Geography and Environment of the Greater Southwest
The boundaries of the American Southwest are often said to extend from the modern community of Durango, Mexico, in the south (24° 01′ 44″ N, 104° 39′ 46″ W) to Durango, Colorado, in the north (37° 16′ 31″ N, 107° 52′ 48″ W), from Las Vegas, Nevada, in the west (36° 06′ 52″ N, 115° 10′ 22″ W) to Las Vegas, New Mexico, in the east (35° 35′ 38″ N, 105° 13′ 26″ W). Geographically it is the region bounded on the north by the Rocky Mountains and the Great Basin, on the east by the American Great Plains, on the west by the Pacific coastal region and on the south by Mesoamerica (Map 2.31.1).
The Southwest is classified as a semi-arid region, much of which receives fewer than 200 mm of precipitation each year. Exceptions to this occur, however, in the many upland areas, where precipitation is significantly higher, often occurring as snowfall in the higher elevations (Cordell 1997: 38). Three main physiographic provinces make up the Southwest. In the north, the landscape is dominated by the Colorado Plateau, which is characterised by a high-elevation, northward-sloping plateau dotted with mountain ranges, mesas and deep-cut canyons like the Grand Canyon and Canyon de Chelly. The southern portion of the Southwest consists of the Basin and Range Province, which comprises a series of low mountain ranges separated by broad alluvial valleys. Separating these two zones is the Mountain Transition Zone, a heavily dissected mountainous region that is bordered on the north by the Mogollon Rim (Cordell 1997; Reid & Whittlesey 1997). In places, the rim is a steep escarpment that demarcates the southern edge of the Colorado Plateau. It is also the point at which rivers in the Southwest change direction.