The Empty Interior

United States Geography

Stretching from the eastern slopes of the Rocky Mountains westward to the Sierra Nevada of California, to the Cascade Range of the Pacific Northwest and into Alaska, is the largest area of sparse population in America. Its low average population density is the key identifying feature of this region. Indeed, there is much variation in other elements of the territory's geography. Portions have rugged terrain interspersed with a series of plateaus, many of which contain extensive flat areas. Annual precipitation ranges from more than 125 centimeters in northern Idaho to less than 25 centimeters in the plateau country. The population of the region is mostly of European origin, although Hispanic-Americans and American Indians are found in significant proportions in the south. Irrigated agriculture is important in some areas, as is ranching, whereas in other areas, lumbering, tourism, and mining are dominant.

This massive expanse of land contains some of America's most strikingly scenic portions. The impact of humans on the region, although locally important, has been overshadowed to a great degree by the varied splendors of the natural 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 Empty Interior 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.

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 Empty Interior. 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. 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.

The Empty Interior 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.


In the state of Nevada, various government agencies control almost 90 percent of all land. Although the percentages are lower elsewhere, the basic pattern of governmental predominance is found throughout the Empty Interior.

It is not surprising that so much of the land remains in government hands. This area and Alaska were the final regions to be occupied by any substantial numbers of people, and federal programs of land distribution, designed to encourage agricultural use, were not relevant because little of the region held any real agricultural promise. The U.S. Bureau of the Census proclaimed the end of the settlement frontier in 1890, a time when much of the Interior West still remained unsettled. Also, by the time other interests, such as lumbering or mining, began to push for greater private land ownership, the federal government was reevaluating earlier programs in which it distributed land almost for free.

A substantial part of America's total national park system is found in the interior West, including such famous parks as Yellowstone, Glacier, and Grand Canyon. But the national parks are only a small portion of the total public land area. The largest share of these lands is held by the Bureau of Land Management, a part of the U.S. Department of the Interior, which puts this land to many uses, grazing being the most important. The bureau has also been the main agent in the construction of irrigation and hydroelectric dams in the area.

The Forest Service, part of the U.S. Department of Agriculture, is the second largest of the federal landholders. The service has traditionally emphasized logging and grazing under its multiple-use charge, and it has increased the quality and quantity of recreational uses of the land.

Two other uses of parts of the Empty Interior say much about the region's past and about America's attitude toward the land's quality and usefulness. First, some of the largest American Indian reservations are found here, especially in northern Arizona and New Mexico. Also, some of the country's largest bombing and gunnery ranges, as well as its only atomic bomb testing facility, are found here. The population is sparse, and alternative demands on the land are not great.

As the agricultural frontier moved westward in the late decades of the 19th century, it largely swept past the Interior West. In fact, were it not for minerals, transportation, and the Church of Jesus Christ of Latter-Day Saints, few people would have chosen the region until well into this century.

The Latter-Day Saints, or, more commonly, the Mormons, were established in upstate New York in 1830. The church and its followers were attacked repeatedly, both verbally and physically, for what were considered their "unusual" beliefs. The Mormons moved several times, searching for a place to practice their religion. Many Mormons, often on foot, pushed into the West, where they hoped to create an independent Mormon state.

The locale they selected for their initial western settlement was the Wasatch Valley, tucked between the Wasatch Mountains and the Great Salt Lake in northern Utah, a location that would later become Salt Lake City. It must have seemed an unlikely spot to begin an agricultural settlement. The climate was dry, the lake saline and useless, and the landscape barren. Nevertheless, the Mormons quickly began their agricultural operations; their settlements expanded as new arrivals came. A high birth rate also pushed their population numbers upward. They dreamed of founding an independent country to be called Deseret, stretching northward into what is now Oregon and Idaho and southward to Los Angeles, California, and Mormon communities were established at greater and greater distances from Salt Lake City.

The Mormons ultimately failed in their hopes for creating Deseret. With the discovery of gold in California and Nevada, American expansion moved through and beyond the Mormon area, and the Mormons again found themselves under the will of the United States. Deseret was divided eventually among a half-dozen different states.

