The two great sets of elements that mold the physical environment of the United States are, first, the geologic, which determines the main patterns of landforms, drainage, and mineral resources and influences soils to a lesser degree, and, second, the atmospheric, which dictates not only climate and weather but also in large part the distribution of soils, plants, and animals. Although these elements are not entirely independent of one another, each produces on a map patterns that are so profoundly different that essentially they remain two separate geographies. First, this page covers the features of the physical environment of the coterminous United States, and then it tells about the physical environment of Alaska and Hawaii.
The dominant topographic features of the United States tend to extend north-south across the country. The interior of the country is a vast, sprawling lowland that stretches from the Gulf of Mexico to the Canadian border and then on to Alaska. Geographers with an interest in landform development place this expanse of flat land and gently rolling hills in three different physiographic regions - the Atlantic and Gulf coastal plains, the interior lowland (which some split into the Great Plains and the interior plains), and the Canadian Shield.
The Atlantic and Gulf coastal plains reach north along the east coast of the United States as far as the southern margins of New England. Underlying this area are beds of young, soft, easily eroded rock deposited in recent geologic time as shallow seas lapped back and forth across the land. These low plains extend well out under the ocean surface to form a continental shelf, which in places extends as much as 400 kilometers beyond the shore.
Northward, the interior lowland, although noticeably hillier than the coastal plains, has almost no rough terrain. This region is like a saucer, turned up at the edges and covered with a deep series of sedimentary rocks. These sedimentary beds are generally quite flat; most topographic variation is the result of local erosion or, in the North, of glacial debris deposited during the Ice Age.
The geologic structure of the Great Plains differs little from that of the interior plains. The sedimentary beds dominate, although in the north they are broken by some eroded domes, most notably the Black Hills of western South Dakota. While nearly horizontal, the sedimentary beds do dip gently toward the west to a trough at the foot of the Rocky Mountains, where the Colorado cities of Denver and Colorado Springs are located.
The boundary between the Great Plains and the interior plains is marked by a series of low escarpments that indicate the eastern edge of the mantle of loose sediments, eroded from the Rocky Mountains, that covers the plains.
The character of this massive interior lowland area has had a number of important influences on the economic and settlement history of the United States. In addition to the vast agricultural potential it provides, fully half the country can be crossed without encountering significant topographic barriers to movement. This facilitated the integration of both this region and the distant West into the economic fabric of the country. Nearly all of the interior lowland is drained by the Mississippi River or its tributaries. This drainage pattern assisted regional integration by providing a transport and economic focus for the land west of the Appalachian Mountains.
North and northeast of the central lowland is the Canadian Shield, where old, hard crystalline rocks lie at the surface. Farther south in the lowlands, similar rocks are covered by the sedimentary beds deposited under the sea that once filled the midsection of the country. Erosion has worn down the surface of the Shield into a lowland of small local relief.
The Shield, more than any other North American physiographic region, has had its landforms remolded and shaped by massive continental glaciers during the last million years. These glaciers covered most of Canada east of the Rocky Mountains and the Coast Ranges, and they reached southward to approximately the present valleys of the Missouri and Ohio Rivers.
The ice could pluck rocks weighing many tons off the surface and carry them great distances: Massive boulders are strewn across the landscape of the Shield, resting where they were dropped by the glaciers. Ice melt along the peripheries of the glaciers created major rivers and cut broad new pathways to the sea.
Glaciation scoured much of the Shield's surface. Today, the soil cover of the region remains thin or nonexistent. The heavily disrupted drainage pattern dammed many streams with debris and led others into the area's labyrinth of lakes and swamps rather than to the sea. Central and northern Minnesota, for example, called the "Land of 10,000 Lakes," is part of the southern lobe of the glaciated shield that extends into the states of Minnesota, Michigan, and Wisconsin.
Southward, where the ice was not as thick and its force correspondingly less, the glaciers were diverted or channeled by higher elevations. For example, the ice was blocked in central New York by the highlands south of the Mohawk River. However, narrow probes did push up the valleys of streams tributary to the Mohawk, gradually broadening and deepening them. Today, the deep, narrow Finger Lakes of New York State fill these glacially enlarged valleys and form one of America's truly beautiful landscapes.
All along and beyond the southern edges of the glaciers, deposition replaced erosion as the prime result of glaciation. Large areas of the interior lowland are covered by a mantle of glacial till (rocks and soil dropped by the glaciers), which covers the land to depths varying from a meter or less to more than 100 meters. Where the glaciers remained stationary for long periods of time, higher hills, called moraines, were created. In the east, Staten Island, Long Island, Martha's Vineyard, Nantucket, and Cape Cod are end moraines that mark the farthest major extension of glaciers toward the southeast. The landscape south of the Great Lakes is laced with long, low, semicircular moraine ridges and other glacial deposits.
