United States

The third group of Eastern rivers drains the coastal strip along the Atlantic Ocean and the Gulf of Mexico. Except for the Rio Grande, which rises west of the Rockies and flows about 1,900 circuitous miles (3,050 km) to the Gulf, few of these coastal rivers measure more than 300 miles (480 km), and most flow in an almost straight line to the sea. Except in glaciated New England and in arid southwestern Texas, most of the larger coastal streams are navigable for some distance.

The Pacific systems

West of the Rockies, nearly all of the rivers are strongly influenced by aridity. In the deserts and steppes of the intermontane basins, most of the scanty runoff disappears into interior basins, only one of which, the Great Salt Lake, holds any substantial volume of water. Aside from a few minor coastal streams, only three large river systems manage to reach the sea—the Columbia, the Colorado, and the San Joaquin–Sacramento system of California’s Central Valley. All three of these river systems are exotic: that is, they flow for considerable distances across dry lands from which they receive little water. Both the Columbia and the Colorado have carved awesome gorges, the former through the sombre lavas of the Cascades and the Columbia Basin, the latter through the brilliantly coloured rocks of the Colorado Plateau. These gorges lend themselves to easy damming, and the once-wild Columbia has been turned into a stairway of placid lakes whose waters irrigate the arid plateaus of eastern Washington and power one of the world’s largest hydroelectric networks. The Colorado is less extensively developed, and proposals for new dam construction have met fierce opposition from those who want to preserve the spectacular natural beauty of the river’s canyon lands.

Climate

Climate affects human habitats both directly and indirectly through its influence on vegetation, soils, and wildlife. In the United States, however, the natural environment has been altered drastically by nearly four centuries of European settlement, as well as thousands of years of Indian occupancy.

Wherever land is abandoned, however, “wild” conditions return rapidly, achieving over the long run a dynamic equilibrium among soils, vegetation, and the inexorable strictures of climate. Thus, though Americans have created an artificial environment of continental proportions, the United States still can be divided into a mosaic of bioclimatic regions, each of them distinguished by peculiar climatic conditions and each with a potential vegetation and soil that eventually would return in the absence of humans. The main exception to this generalization applies to fauna, so drastically altered that it is almost impossible to know what sort of animal geography would redevelop in the areas of the United States if humans were removed from the scene.

Climatic controls

The pattern of U.S. climates is largely set by the location of the conterminous United States almost entirely in the middle latitudes, by its position with respect to the continental landmass and its fringing oceans, and by the nation’s gross pattern of mountains and lowlands. Each of these geographic controls operates to determine the character of air masses and their changing behaviour from season to season.

The conterminous United States lies entirely between the tropic of Cancer and 50° N latitude, a position that confines Arctic climates to the high mountaintops and genuine tropics to a small part of southern Florida. By no means, however, is the climate literally temperate, for the middle latitudes are notorious for extreme variations of temperature and precipitation.

The great size of the North American landmass tends to reinforce these extremes. Since land heats and cools more rapidly than bodies of water, places distant from an ocean tend to have continental climates; that is, they alternate between extremes of hot summers and cold winters, in contrast to the marine climates, which are more equable. Most U.S. climates are markedly continental, the more so because the Cordillera effectively confines the moderating Pacific influence to a narrow strip along the West Coast. Extremes of continentality occur near the centre of the country, and in North Dakota temperatures have ranged between a summer high record of 121 °F (49 °C) and a winter low of −60 °F (−51 °C). Moreover, the general eastward drift of air over the United States carries continental temperatures all the way to the Atlantic coast. Bismarck, North Dakota, for example, has a great annual temperature range. Boston, on the Atlantic but largely exempt from its influence, has a lesser but still-continental range, while San Francisco, which is under strong Pacific influence, has only a small summer–winter differential.