North America is the larger of the two continents of the Western Hemisphere. The main body of this somewhat rectangular continent is extended in the northwest by the peninsula of Alaska and its Aleutian Island chain, in the northeast by the world's largest island (Greenland), in the southeast by Florida's peninsula, and in the southwest by Mexico and the land bridge to South America. In addition, the Caribbean islands are also often included in the physical definition of North America. Altogether, the continent's land area places it third in size among the seven continents, smaller only than Asia and Africa. Alternative definitions of North America are also made on political or cultural grounds. Greenland, under Danish sovereignty and virtually without permanent human settlement, is often considered an extension of Europe. The small countries occupying the narrow isthmus between Mexico and South America may be treated separately as \TCentral America\t. The Caribbean islands may be combined with Mexico and Central America and described, collectively, as Middle America. In the broadest definition, North America extends across 5,300 km (3,300 mi) from St. John's, Newfoundland, to the Pacific coast of Washington's Olympic Peninsula; and 5,600 km (3,500 mi) between the Arctic and the Gulf of Mexico, with an additional 2,300 km (1,400 mi) south across the land bridge to the Panama-Colombia border. From the scant evidence accumulated, it is believed that humans first entered North America more than 20,000 years ago. They probably crossed a land bridge that appeared between Siberia and Alaska as sea levels dropped during the period of continental glaciation. Small groups of hunters and gatherers pushed human settlement south and eastward, eventually across the Central American land bridge and on into South America. Agriculture and permanent settlement in villages in the Western Hemisphere are thought to have begun in central Mexico about 7000 BC. During the 10th century AD Norse and probably Irish explorers reached North America. The first uninterrupted European settlement, however, awaited Spanish occupation of the Caribbean and Middle American lands during the 16th century. French and English permanent settlements (begun in 1605 and 1607, respectively) proved to be the most durable of the other European contacts. Africans were carried to Caribbean plantations early in the Spanish occupation and introduced into English colonial holdings in 1619. The tide of European migration continued for the next three centuries to fill much of the rest of North America. Political independence in North America began with the United States (1776), and now most states are independent. By 1850 all of Middle America except Panama (1903) and Belize (1981) was independent. In the Greater Antilles, Haiti achieved independence in 1804, Cuba in 1898, and Jamaica in 1962; Puerto Rico remains a U.S. territory. The Bahamas and many of the larger islands in the Lesser Antilles are independent, but Caribbean islands remain dependencies of the United States, Britain, France, or the Netherlands. Canada became essentially independent of Britain in 1867. LAND The natural regions of North America are typified by broad areas of apparently similar appearance and morphology. The entire western margin of the continent is paralleled by a series of mountain chains with intermontane basins and plateaus. The vast interior of the continent is dominated by gently rolling plains. The eastern side of the continent has both low mountain relief and, along the southeast coast, almost flat topography. The general areas of physiographic similarity, however, contain important structural differences. In addition, climatic zones do not always coincide with the physiographic patterns. The main landform zones extend north-south, whereas some of the major climatic differences cut across this trend with latitudinal zones. Western Uplands The entire western margin of the continent is occupied by a massively crumpled landscape with long ranges of mountains interspersed with high plateaus, narrow valleys, and broad interior basins. This region extends from the Arctic (in Alaska) through Mexico and the Central American isthmus to South America. There are two linear mountain regions: the more eastern is the \TRocky Mountains\t, and the more western section is a succession of rugged ranges (\TCascade Range\t, \TSierra Nevada\t, and \TCoast Ranges\t) that parallel the Pacific coast. Between (and among) these mountainous margins are areas of lower relief punctuated in some places by smaller mountain ranges or cut deeply by rivers and streams to form spectacular canyonlands, such as the \TGrand Canyon\t of the \TColorado River\t. The southern extension of the continent is largely within this region, with the two major ranges in Mexico (Sierra Madre Occidental and Sierra Madre Oriental; see \TSierra Madre\t) forming the margins of an interior plateau in the north and merging in the rugged south central portion of the country. Except for a narrow high-rainfall zone along the northern Pacific margin, most of the region is dry but also subject to altitudinal climatic zonation. Thus, hot or dry lowlands grade into moist, cool subalpine conditions at higher elevations. These altitude variations are furthermore modified by latitude, because this region includes the full north-south (cooler-warmer) range