One section of the interior lowland escaped glaciation. The southwestern quarter of Wisconsin and the adjoining 400-kilometer stretch of the Mississippi River valley were apparently spared by the barrier effect on the flowing ice of the Superior upland to the north and by the channeling of the ice by the deep valleys of Lakes Michigan and Superior. The result is the "driftless area" (drift is another name for till), a local landscape that is more angular, with fragile rock formations like natural bridges and arches.
As the ice retreated, massive lakes were created along the glacial margins. On the northern Great Plains, two huge lakes, Agassiz and Regina, together covered an area larger than today's Great Lakes. With continued glacial retreat, these lakes mostly disappeared. Their existence is now marked by the former lake bed, a flat area covering parts of North Dakota and Minnesota.
Sea level was significantly lower during periods of widespread glaciation. This lowered the base level of many rivers and thus fostered increased erosion by those streams. Furthermore, many of these stream valleys extended well into what is today the ocean. Along with many others, the Susquehanna and Hudson Rivers cut much deeper valleys during this period. As the ice retreated and sea level rose, the ocean filled these deepened valleys. Two of the world's finest harbor areas were formed in this way: New York Bay, with the deep Hudson River and the protective barriers formed by Staten Island and Long Island; and Chesapeake Bay, the drowned valley of the Susquehanna River and some of its major former tributaries, such as the Potomac and James Rivers.
In the East, the coastal plains are gradually squeezed against the coast northward along the ocean by the Appalachian Highlands until the lowland disappears entirely at Cape Cod. From there northeastward, the coastal landscape is a part of the northern extension of the Appalachian Mountain system. The Appalachians - eroded remnants of what were once much higher mountain ranges - separate the seaboard from the interior lowlands along much of the eastern United States.
Soils in most parts of this region are shallow, and the steep slopes, difficult to farm under any circumstances, are totally unsuited to modern agricultural practices that emphasize mechanization. Large-scale urban or industrial growth is cramped by the small, local lowlands. Early settlers found the Appalachians from the Mohawk River in New York southward to northern Alabama to be a surprisingly effective barrier to western movement; there are few breaks in the mountains' continuity.
The western United States is a land of mountains and of sudden, great changes in elevation. The physiography, again, is arranged in a series of three large north-south trending bands, with the Rocky Mountains on the east separated from the mountains and valleys of the Pacific coastlands by a series of high, heavily dissected plateaus.
Starting in the east, the Rocky Mountains generally present a massive face to the Great Plains, with peaks occasionally rising 2 kilometers or more. Elsewhere, as in south-central Wyoming, the Rockies almost seem not to exist at all. In the northern Rockies in Idaho, the north-south linearity of most of the region's mountains is replaced by massive igneous domes irregularly eroded into a rugged, extensive series of mountain ranges that contain the largest remaining area of wilderness in the United States outside Alaska.
The high plateaus of the interior West are also varied in their origin and appearance. The southernmost subsection, the Colorado Plateau, is a series of thick beds of sedimentary rocks rising more than 1,000 meters above the lowlands' elevation and tilted upward toward the northeast. The plateau is a land of spectacular canyonlands, volcanic peaks, and sandy deserts.
Farther north, the Columbia-Snake Basin has been filled by repeated lava flows to a depth of more than 1,000 meters. Rivers, both past and present, have eroded into the rock. The resultant landscape is similar to that of the Colorado Plateau, although the stepped appearance resulting from the variable resistance to weathering of the eroded sedimentary rocks of the Colorado Plateau is missing. Volcanic cones also dot portions of the region, especially across south-central Oregon and in the Snake River Valley in Idaho.
The plateaus gradually widen northward, encompassing the valley of the Yukon River in Alaska. In comparison, much of central Alaska is a broad, flat lowland that is poorly drained.
In the conterminous United States (excluding Alaska and Hawaii), the Pacific Coast seems to consist largely of two north-south trending mountain chains separated by a discontinuous lowland. In southern California, the Coast Range is fairly massive, with peaks reaching 3,000 meters. From there almost to the Oregon border, the mountains are low and linear, seldom rising above 1,000 meters. This also is the major fault zone of the state and a region of frequent earthquake activity. Along the California-Oregon border, the Klamath Mountains are higher, more extensive, and much more rugged and irregular. Except for the Olympic Mountains in northwestern Washington, the Coast Ranges in the rest of Oregon and Washington State are low and hilly rather than mountainous.