The Mormons were the first Americans to face the problems of life in the Interior West, and they solved the majority of them. None of their solutions was more important than irrigation. Americans had previously had no need for extensive irrigation, and the techniques and central control necessary to collect and move water to a large number of agricultural users were almost unknown. The Mormons constructed a large number of storage dams on the western slopes of the Wasatch Range, and many kilometers of canals moved the water to the users in the valley below. The results of these efforts today cover much of the valley with agricultural crops, trees, and green lawns. These early efforts at large-scale irrigation were the beginning of an irrigation boom in the interior West.

Mormons continue to have a substantial impact on the Interior West. Of the roughly 11 million persons found in this region, over 1.5 million are Mormons.


Irrigation and Agriculture: Much of the flow of several of the more important rivers in the Empty Interior is diverted for various uses, with irrigation claiming the largest share. The Reclamation Act of 1902 provided for federal support for the construction of dams, canals, and, eventually, hydroelectric systems for the 17 western states (excluding Alaska and Hawaii). Today, over 80 percent of the water from these federally supported projects is used to irrigate over 4 million hectares. While most of this irrigated land is in California, large irrigation projects are nevertheless scattered throughout the region.

The 1 million hectares irrigated in the Snake River Plain makes Idaho the region's leader in terms of the amount of land irrigated. This enables the state to be among America's leaders in potato and sugar beet production; alfalfa and cattle are also important. The Columbia Valley Reclamation project, supplied by the bountiful waters of the Columbia River impounded behind Grand Coulee Dam in central Washington, contains well over 400,000 hectares, producing such crops as alfalfa, sugar beets, and potatoes. Irrigation along the Wasatch Valley has expanded little since the first decades of Mormon settlement. About 400,000 irrigated hectares there are devoted primarily to sugar beets and alfalfa. The Grand Valley, along the Colorado River in west-central Colorado, produces alfalfa and potatoes as principal crops, although tree fruit, especially peaches, are also important. In Washington, tributaries of the Columbia River, notably the Yakima and Wenatchee, supply water for America's most famous apple-producing regions.

Each of these areas produces a limited set of crops. The region's short growing season precludes production of most long-season crops. And local demand is limited, minimizing the need for dairying or many fresh vegetables.

The more southerly irrigated districts, although affected by irrigation water shortages, nevertheless have one major advantage over their northern counterparts--a far longer growing season. California's Imperial Valley, with a frost-free period in excess of 300 days, is one of America's premier crop producing areas. Much of America's winter head-lettuce supply comes from here, as do grapes, cotton, and alfalfa for fattening beef. The cattle population of the valley is over 250,000 head. A newly constructed electric power generating facility there uses locally abundant cattle manure for fuel. The Imperial Valley's growing season is long enough to support double-cropping, and of course this increases overall productivity.

The Coachella Valley north of the state's Salton Sea produces such crops as dates, grapes, and grapefruit. The Yuma Valley along the lower Colorado River supplies cotton, sugar beets, and oranges. In the Salt River Valley, near Phoenix, Arizona, winter lettuce, oranges, and cotton are the major crops. These southern crops, unlike those grown farther north, face little direct competition from the agricultural centers in the major market areas of the eastern United States.

Transportation Services: Because little traffic is generated within the Interior West, a prime goal of transport developers has been to permit movement across the region as speedily and inexpensively as possible. Consequently, most major highway and rail routes pass through the region east-west, from the urban centers of the Midwest to those of the West Coast.

Despite these requirements of transport design, the great width of the region demands development of many service facilities for the traffic passing through. Many of the towns of the region began as centers established to service and administer the railroads. The centers were founded wherever railroad personnel were needed, whether the region was populated or not. Although fewer local railroad workers were needed as technological innovations were implemented beginning in the late 1940s, this population reduction was more than compensated for by the growing need for people to serve truck and automobile traffic with gas stations, car repair facilities, motels, and restaurants.

Although transport services represented the principal early influence on the growth of urban centers, cities that have become the largest have usually been aided by some additional attribute. Spokane, Washington, with a population of over 350,000, has, for example, become the principal center for the "Inland Empire" of Washington. That area, geographically defined and half encircled by the sweep of the Columbia River across central Washington State, has long had substantial agricultural production. Albuquerque, New Mexico, with a population of about 500,000, has gained a role similar to Spokane's through its centrality and accessibility in that state. Phoenix, Arizona, grew initially as an agricultural center and then boomed as Americans flocked to its warm, dry environment. It has become a retirement center as well as a focus of manufacturing activities, with industries that produce small, high-value products, such as the electronics industry, being particularly important in the city's growth.