of climatic conditions. The Rocky Mountains form the continental divide between Atlantic and Pacific Ocean drainage, but the arid \TGreat Basin\t of the western United States and a smaller portion of north central Mexico have interior drainage. Throughout the entire region, settlement is especially dependent on local water supplies or on the importation of water. Canadian Shield The largest landscape region in North America is occupied by the smallest number of people. Located north of the \TGreat Lakes\t and the estuary of the \TSt. Lawrence River\t, and including Greenland, the \TCanadian Shield\t is a vast region of Precambrian metamorphic rock. Along the eastern side of the shield, as in the \TLaurentian Mountains\t of Quebec and Labrador and in the mountains on \TBaffin Island\t and Greenland, shield topography remains rugged. Most of the remainder between the Arctic and the Great Lakes is a low-lying, moderately hilly landscape scoured by repeated glaciation and now dotted with millions of small lakes and swampy bogs that freeze during the long winters. The general drainage is northward into \THudson Bay\t or the Arctic Ocean, although the southern margins of the shield drain toward the Great Lakes and the St. Lawrence River. Soils that are not waterlogged in the summer and frozen all winter are thin, rocky, and acidic. The southern portion of the shield is covered by a forest of needleleaf evergreens, whereas the northern portion (except where permanently ice-covered) is characterized by the low, brushy tundra. Interior Plains The immense Interior Plains of the continent reach from the Arctic almost to the Gulf of Mexico and have flat to moderately rolling topography. The region has a continental climate, characterized by a wide seasonal temperature range and periods of intermittent aridity. Because of their extent, the northern plains are colder than the southern portion, and the eastern portion of the region is more humid (shorter dry spells and more rainfall) than the western. Because of the east-west climatic division of the Interior Plains, the drier western portion is often referred to as the \TGreat Plains\t, with the remainder still called the Interior Plains. The shift from one portion of the larger region to the other is generally gradual. The vegetation consists of grassland throughout the drier portion of the region, merging into what was originally a deciduous or a mixed coniferous-deciduous woodland. The small portion of the plains in the far north shares the coniferous-to-tundra pattern found across the shield. The region's drainage pattern is largely into the \TMississippi River\t system, but some of the rivers flow across the shield into the Hudson Bay and eventually make their way into the Arctic-bound \TMackenzie River\t. Soils of the plains, responsive to climate and vegetation, are of the grey brown forest type in the wood portions of the region, fertile brown-to-black grassland earths in the drier western section, and poorly developed and boggy in the far north. Appalachian and Interior Highlands The \TAppalachian Mountain\t system occupies an area from Newfoundland and the Gulf of St. Lawrence almost to the Gulf of Mexico. Older than the western mountains but far younger than the shield, most of the Appalachian region (and the small area of interior highlands formed by the \TOzark Mountains\t and the Ouachita and Boston ranges) is composed of a series of low, well-worn mountains or deeply dissected plateaus. Local relief may be considerable, but the scale is more modest than in the west. The northern portion of the region offers a more jumbled and rounded relief pattern than the \TBlue Ridge Mountains\t and Ridge and Valley sections of the southern Appalachians. The natural vegetation is generally a mixed coniferous-deciduous forest; the soils are rocky and infertile on the upland portions and more fertile, but subject to flooding, in many of the valleys. The region cuts across several climatic zones, all relatively humid, ranging from the subarctic regime of Newfoundland to the humid subtropical conditions of the southern interior United States. Atlantic and Gulf Lowlands Bordering the continent on the south and east are a series of plains and low, rolling topography. From \TLong Island\t to Mexico's \TYucatan Peninsula\t and beyond, this region is primarily characterized by its low elevation and its proximity to the coast, although the Atlantic plain gives way inland to the gently hilly, fertile \TPiedmont Plateau\t. These conditions, together with the region's southerly location within the continent, mean that summers are hot and humid and that winters, where seasonal variation is experienced, are relatively short and mild. The vegetation and the soil patterns are extremely variable, but many of the soils are productive when fertilized. The Caribbean Located almost entirely within the tropics (only some of the islands of the Bahamas lie to the north), the Caribbean islands experience a humid, tropical climate. It is this area, together with the Atlantic and Gulf Lowlands, that is most susceptible to hurricanes. Many of the islands represent only the tips of a long mountain range that rises from the ocean floor; thus, however small the islands may be, almost all have a narrow coastal lowland that rises toward a mountainous interior. The dominant vegetation pattern is broadleaf evergreen