United States

The third intermontane region, the Columbia Basin, is literally the last, for in some parts its rocks are still being formed. Its entire area is underlain by innumerable tabular lava flows that have flooded the basin between the Cascades and Northern Rockies to undetermined depths. The volume of lava must be measured in thousands of cubic miles, for the flows blanket large parts of Washington, Oregon, and Idaho and in southern Idaho have drowned the flanks of the Northern Rocky Mountains in a basaltic sea. Where the lavas are fresh, as in southern Idaho, the surface is often nearly flat, but more often the floors have been trenched by rivers—conspicuously the Columbia and the Snake—or by glacial floodwaters that have carved an intricate system of braided canyons in the remarkable Channeled Scablands of eastern Washington. In surface form the eroded lava often resembles the topography of the Colorado Plateau, but the gaudy colours of the Colorado are replaced here by the sombre black and rusty brown of weathered basalt.

Most large mountain systems are sources of varied mineral wealth, and the American Cordillera is no exception. Metallic minerals have been taken from most crystalline regions and have furnished the United States with both romance and wealth—the Sierra Nevada gold that provoked the 1849 gold rush, the fabulous silver lodes of western Nevada’s Basin and Range, and gold strikes all along the Rocky Mountain chain. Industrial metals, however, are now far more important; copper and lead are among the base metals, and the more exotic molybdenum, vanadium, and cadmium are mainly useful in alloys.

In the Cordillera, as elsewhere, the greatest wealth stems from fuels. Most major basins contain oil and natural gas, conspicuously the Wyoming Basin, the Central Valley of California, and the Los Angeles Basin. The Colorado Plateau, however, has yielded some of the most interesting discoveries—considerable deposits of uranium and colossal occurrences of oil shale. Oil from the shale, however, probably cannot be economically removed without widespread strip-mining and correspondingly large-scale damage to the environment. Wide exploitation of low-sulfur bituminous coal has been initiated in the Four Corners area of the Colorado Plateau, and open-pit mining has already devastated parts of this once-pristine country as completely as it has West Virginia.

Drainage

As befits a nation of continental proportions, the United States has an extraordinary network of rivers and lakes, including some of the largest and most useful in the world. In the humid East they provide an enormous mileage of cheap inland transportation; westward, most rivers and streams are unnavigable but are heavily used for irrigation and power generation. Both East and West, however, traditionally have used lakes and streams as public sewers, and despite efforts to clean them up, most large waterways are laden with vast, poisonous volumes of industrial, agricultural, and human wastes.

The Eastern systems

Chief among U.S. rivers is the Mississippi, which, with its great tributaries, the Ohio and the Missouri, drains most of the midcontinent. The Mississippi is navigable to Minneapolis, nearly 1,200 miles (1,900 km) by air from the Gulf of Mexico, and along with the Great Lakes–St. Lawrence system it forms the world’s greatest network of inland waterways. The Mississippi’s eastern branches, chiefly the Ohio and the Tennessee, are also navigable for great distances. From the west, however, many of its numerous Great Plains tributaries are too seasonal and choked with sandbars to be used for shipping. The Missouri, for example, though longer than the Mississippi itself, was essentially without navigation until the mid-20th century, when a combination of dams, locks, and dredging opened the river to barge traffic.

The Great Lakes–St. Lawrence system, the other half of the midcontinental inland waterway, is connected to the Mississippi–Ohio via Chicago by canals and the Illinois River. The five Great Lakes (four of which are shared with Canada) constitute by far the largest freshwater lake group in the world and carry a larger tonnage of shipping than any other. The three main barriers to navigation—the St. Marys Rapids, at Sault Sainte Marie; Niagara Falls; and the rapids of the St. Lawrence—are all bypassed by locks, whose 27-foot (8-metre) draft lets ocean vessels penetrate 1,300 miles (2,100 km) into the continent, as far as Duluth, Minnesota, and Chicago.