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The Mountain West region encompasses four states: Colorado, Wyoming, Montana, and Idaho. These states represent the northern, middle, and southern Rocky Mountain physiographic regions in the United States. This area is one of the most diverse regions in the United States because it includes eight different physiographic provinces: The Great Plains, the northern Rockies, the middle Rockies, the southern Rockies, the Wyoming Basin, the Columbia Plateau, the Basin and Range, and the Colorado Plateau. This diversity in geology and geomorphology also translates into differences in climate, vegetation, and wildlife habitat. In addition, the natural resources in this area attracted the early pioneers and the different amenities are attracting a new and expanding western population.
This portion of the United States occupies over 432,538 square miles (1.12 million square kilometers) and stretches approximately 860 miles (1,400 kilometers) along the crests of the Rocky Mountains. The northern border of Idaho and Montana follows latitude 49 degrees north separating the United States from Canada, while the southern border is Colorado at latitude 37 degrees north. The eastern limit of this region is longitude 102 degrees west-the eastern border of Colorado-and the western edge is longitude 117 degrees west-the western border of Idaho.
The Mountain West is a region dominated by the Rocky Mountains, a complex of mountains, valleys, and basins formed during the Cretaceous Period (140-65 million years ago). However, portions of the southern Rockies were uplifted more than 3.9 billion years ago during the Precambrian Period. The backbone of the Rockies was created by a combination of igneous and metamorphic rock and the edges are tilted sedimentary layers forming long ridges or hogbacks. The movements of the earth’s crust have created a series of folded and faulted mountains. The Rockies have also experienced several volcanic episodes and numerous intrusions, lava flows, and other magmatic features are evident throughout the four states. The various uplifts, folds, and faults have occurred over more than three billion years and the mountains have experienced a multitude of erosional periods creating several flat basins filled with sedimentary material, the most notable of which is the Wyoming high basin. Finally, the three ice and intermittent ice advances have scarred the mountains with spectacular glacial features and remnants still present today in Glacier, Rocky Mountain, Grand Teton, and Yellowstone National Parks.
Adjacent to the Rocky Mountains to the east is the Great Plains, an area of low relief except for outlier uplifted mountains. The majority of the geologic formations were created during the Mesozoic and Cenozoic periods (225-70 million years ago). The surface material is mainly the erosional deposits from the Rocky Mountains. In addition, the northern portions display the large impacts of the continental glaciers, and in western Montana, both continental glacial and the long reach of alpine glacial features.
The western side of the Rocky Mountains border several physiographic provinces. The northern Rockies are adjacent to the Columbia Plateau, an area dominated by volcanic materials of the Miocene-Pliocene periods (25-2 million years ago). Some of the lava flows extend over 100 miles (160 kilometers) and experienced folding and faulting creating ridges and steep hillsides. To the west of the middle Rocky Mountains is the great expanse of the Basin and Range Province, a large area occupying over 297,800 square miles (771,300 square kilometers) of recently faulted mountains and valleys. This province consists of a series of parallel mountain ranges with wide valleys of low relief. The southern Rocky Mountains in Colorado blend into the Colorado Plateau-a long, high area above 4,920 feet (1,500 meters) of mainly horizontal sedimentary rock eroded into steep-walled valleys, exposing folded and faulted rock formations. Scattered throughout the area are igneous structures, including large shield and conic volcano mountains, lavacapped mesas and tables, and lava flows.
The mountains, plateaus, valleys, and plains in this region display the radical topographic variations of the area. Colorado has the highest average elevation among the coterminous states of over 6,800 feet (2,070 meters). Colorado also has 59 peaks that soar over 14,000 feet (4,268 meters), also known as the “14’ers.” Wyoming also has a very high average elevation of 6,700 feet (2,040 meters), yet does not have the multitude of high peaks similar to Colorado. Both Montana and Idaho have peaks that are slightly over 12,500 feet (3,811 meters), yet they also have low lands that are below 1,800 feet (549 meters). Overall, there are more than 20 major mountain ranges in the Mountain West, along with many isolated mountains. To the east and west of the Rocky Mountains are the flat plains and plateaus that generally slope away from the high peaks.
The elevation changes, differences between windward and leeward sides of mountains, the rain shadow effect, and the large latitudinal differences between the northern and southern portions of the four states provide diversity in climatic conditions. The northern portions of Idaho and Montana experience the most diversity in temperature and precipitation. Parts of Idaho receive over 60 inches (1,500 millimeters) of precipitation while adjacent valleys may receive less than five inches (125 millimeters) with temperatures ranging from 48 degrees C to negative 51 degrees C. All of the states experience high temperature differences between mountain passes and lowlands along with large ranges in precipitation, both rainfall and snowfall. The eastern portions of Colorado, Wyoming, and Montana display the westerly longitudinal progression of drier conditions in the Great Plains. The orographic effect of the Rocky Mountains produces higher precipitation on the western slopes-the windward side-and drier conditions on the eastern slopesthe leeward side.