with some broadleaf deciduous intermixture. RESOURCES The agricultural resources of North America are immense, with much of this potential located in the United States and southern Canada. Almost all of the Interior Plains and the Atlantic and Gulf Lowlands regions are suitable for agriculture. Significant areas within the Western Uplands--wherever water is available--are valuable crop or pasture lands as are smaller sections of Middle America. The population pressure on agricultural resources is uneven, with the greatest demand in Middle America, especially the Caribbean. The United States and Canada, however, are among the world's primary food exporters. Mineral fuel resources--coal, petroleum, and natural gas--are found in abundance in North America, although they too are unevenly distributed. Three extensive coalfields (Appalachian, and Eastern and Western Interior) are located in the eastern half of the United States, and many other, smaller and scattered, high-quality deposits are located throughout the eastern Rockies. Only the \TUSSR\t and possibly China possess coal deposits that might approach the size and quality of those located in the United States. Canada, too, has extensive reserves, although not generally as of high quality or in the same quantities as in the United States. The small deposits in northeastern Mexico (near Monterrey) are the only significant coalfields in Middle America. About 13% of the world's reserves of petroleum and natural gas are found in North America. Deposits currently recognized in the United States represent roughly half of the total, but these have been more intensively developed than deposits elsewhere. The leading U.S. producing regions are in the south central plains and in a long arc along the Gulf Coast, although significant deposits also occur in California, Alaska, and elsewhere. Canada continues to discover sizable deposits, especially along the eastern margin of the Rockies. Mexico has long known of major deposits along its portion of the Gulf Coast, but recent exploration has boosted official estimates of proven reserves well over tenfold between 1976 and 1983. Mexico now has almost twice the proven oil reserves of the United States. North America's water resources are important for hydropower, irrigation, and transportation. Canada's hydropower potential is concentrated in the Laurentian Mountains of the shield and in the major rivers of the far West. Most of the remaining undeveloped hydro potential is in the West, concentrated principally on the \TColumbia River\t and its major tributaries. Much of Middle America's hydro potential remains to be developed. In the arid western region aqueducts carry water for irrigation and municipal needs across many hundreds of kilometers, but much of the available water has already been used. The navigable waterways of North America are exceptional. With relatively little modification, the Mississippi River and its tributaries provide access between the Gulf of Mexico and the heart of the Interior Plains. The Great Lakes are also navigable, with only two natural interruptions (rapids at \TSault Sainte Marie\t and \TNiagara Falls\t), and, with the development of the \TSt. Lawrence Seaway\t, the Great Lakes now connect the north central Interior Plains with the Atlantic Ocean. North America also possesses an impressive array of high-quality metallic mineral deposits. Iron, nickel, copper, uranium, gold, and most other major metals are found in minable deposits, mostly in the shield and the Western Uplands. Bauxite, in short supply in North America, is found in the Ozarks and Jamaica. Of the economically important metallic minerals, only tungsten, tin, and chromium are insufficiently concentrated on the continent. As rich in resources as North America is, its people--especially those of the United States--impose high demands on these resources. The high consumption by the U.S. population makes North America a net importer of virtually all major resources. Within the continent, raw materials and primary foodstuffs flow from Canada and Middle America into the United States. These high demand levels create environmental stress where less-accessible deposits are tapped and where concentrated waste is returned to the environment, and economic stress as more expensive means must be used for extraction and higher prices paid for imports. PEOPLE Of the 410 million people (1987 est.) in North America, more than 60% are located in the United States and almost another 20% live in Mexico. Almost all of Canada's 26 million people live within 200 km (125 mi) of the U.S. border. The remainder of the continent's population is divided between the Caribbean and Central American countries, with Cuba by far the most populous (1987 est. pop., 10.3 million). The pre-European population was also distributed unevenly. Indigenous groups totaling perhaps 15 million (estimates vary widely) lived in the mainland portion of Middle America by the end of the 15th century, with the largest concentration in south central Mexico. The indigenous population of all the rest, including the Caribbean, is estimated at less than 2 million. During the first century of Spanish colonial rule, the indigenous mainland population was reduced to about 3 million, while the native Caribbean inhabitants virtually disappeared (see \TIndians, American\t). Of the Europeans, the Spanish