The interior lowlands along the coast - the Central Valley of California, the Willamette Valley in Oregon, and the Puget Sound lowland in Washington - are the only extensive lowlands near the West Coast. Filled with relatively good soils, these lowlands have supported much of the Pacific Coast's agriculture.
East of the lowlands are the Sierra Nevada and the Cascade mountain ranges. The Sierra Nevada appears as though a massive section of earth was tilted upward relative to the areas to the east and west in what is called a fault block, with the highest, sharpest exposed face toward the east. Although the western approaches into the Sierra Nevada are reasonably gentle, on the eastern side the mountains rise in some places more than 3,000 meters. Volcanic activity was important in the formulation of the Cascades. Some of America's best known volcanic peaks, such as Mt. Rainier and Mt. St. Helens in Washington, are found there.
There is a distinct association between the location of minerals that meet the needs of heavy manufacturing and the land's subsurface rock structure. Each of the three major forms of rock - sedimentary, metamorphic, and igneous - is capable of containing a type of mineral economically useful to humans. Sedimentary and metamorphic rocks are the most prevalent rock substructure and are more likely to contain minerals of broad utility than are igneous rocks.
Sedimentary rock is the result of the gradual settling of small solid particles in stationary water. For example, if a shallow sea were located adjacent to an arid landscape subject to occasional rainstorms, sand particles would be washed into the sea and spread across its bottom by water currents and the force of gravity. As this process continued, each layer of sand would press down on the layers beneath it, squeezing and solidifying the sandy mass that had been deposited only a few thousand years before. When this seabed was raised and folded into mountains by shifts in the earth's crust, the method by which at least some of the rocks were formed was betrayed by the presence of layers of sandstone.
About 300 million years ago during what earth historians call the Carboniferous period of the Paleozoic Era, conditions present in most existing land areas were such that unusual sedimentary sequences were created. Heavily vegetated and thick, swampy regions were drowned and covered with another layer of sediment. In some cases, the organic matter came to be represented in liquid form, trapped between folds of impermeable rock and eventually drawn off as petroleum. Most of these petroleum deposits are found in conjunction with another by-product of the period - natural gas. In other cases, the organic matter became solid layers of coal that were sometimes only centimeters thick but occasionally found dozens of meters thick.
In North America, vast regions are underlaid by sediments formed during the Carboniferous period. These areas where coal, oil, or natural gas might be found are located in the interior and Great Plains, sections of the Gulf coastal plain, portions of the Pacific mountains and valleys, the Arctic rimland, and in folded and broken form along the western margins of the Appalachian Highlands and into the eastern Rockies.
Large deposits of mineral fuels have been identified across extensive portions of these sedimentary lowlands. The most important coal deposits in America have been mined in the more rugged Appalachian field. Mines throughout this nearly continuous field in eastern Kentucky, West Virginia, and western Pennsylvania were the earliest to be brought into production, and they continue to supply over half of America's coal needs.
Until recently, much of the remaining coal mined in the United States has been obtained from the Eastern Interior Field, which underlies most of Illinois and extends into western Indiana and western Kentucky. Although some of the Eastern Interior Field's coal has been used in iron and steel production, its higher sulfur content has restricted most use to heating and electric-power generation.
The Western Interior Field is also large, located under Iowa and Missouri with a narrowing extension southward into eastern Oklahoma. The coal found in this field is of slightly poorer quality than that found in the eastern fields and has only recently begun to be mined.
There are many small and a few large bituminous deposits scattered through and along the eastern margins of the Rocky Mountains. Extensive deposits in Wyoming and Montana have come into production in the last two decades. There are also several extensive fields of lignite (brown coal) in the northern Great Plains.
Scattered deposits of petroleum and natural gas are found throughout the Appalachian coal field. Southern Illinois and south-central Michigan produce some petroleum, as do scattered sites across the northern Great Plains and the northern Rockies.
Easily the most important petroleum fields, however, have been those in the southern plains, along the Gulf coast, and in southern California. One great arc of producing wells is located along the full length of the Texas and Louisiana coasts. Another slightly broken arc extends from central Kansas south through Oklahoma and westward across central Texas to New Mexico. Between and beyond these two large areas lie two more fields of great importance, the East Texas field and the Panhandle field in northwest Texas. Separate from these fields but also of major importance are those located in southern California. In the mid-1960s, exploitation of deposits of petroleum and natural gas was begun along the north Alaska slope.