The precipitation regime of the Rocky Mountains produces the headwaters of six major drainage systems: the Missouri, Arkansas, Rio Grande, Snake, Bear, and Colorado Rivers. The Missouri and Arkansas Rivers are some the largest contributors to the Mississippi River. From the southern portion of the Rocky Mountains, the Rio Grande drains southward into the Gulf of Mexico. The flow of the Rio Grande is interrupted by a number of reservoirs and over-allocation to municipalities, agriculture, and industries. From the western slope of the Rockies, the Colorado, Bear, and Snake Rivers drain across the Colorado Plateau, the Basin and Range, and the Columbia Plateau provinces. The Colorado River has its beginnings in northern Colorado and western Wyoming, traversing more than 1,429 miles (2,300 kilometers) to the Gulf of California in Mexico. The Colorado River, like the Rio Grande, feeds into a number of reservoirs created by large dams, the most notable of which is the Hoover Dam producing Lake Mead. Similarly, the Snake River has a number of dams and reservoirs along its route to the Columbia River regulating its maximum and minimum flows. Finally, the Bear River is only a small and short river compared to the other five rivers, traveling only 347 miles (560 kilometers). It is also different from the other rivers because of the fact that it drains into a closed drainage basin, the Great Salt Lake, with no outlet to the ocean or sea.
The vegetation patterns correspond to the elevation and precipitation complexes of the Rocky Mountains, the Great Plains, the Basin and Range, and Plateau provinces. An increase in elevation creates cooler temperatures along with slopes away from direct solar radiation, while precipitation will generally be higher on the windward sides of mountains and lower on the leeward side. There are five vegetation zones through this region: Prairies, foothills, montane, sub alpine, and alpine. East of the Rocky Mountains, the Great Plains are dominated by mixed-grass prairies in the northern portions (Montana and Wyoming) and short grass prairies in the south extending into Colorado. The mixed grass prairie includes little bluestem, needle grasses, wheat grasses, sand-reeds, and gramas. The short grass prairie also has buffalo grass, ring grass, needle-and-thread, June grass, and galleta. The foothills zone is a transition from the prairies to the montane and can have a mix of sagebrushes and scattered woodlands of ponderosa pine, limber pine, juniper, pinion, Gambel oaks, and shrubs. The lower and warmer montane zone is dominated by Douglas firs. In the higher and cooler montane zone, lodge pole pines and dispersed aspen stands are found. The sub-alpine zone varies between spruce fir, white spruce, Englemann spruce, and white pine, depending on micro-climatic conditions. The alpine region transforms trees into shrub-like krummholz with wind, growing-day length, and soil conditions. In addition, grasses, sedges, sagebrushes, mosses, and lichens along with hundreds of flowering plants add to the vegetation diversity in this highest elevation zone, in conditions similar to the Arctic.
Adding to the complexity of the vegetation pattern are two major factors: The micro and local variations in topography, climate, soils, and geology and the drainage patterns with their corresponding riparian and river ecosystems. Changes in solar radiation, small patches of soil nutrients, and exposed bedrock generate mixes of vegetation in short distances. Access to moisture in the floodplain allows plants to extend their regime into the drier portions of the plains, the higher elevations, and the southern slopes of mountains. The major drainage systems flowing out of the Rocky Mountains extend the riparian species of broad-leaf cottonwoods, alders, and willows far into the adjacent plains and plateaus for hundreds of miles.
The combination of the physiography, climatic conditions, soils, fauna, and vegetation create broad areas of similar characteristics called ecoregions. The four-state mountain area is part of the Dry Domain and has elements of the Great Plains, southern Rocky Mountain Steppe, middle Rocky Mountain Steppe, northern Rocky Mountain Steppe, and Intermountain Semi-Desert and Desert. These elements are the basic components that compose the major habitats of the area and explain the considerable number of mammals, birds, and amphibians found throughout the area. The ecozones, coupled with the U.S. Fish and Wildlife Service’s Gap Analysis program-a series of statewide terrestrial vertebrate inventories using GIS and remote sensing techniques to model species distribution and species richness-provide regional details of fauna through the area. The number of terrestrial vertebrates found range from a low of 375 in Idaho to almost 600 in Colorado. The majority of these are bird species (60-70 percent) with mammals (24-28 percent) the next largest category. Amphibians and reptiles are found throughout the region and account for 10-20 percent of the total species. Fish species, though not part of the Gap Analysis, are critical to the region’s aquatic ecosystem. There are approximately 30-50 fish species; however, this number will vary based on the dominance of native and non-native varieties.