arrived first and by the early 16th century had established their control over the already organized wealth of the Aztec empire in the Valley of Mexico and the strategically important isthmus of Central America. In addition, Spain occupied several of the larger islands in the Caribbean. Other European countries laid claim to sections of the Caribbean and subsequently established outposts along the Atlantic coast. The French early claimed the mouths of the two water routes to the interior, the St. Lawrence and Mississippi rivers, and the Dutch temporarily held the mouth of the Hudson River. By 1750 the dominant European cultural patterns were established. Mexico and Central America were largely Spanish with a significant but diminished Amerindian influence. This Spanish region reached up the Rio Grande, into what is now New Mexico, and along much of the rest of the Gulf Coast. The Caribbean islands were very mixed culturally, and the composition was complicated further by the introduction of Africans and, later, Asian Indians in some colonial holdings. The French, soon to lose political control of all their North American claims, were well established along the middle St. Lawrence in what became Quebec and had a foothold in New Orleans at the mouth of the Mississippi. The area of British settlement, scattered along the entire Atlantic coast between Spanish Florida and the St. Lawrence, included pockets of Dutch, French Huguenot, German, and Swedish settlers. The only other European presence of consequence was a thin strip of Russian control along the north Pacific coast. During the 150 years following U.S. independence, many millions of immigrants arrived from Europe. During the first century of this period, most immigrants came from northwestern Europe. The British Isles continued to be a major source of settlers, with the Irish arriving in a strong wave between 1830 and 1860. Immigrants from Germany were the next most numerous. Late in the 19th century, following a small wave from Scandinavia in the 1880s, most new immigrants came from eastern and southern Europe. Poland, Hungary, Russia, Bohemia, and other areas of Eastern Europe sent large numbers of migrants after 1890, as did Italy and Greece. Many Chinese arrived in the mid- to late-19th century to help build the railroads (see \TOriental Americans\t). The European migration stream ended with World War I and the restrictive immigration laws that were passed during the 1920s. Since the 1920s most immigration to the United States has come from Latin America, Canada, and, more recently, Asia. Canada, too, has received millions of immigrants, although the numbers have been much smaller than for the United States. Canadian immigrants have come primarily from Britain and the United States, with significant numbers from Eastern Europe, the \TUSSR\t, Ireland, and Italy during some decades. Most immigrants settled where economic opportunity appeared to be greatest at the time of their arrival in the United States. Many of the Irish settled in the major urban areas, along the East coast. The later German and Scandinavian settlers moved into the agricultural frontier of the Interior Plains. By the time the East European and Italian immigrants arrived, the agricultural frontier was closed; it was into the major industrial centers that these new North Americans settled. The most recent wave of migration--from Latin America--has also been largely urban bound. A large Cuban influx has settled in southern Florida. The movement out of Puerto Rico during the 1950s and '60s was focused on New York and other Atlantic coast cities. Recent migrants from Mexico and other Central American countries have moved primarily into the major urban concentrations in the U.S. South (see \THispanic Americans\t). Most of the large Canadian cities have a mixture of European cultural heritages among their populations, but the French-Anglo division is most pronounced. The original French settlement probably totaled fewer than 20,000 persons, but their numbers have increased during the last two centuries to almost 7 million. About 5.3 million of these live in Quebec province. In the rest of Canada--except for New Brunswick, in which about one-third of the population speak French, and in the far north, where Eskimo is important--English predominates. Africans were imported as slaves to the Caribbean plantations and to the southern British colonies, probably by the millions. Although importation ceased early in the 19th century, this population remained concentrated in the southern United States and the Caribbean regions until early in the 20th century. Following the virtual end of European immigration to the United States, blacks began to move to northern and western cities in large numbers. This migration wave, from the agrarian South to the urban North and West, apparently peaked about 1970 and has begun to taper off. Almost half of the black population of the United States continue to reside in the Southeast (see \Tblack Americans\t). Demography and Urbanization Differences in population growth rates are most marked between Middle America and the two larger northern countries. Annual rates of increase are 0.7% and o.8% for the United States and Canada, respectively (1987 est.). Rates for mainland Middle America are 3 to 5 times the U.S. level, whereas rates for the Caribbean differ widely, with some