Metamorphic rock is formed in a quite different manner than sedimentary rock. Under the tremendous pressure exerted through the gradual deformation of the earth's crust, the internal structure of previously formed rocks can be metamorphosed, or changed. So great is the pressure exerted over thousands of years and so great is the heat generated that the very molecular structure of the rock is altered. This transformation indicates why metallic minerals in economically extractable quantities are located most often in areas of metamorphic rock.
Many of the mining sites for early exploitation of the metallic minerals were located near the margins of the Canadian Shield. The pattern of mineral production follows a long arc extending from the North Atlantic and St. Lawrence River estuary across the Great Lakes and northward through Canada to the Arctic Ocean. The arc continues on both sides of Lake Superior: in northern Michigan, Wisconsin, and Minnesota with copper and iron.
A second zone of metamorphic rock is located along the eastern Appalachian Mountains. Copper and iron were important minerals found locally by New England colonists.
A third and extensive region of metallic minerals is formed by the western mountains. Scatter deposits of gold and silver, a few of them rich, drew prospectors and mining companies to isolated locations from south of the Mexican border to central Alaska. Of great industrial importance are the large deposits of copper, zinc, lead, molybdenum, and uranium found in this western region, as well as smaller deposits of tungsten, chromite, manganese, and other minerals.
It should not be assumed that America's industrial requirements are met fully by the tremendous variety of minerals found in these three zones of metamorphic rock. A few minerals needed by modern industry (for example, tin, manganese, and high-grade bauxite for aluminum) have not been located in America in sufficient quantities to satisfy domestic needs. In addition, the growth of industrial capacity has been matched by a growth in demand for many minerals. Nevertheless, few countries have equaled or even approached the original quantity and diversity of metallic minerals and mineral fuels located in the United States.
This abundance of minerals has been critical in assisting the development of the immense American manufacturing-industrial complex.
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.
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 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 draft lets ocean vessels penetrate 1,300 miles into the continent, as far as Duluth, Minnesota, and Chicago.
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 to the Gulf, few of these coastal rivers measure more than 300 miles, 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.
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 colored 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.
Due to its large size and wide range of geographic features, the United States contains examples of nearly every global climate. The climate is temperate in most areas, tropical in Hawaii and southern Florida, polar in Alaska, semiarid in the Great Plains west of the 100th meridian, Mediterranean in coastal California and arid in the Great Basin. Its comparatively generous climate contributed (in part) to the country's rise as a world power, with infrequent severe drought in the major agricultural regions, a general lack of widespread flooding, and a mainly temperate climate that receives adequate precipitation.
The main influence on U.S. weather is the polar jet stream, which brings in large low pressure systems from the northern Pacific Ocean. The Cascade Range, Sierra Nevada, and Rocky Mountains pick up most of the moisture from these systems as they move eastward. Greatly diminished by the time they reach the High Plains, much of the moisture has been sapped by the orographic effect as it is forced over several mountain ranges. However, once it moves over the Great Plains, uninterrupted flat land allows it to reorganize and can lead to major clashes of air masses. In addition, moisture from the Gulf of Mexico is often drawn northward. When combined with a powerful jet stream, this can lead to violent thunderstorms, especially during spring and summer. Sometimes during late winter and spring these storms can combine with another low pressure system as they move up the East Coast and into the Atlantic Ocean, where they intensify rapidly. These storms are known as Nor'easters and often bring widespread, heavy snowfall to the Mid-Atlantic and New England. The uninterrupted flat grasslands of the Great Plains also leads to some of the most extreme climate swings in the world. Temperatures can rise or drop rapidly and winds can be extreme, and the flow of heat waves or Arctic air masses often advance uninterrupted through the plains.
The Great Basin and Columbia Plateau (the Intermontane Plateaus) are arid or semiarid regions that lie in the rain shadow of the Cascades and Sierra Nevada. Precipitation averages less than 15 inches (38 cm). The Southwest is a hot desert, with temperatures exceeding 100°F (38°C) for several weeks at a time in summer. The Southwest and the Great Basin are also affected by the monsoon from the Gulf of California from July-September, which brings localized but often severe thunderstorms to the region.
Much of California consists of a Mediterranean climate, with sometimes excessive rainfall from October-April and nearly no rain the rest of the year. In the Pacific Northwest rain falls year-round, but is much heavier during winter and spring. The mountains of the west receive abundant precipitation and very heavy snowfall. The Cascades are one of the snowiest places in the world, with some places averaging over 600 inches (1,520 cm) of snow annually, but the lower elevations closer to the coast receive very little snow. Another significant (but localized) weather effect is lake-effect snow that falls south and east of the Great Lakes, especially in the hilly portions of the Upper Peninsula of Michigan and on the Tug Hill Plateau in New York. The lake effect dumped well over 5 feet of snow in the Buffalo, New York area throughout the 2006-2007 winter The Wasatch Front and Wasatch Range in Utah can also receive significant lake effect accumulations off of the Great Salt Lake.