Ogden, Utah, is one community that exists as a major rail center and was early among the most important such places in the region, but it has not become a major urban place. It is only about 55 kilometers north of Salt Lake City, a city whose continued dominance derived from its key functions as the capital of Utah and of Mormonism.

Tourism: The variety and appeal of the Empty Interior's scenic wonders attract millions annually. Visitors to most of the major parks must first pass a long, garish strip of motels, snack bars, gift shops, and other sources of local color. In addition, distances between attractions are usually great, and services are thus needed in countless locations. When legal gambling operations in the state of Nevada are included as part of the area's tourist industry, the overall regional impact of tourism becomes even greater.

Lumbering and Ranching: Ranching and lumbering depend on governmental land for many of their basic materials. The holdings of the Forest Service and the Bureau of Land Management are open to grazing, and most lumbering in the Empty Interior is carried out on Forest Service lands. Levels of production per hectare for both ranching and lumbering products are relatively low, especially when compared with land held in private hands.

One reason for this apparent inefficiency is the limited quality of the land. In many drier areas, 40 hectares of land per head is needed for satisfactory cattle grazing. The great seasonal climatic variations found in much of the region make this one of the few areas in America where transhumance is practiced--a seasonal movement of animal flocks, by those who tend them, from the lowlands in winter to mountain pastures and meadows in summer. It is especially important in the sheep-ranching economy. Many Basques, expert shepherds from the Pyrenees of Spain and France, came to this area as contract laborers to manage the herds. Today, descendants of the Basques are a substantial part of the population of several states, especially Nevada.

Mining: In the late 19th century, gold miners followed shortly on the heels of the Mormons to become the second largest group of settlers in the region. The discovery of the Comstock Lode in Nevada gave rise to Virginia City, which grew into a city of 20,000 during its heyday around 1870 before nearly disappearing with the decline of high-quality ore.

The boom in gold and silver mining in the years immediately following the Comstock discovery created rapid population growth in Nevada. This culminated in the admission of the state into the union in 1864, long before most of its neighbors. The depletion of much of this mineral resource by the late 19th century resulted in a widespread population decline in Nevada, one from which it did not fully recover until well into the 20th century. Today, the mining economy is of little significance to the state--or to any part of the interior West--although some of the abandoned mining centers are important tourist attractions.

Leading today's list of mineral contributions to the region's economy is copper, with production concentrated in Arizona and Utah. The vast open pit of the Bingham mine outside Salt Lake City, said to be the largest human-made excavation in the world, has yielded some 8 million tons of copper. Among the several score major and minor copper-mining centers in Arizona, the most important is at Morenci in the eastern part of the state. Other important mines are at San Manuel, Globe, and Bisbee, all in southern Arizona.

Most of the copper ore mined in the Empty Interior is low grade, with a metal content of under 5 percent. Consequently, most mines have a smelting or concentrating facility located nearby to lessen shipment costs by greatly reducing the weight of the material being shipped. Refining is thus a major manufacturing industry in the region.

Lead and zinc follow copper in regional importance, with the two often joined by several other metals mined at the same location. The Butte Hill mine in Montana, for example, long was a significant producer of lead and zinc as well as copper. The Coeur d'Alene district in northern Idaho produces gold, silver, lead, and zinc; the Leadville district in Colorado has those four plus molybdenum, used in the manufacture of steel products. In fact, some three-quarters of the world's supply of molybdenum comes from the Leadville district. Uranium exploration has also been widespread in the region, and today Utah and Colorado are the principal producing states. Approximately 25 million tons of coal is mined annually.

Spread across thousands of square kilometers of the area where Utah, Colorado, and Wyoming meet are the vast oil shale deposits of the Green River Formation. Locked in these rocks are as much as a trillion barrels of oil, vastly more than the entire proven alternative oil reserves of the world. However, operational and environmental problems have put development of this industry largely on hold.

There has been little sustained or substantial urban growth based on these mineral resources. Butte, Montana, with a population of 34,000 in 1990, is perhaps the region's largest city developed with mining (copper) as the main base of the economy, yet it has long been an important processing center for agricultural products as well.

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Source: U.S. Department of State