relatively high and others roughly equal to the Canadian and U.S. rates. Countries with high rates of increase also have young populations. An estimated 42% (1987) of Mexico's population is younger than 15 years, which, except for Costa Rica (35%) and Panama (38%), is the lowest proportion among the mainland Middle American states. In contrast, only about 22% of the 243.8 million (1987 est.) U.S. residents are that young. In addition, the U.S. population has an unusually large number of persons born between 1946 and the early 1960s (the "baby boom" years), after which the birthrate declined sharply. As this population ages, the number of people in older age categories will also increase. In the United States, the 20-30-year-old age group is the largest, whereas for most Middle American countries the largest group is between the ages of 5 and 15 years. Insofar as different age groups require different services from society, including education, health, and employment, the demands on national governments also vary in North America. Levels of urbanization vary sharply, with generally lower, although increasing, rates in Middle America than in the United States and Canada. Approximately three-quarters of the two northern countries' populations live in urban areas, whereas in the rest of North America the proportion ranges between 45% and 60%. Some countries have even lower rates, for example, Haiti at about 25%. There are strong rural-to-urban migration streams in Middle America. Only slightly more than 25% of Mexico's population were considered urban in 1940, for example, but by 1987 this proportion approached 70%. Mexico City alone contains more than 22% of the national population in its total urban area. The U.S. and Canadian populations have been urban for decades, with major population shifts occurring among the large urban centers, and from central cities to the \Tsuburbs\t. In Canada the movement is from the cities in Ontario and Quebec to those in Alberta and British Columbia. In the United States the major shift has been from northeastern metropolitan areas to those located in the West and South, and since 1970 a small but noticeable migration from urban areas to very small towns and rural settings has taken place. Education The educational levels of North Americans are exceeded on a continental scale only in Europe and in Australia. In the United States and Canada more than 50% of the persons older than 25 years have completed at least 12 years of schooling. In addition, more than 15% have completed at least 4 years of college. Virtually the entire populations of both countries are considered literate. In contrast, a large portion of the Middle American population does not finish primary school. Recent estimates place the proportion of Mexican children finishing primary school at less than 40%. The rates are considerably lower in other parts of Middle America. Illiteracy rates, therefore, remain high, probably exceeding 50% in some countries, including Haiti and Guatemala. Health Using infant mortality as a general indicator of public health, only Northern and Western Europe have infant mortality rates as low as or lower than the two countries of temperate North America (about 13 deaths per 1,000 live births). Partly because their life expectancy exceeds 70 years, the populations of the United States and Canada experience relatively higher mortality from the diseases of aging (heart disease, cancer, and stroke) than from infectious diseases, which are more likely to strike down the very young. The rest of North America, on the other hand, must contend with many of the enervating and often fatal illnesses typical of the tropics. The infant mortality rate often exceeds 30 deaths per 1,000 live births. Religion Of those who regularly attend religious services in North America, the overwhelming majority are Christian. Attendance estimates for the United States and Canada range from less than 15% of the adult population to nearly 50%. Estimates for Mexico range from 20% to almost 75%. Together, the various Protestant religions dominate in the United States except in the Northeast and large eastern cities and the Southwest, where Roman Catholics are more numerous. In Canada, Quebec is largely Catholic while the other provinces are predominantly Protestant. Almost all of Middle America is predominantly Catholic--a legacy of the Spanish colonial period. Sprinkled throughout North America, especially in Canada and the United States, are sizable Jewish communities and smaller groups of virtually every other religion. ECONOMY Levels of economic development vary greatly over the continent. The postindustrial economies of Canada and the United States employ well under one-tenth of their labor forces in agriculture and one-third or less in industry, with the rest primarily employed in service industries. Fueled by oil revenues, Mexico's economy developed more rapidly in the 1970s than any other country's on the continent. The fall of oil prices in the 1980s, however, stalled this growth and plunged Mexico deeply into debt. About one-third of the workers remain engaged in agriculture, resulting in the existence of a large rural peasant class. In the Caribbean the pace of economic development is uneven, and unemployment rates are generally high. The agricultural sector employs about three-quarters of the labor force in some