean, lakes in the basin and range area have increased their salt concentration. The Great Salt Lake, covering about 5,000 square kilometers, is the remnant of Lake Bonneville and today has a salt content much higher than that of the oceans.
North of the basin and range region, the Columbia Plateau is the result of a gradual buildup of lava flows. Contained by the surrounding mountains, these repeated flows, each averaging 3 to 6 meters thick, have accumulated to a depth of 650 meters in some areas. A few small volcanoes and cinder cones dot the area, but the primary features of volcanic activity here are the vast flows of formerly molten material. Here, too, streams have eroded deep, steep-sided canyons.
With some gaps, the pattern of eroded plateaus continues northward into the Yukon Territory in the area between the Rocky Mountains and the Pacific Ranges. In central Alaska, the drainage basin of the Yukon River occupies the territory from the Alaska Range to the Brooks Range. Surface materials are mostly sedimentary rocks.
The Intermountain West supports a growing wildlife population that includes the bison (buffalo), the North American elk, the pronghorn antelope, the wild bear, the white-tailed deer, and the wild turkey.
There is a strong association between precipitation and elevation throughout the Interior West. Low-lying areas are generally dry. Heaviest precipitation amounts are usually found on the mid-slopes of mountains. The entire region is almost totally dependent for surface water on the exotic streams.
The association between topography, temperature, and precipitation results in a marked altitudinal zonation of vegetation throughout the Intermountain West. The lowest elevations are generally covered with desert shrub vegetation, most notably sagebrush. In the far south, there is a modest late summer increase in precipitation that allows a sagebrush/grasslands combination. Elsewhere, this combination is found at elevations above the desert shrub.
The Intermountain Region of the Forest Service includes 12 national forests: Ashley, Boise, Bridger-Teton, Dixie, Fishlake, Manti-LaSal, Payette, Salmon-Challis, Sawtooth, Caribou-Targhee, Humboldt-Toiyabe, and Uinta-Wasatch-Cache National Forests.
Upslope from the sagebrush is a tree line, above which precipitation is sufficient to support tree growth. The forests are at first a transitional mix of grass and small trees, like pinon pine and juniper. At higher elevations, these blend into more extensive forests of larger trees, such as ponderosa pine, lodgepole pine, and Douglas fir. If the mountains are high enough, smaller trees such as subalpine fir varieties and then a second tree line are encountered. Above this upper tree line, a combination of high winds and a short, cool growing season render tree growth impossible, and the trees are replaced by tundra.
Increasing air temperatures are expected to reduce available soil moisture and cause gradual changes in the abundance and distribution of tree, shrub, and grass species throughout the Intermountain Region. Conditions are expected to favor more drought tolerant species. The earliest changes will be on the edges of lifeforms, for example, where upper and lower treelines meet. Ecological disturbances, including wildfire and insect outbreaks, will be the primary facilitator of vegetation change, and future forest landscapes may be dominated by younger age classes and smaller trees. High-elevation forests will be especially vulnerable if disturbance frequency increases significantly.
Most strategies for conserving native tree, shrub, and grassland systems focus on increasing resilience to anticipated persistent low soil moisture. These strategies generally include managing landscapes to reduce the severity and extent of disturbances, encouraging fire to play a more natural role, and protecting refugia where fire-sensitive species can persist. Adaptation strategies that increase species, genetic, and landscape diversity (spatial pattern, structure) will reduce the risk of major forest loss. Adaptation tactics include using silvicultural treatments that reduce stand density management, fuel treatments that reduce fuel continuity, reducing populations of non-native species, potentially using multiple genotypes in reforestation, and revising grazing policies and practices.
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