The Mountain West is one of the fastest growing areas in the United States. Between 2000 and 2005 the population of the United States grew by 5.3 percent while Colorado and Idaho statewide grew by over 8.5 percent, and portions of Montana and Wyoming grew at rates approaching this number. Typical of the west, populations are centered in urban areas, with extremely low densities found in between the concentrations.
Colorado has the largest population (4.67 million) in the region with the majority along the eastern slope, the Front Range, in Denver (554,636), Colorado Springs (360,879), and Aurora (276,393). Idaho’s population (1.43 million) is more dispersed across the state, but it is still in urban areas with Boise in the west-central portion the largest (185,787) and the other cities located in the central portions of the state: Nampa (51,867), Pocatello (51,466), and Idaho Falls (50,730).
The highest density of large cities in Montana (935,670) are along the eastern slopes of its western mountains; Missoula (57,053), Great Falls (56,690), and Butte-Silver Bow (34,606). Montana’s largest city, Billings (89,847), is located in the west-central plains. Wyoming has the smallest population (509,294) with only Cheyenne (53,011) and Casper (49,644) as population centers. However, the next tier of cities is dispersed across the state with no concentrations except along the historic railroad lines. The population migration into the Mountain West is predominately to the urban areas; however, rural second homes are scattered in the more recreationand amenity-oriented landscapes.
Historically, mining, agriculture, forestry, and ranching were the focus of economic activities in the Mountain West. However, as the population shifted to urban areas in the mid-20th century, citycentered economic activities began to increase in importance. The U.S. Bureau of Economic Analysis identifies 10 major domestic production activities: Agriculture, forestry, fishing, and hunting; mining; utilities; construction; durable goods manufacturing; nondurable goods; wholesale trade; transportation; information; and fire. Of these activities, three categories can be considered non-urban: Agriculture, forestry, fishing, and hunting; transportation; and mining. As a percent of the total gross domestic production, agriculture, forestry, fishing and hunting are a major production activity in Montana and Idaho (eight to nine percent), while mining is dominant in Wyoming (47 percent) and important in Colorado and Montana (eight to nine percent). Transportation is also significant in Wyoming and Montana (nine percent). In all four states, recreation is a very important component of the economy, though it is not considered a production activity but rather a service industry. These activities are important because of the amount of land they require and their role in the domestic economy. Crop and grazing lands dominate the landscape in all four states, with forested areas the next largest category.
Public And Private Lands
The Mountain West is part of the transition area between the dominance of public and private land ownership. To the east, the majority of the lands are in private ownership, while to the west and south more of the land is in public ownership. This is an important factor because as more lands become public, it decreases the potential for private enterprise, but increases protection of natural resources. Idaho has the highest percent of public lands (50 percent), mainly in the northern half of the state. Wyoming (42 percent), Colorado (37 percent), and Montana’s public lands are located in the western portions of each state. Each of the four states has approximately the same amount of public lands, between 38-43,700 square miles (98-113,000 square kilometers), however, because of their size differences, the percentages change. The public lands are managed by several federal agencies, mainly the U.S. Forest Service, the U.S. Bureau of Land Management, the U.S. National Park Service, and the U.S. Fish and Wildlife Service.
There are 50 national forests, 34 national parks, national historic sites, national trails and national monuments, and five national recreation areas distributed between the four states. Lands managed by the Bureau of Land Management (BLM) are generally distributed in the plains and plateaus between the mountains, sometimes referred to as the “checker-board” area, for the pattern of every other land section (one square mile).
The diversity of the landscape, variety of fauna and flora, and the importance of water resources make the Mountain West one of the most vital areas to the United States. The physical amenities and natural resources of the Mountain West will continue to attract people for residency, employment, or recreation. However, the intensity of activities has the potential to have large impacts on the environment. The conservation management of federal lands, the responsibility of private enterprise to sustain the environment, and the participation of local citizens to monitor their impacts can mitigate these impacts. The types of interactions between the environment and the population are critical to sustain this area for future generations.
- Bailey, Ecoregions of the United States (U.S. Department of Agriculture, U.S. Forest Service, 1980);
- S. Baron, ed., Rocky Mountain Futures: An Ecological Perspective (Island Press, 2002);
- Blood, Rocky Mountain Wildlife (Hancock House Publishers, 1976);
- Hunt, Physiography of the United States (W.H. Freeman and Co., 1967);
- Kershaw, A. MacKinnon, and J. Pojar, Plants of the Rocky Mountains (Lone Pine Publishing, 1998);
- H. Knight, Mountains and Plains: The Ecology of Wyoming Landscapes (Yale University Press, 1994);
- J. Mac, P.A. Opler, C.E. Puckett Haecker, and P. D. Doran, Status and Trends of the Nation’s Biological Resources (U.S. Department of the Interior, U.S. Geological Survey, 1998);
- W.D. Thornbury, Regional Geomorphology of the United States (John Wiley & Sons, , 1965).