areas, although the tourism industry results in high service-sector employment. Agriculture Measured as yield per worker-hour, North America's agriculture may be the most productive on earth. The United States and Canada regularly export more wheat than the rest of the world combined. Wheat, corn, oats, soybeans, barley, and many other feed crops are grown regionally throughout the vast Interior Plains, with most used to feed the two countries' millions of cattle, hogs, and dairy cows. Specialty crops, such as cotton, tobacco, rice, and peanuts, are grown across the U.S. South. Fruit and vegetable production is concentrated in California, Texas, and Florida, because of the long growing season. Sugarcane is prominent in the Caribbean, and coffee, cotton, and bananas are important in Central America. Farming has traditionally been a family-owned operation in the United States and Canada, with farms of 60-160 ha (150-400 acres) the norm in more humid sections and larger farms in the drier interior. Farm sizes are increasing, however, and ownership patterns are changing as agribusiness, or large-scale corporate agriculture, becomes more common. Plantations are important among regions of lowland and island agriculture. In Central America ownership of large estates (haciendas), farmed by landless peasants and ranging in size from 600 ha to more than 30,400 ha (1,500-75,000 acres), is concentrated in the hands of a small, wealthy, and politically powerful class. Scattered among the large holdings are very small subsistence plots farmed by low-income peasants. Following the Revolution of 1910 in Mexico, a land-reform program subdivided many of the largest haciendas into communally held farm villages known as ejidos. Manufacturing In North America the primary area of traditional industry, which produces such products as steel, automobiles, and machine tools, lies around the southern Great Lakes (Chicago, Gary, Detroit, Cleveland, Pittsburgh, Toronto, Hamilton). Other, smaller areas are numerous in the United States and Canada, but except for Mexico (Monterrey and Mexico City), there is little heavy industry in Middle America. The continental distribution of lighter industry is similar, with most located in the two temperate countries. The traditional pattern of Great Lakes-Northeast United States concentration is changing rapidly. The new pattern is one of decentralization and dispersion with an emphasis on high-technology industries. Urban regions in the South (Atlanta, Dallas, Houston) and the West (Phoenix, Denver, Vancouver, Los Angeles, San Diego) are growing. Rapid industrialization is occurring in Mexico and, to a lesser degree, in Puerto Rico and Jamaica, but the rest of Middle America remains far behind. Forestry and Fishing More than one-third of Canada is forested, but most forests--with those in British Columbia the leading exception--are not easily accessible and are slow-growing woodlands of the northern boreal region. Almost 200 million ha (500 million acres) of commercial timberland are available in the United States; the most used are the pine forests of the Southeast and Pacific Northwest. U.S. domestic consumption of wood as lumber and pulp exceeds supply, so much is imported from Canada to make up the difference. Central America has vast forest resources, but they are currently used selectively. In tropical forests the mixture of exploitable species with others and transportation difficulties impede logging. There are four major zones of fishing around North America. Shallow shelves, such as the Grand Banks and Georges Bank off the New England and Newfoundland coasts, are fished for cod, herring, flounder, and scallop. The southern Atlantic and Gulf coasts are fished for herring, sardines, shrimp, and oysters. Along the Pacific, in addition to cod and herring, salmon and flounder are prominent north of Point Reyes, Calif., while tuna is more important to the south. Fishing is locally important in the Caribbean and elsewhere. Trade The countries of North America, dominated economically by the United States, trade with each other more than with most other parts of the world. Canada's chief trading partner for both imports and exports is the United States. Similarly, the primary trading partner of the United States is Canada. All Middle American countries, except Cuba, trade more intensively with the United States than with any other country. In many cases, more than half of Middle America's exports and imports go to or come from the United States. In general, raw materials and specialty crops flow into the United States from its neighbors, while manufactured products and basic foodstuffs are exported. Many countries, particularly oil importers with developing economies dependent on export revenues from one or two primary products, suffer from severe trade deficits. RECENT DEVELOPMENTS The United States has long held a proprietary attitude toward the Western Hemisphere and especially toward North America. Partly as a result of its traditional self-imposed isolation from Europe's political power struggles combined with a sense of Manifest Destiny, and partly in recognition of its clear economic dominance, the United States has rarely hesitated to involve itself directly in the political and economic affairs of other states on the continent. Central America, where extremes of wealth and poverty have resulted in much political instability, continues to attract U.S. intervention, although changing conditions have forced a modification in this approach to continental affairs. Mexico's size and growth since 1960 have made it the dominant force within Middle America. This dominance is not even or constant, but when Mexico's national policy toward other states in the region does not coincide with that of the United States, more U.S. accommodation is necessary than was formerly the case. Revolutions in Cuba (1959) and Nicaragua (1979) and the signing of a treaty (1978) for gradual U.S. withdrawal from the \TPanama Canal Zone\t all showed that strains within the political economies of the region were no longer likely to be settled solely from Washington. Nevertheless, strong indirect U.S. involvement in Nicaragua and the U.S. intervention (1983) on the tiny island of Grenada demonstrated that the United States still considered the region part of its sphere of influence. Two issues between the United States and Mexico illustrate their interdependence. Northwestern Mexico, like the U.S. Southwest, is dry, and most of the irrigation water in the region is drawn from the south-flowing Colorado River. As U.S. demands on this limited supply have increased, farmers south of the border have complained that an international river has been appropriated for only one country's needs. Bilateral discussions were begun to settle the issue. Second, despite rapid economic growth in Mexico, the country's fast-growing population has maintained many people in poverty. The booming growth of southwestern U.S. cities has led many Mexicans, perhaps by the hundreds of thousands annually, to cross the border illegally in search of employment. Even the most stringent efforts by the United States to stem this flow have failed, and the social, economic, and political issues involving these illegal migrants remain tangled and sensitive. Mexico's ability to grow economically at a rate rapid enough to absorb its own population increase was jeopardized by the fall in oil prices during the 1980s. A U.S. law passed in 1986 requires U.S. employers to verify the legal status of all newly hired workers. This may slow the illegal migration stream from Mexico, but the long-term effects on Mexico and on the United States remain to be seen. Canada's development of its oil and gas reserves in Alberta and British Columbia has also been accompanied by a growing caution in making long-term commitments to U.S. market demand. In addition, within Canada, the boom economies of its western provinces threaten to draw the center of the national economy away from the traditional eastern Toronto-Montreal axis. The pull from the West has also exaggerated the country's regionalisms. Conflicts on provincial versus federal sovereignty emerged (1980) over the issues of drafting the federal constitution and the pricing system for fossil fuels from Alberta. A plebescite in Quebec reaffirmed (1980) its inclusion within the confederation of Canada but did not eliminate the need to address the underlying issue of bicultural equality. Regional rivalries within the United States--characterized by such labels as Snowbelt v. Sunbelt and the Sagebrush Rebellion--highlight major relocations in population and economic activity. Shifts in employment from heavy industry, dependent on raw materials and transportation, to lighter, more footloose high-skill, high-technology industry have stimulated major growth spurts in the South and West, largely at the expense of the Northeast. Accompanying the clear benefits of regional economic growth, however, have been strains on local service structures and local cultures unaccustomed to such a rapid influx of people and economic demand. In the arid West, where a major share of the land remains in federal ownership and where the natural environment is less resilient than in the more humid East, debate has become more vigorous between proponents of full-scale development and those arguing for maintenance of balance with the environment's ability to absorb human demands. Ultimately, the supply of available water in the West will limit the number of people and economic activities within the region, but those who argue for slower regional growth are concerned that irreparable damage will have been caused by then. Stephen S. Birdsall Bibliography: Birdsall, Stephen S., and Florin, John W., Regional Landscapes of the United States and Canada (1981); Bryson, Reid A., and Hare, F. Kenneth, eds., Climates of North America (1974); Callwood, June, Portrait of Canada (1981); Farb, Peter, Face of North America: The Natural History of a Continent (1963); Garreau, Joel, The Nine Nations of North America (1981); Helms, Mary W., Middle America: A Cultural History of Heartland and Frontiers (1975); Hunt, Charles B., Physical Regions of the United States and Canada (1974); Inter-American Development Bank, Economic and Social Progress in Latin America (1980); National Geographic Society, We Americans (1975); Paterson, John H., North America, 8th ed. (1989); Spencer, Robert F., et al., The Native Americans (1977); West, Robert C., and Augelli, John P., Middle America: Its Land and Peoples (1976). See also: articles on individual physical features, cities, countries, states, and provinces; \TCanada, history of\t; \TLatin America, history of\t; \TMexico, history of\t; \TUnited States, history of the\t.