In northern Alaska, tundra and arctic conditions predominate, and the temperature has fallen as low as -80°F (-62°C). On the other end of the spectrum, Death Valley, California once reached 134°F (56.7°C), the second-highest temperature ever recorded on Earth.
On average, the mountains of the western states receive the highest levels of snowfall on Earth. The greatest annual snowfall level is at Mount Rainier in Washington, at 692 inches (17,580 mm); the record there was 1,122 inches (28,500 mm) in the winter of 1971-72. This record was broken by the Mt. Baker Ski Area in northwestern Washington which reported 1,140 inches of snowfall for the 1998-99 snowfall season. Other places with significant snowfall outside the Cascade Range are the Wasatch Mountains, near the Great Salt Lake, and the Sierra Nevada, near Lake Tahoe. In the east, while snowfall does not approach western levels, the region near the Great Lakes and the mountains of the Northeast receive the most. Along the northwestern Pacific coast, rainfall is greater than anywhere else in the continental U.S., with Quinault Ranger in Washington having an average of 137 inches (3480 mm). Hawaii receives even more, with 460 inches (11,680 mm) measured annually on Mount Waialeale, in Kauai. The Mojave Desert, in the southwest, is home to the driest locale in the U.S. Yuma, Arizona, has an average of 2.63 inches (66.8 mm) of precipitation each year.
In central portions of the U.S., tornadoes are more common than anywhere else on Earth and touch down most commonly in the spring and summer. Deadly and destructive hurricanes occur almost every year along the Atlantic seaboard and the Gulf of Mexico. The Appalachian region and the Midwest experience the worst floods, though virtually no area in the U.S. is immune to flooding. The Southwest has the worst droughts; one is thought to have lasted over 500 years and to have decimated the Anasazi people. The West is affected by large wildfires each year.
The United States is affected by a large variety of natural disasters yearly. Although severe drought is rare, it has occasionally caused major problems, such as during the Dust Bowl (1931-1942), which coincided with the Great Depression. Farmland failed throughout the Plains, entire regions were virtually depopulated, and dust storms ravaged the land. More recently, the western U.S. experienced widespread drought from 1999-2004, and signs of a major, long-term drought across the Great Plains have developed. In the past year, drought has spread from the Southern Plains westward through the Southwest and east along the Gulf Coast to Florida.
The United States also experience, by a large margin, the most frequent and powerful tornadoes in the world. The Great Plains and Midwest, due to the contrasting air masses, sees frequent severe thunderstorms and tornado outbreaks during spring and summer. The strip of land from north Texas north to Kansas and east into Tennessee is known as Tornado Alley, where many houses have tornado shelters and many towns have tornado sirens. Another natural disaster that frequents the country are hurricanes, which can hit anywhere along the Gulf Coast or the Atlantic Coast as well as Hawaii in the Pacific Ocean. Particularly at risk are the central and southern Texas coasts, the area from southeastern Louisiana east to the Florida Panhandle, the east coast of Florida, and the Outer Banks of North Carolina, although any portion of the coast could be struck. Hurricane season runs from June 1 to November 30, with a peak from mid-August through early October. Some of the more devastating hurricanes have included the Galveston Hurricane of 1900, Hurricane Andrew in 1992, and Hurricane Katrina in 2005. The remnants of tropical cyclones from the Eastern Pacific also occasionally impact the southwestern United States, bringing sometimes heavy rainfall.
Like drought, widespread severe flooding is rare. Some exceptions include the Great Mississippi Flood of 1927, the Great Flood of 1993, and widespread flooding and mudslides caused by the 1982-1983 El Niño event in the western United States. The Great Flood of 1993 was among the most costly and devastating ever to occur in the United States, with $15 billion in damages. The flooded area totaled around 30,000 square miles (80,000 square kilometers). Localized flooding can, however, occur anywhere, and mudslides from heavy rain can cause problems in any mountainous area, particularly the Southwest. Large stretches of desert shrub in the west can fuel the spread of wildfires. The narrow canyons of many mountain areas in the west and severe thunderstorm activity during the monsoon season in summer leads to sometimes devastating flash floods as well, while Nor'Easter snowstorms can bring activity to a halt throughout the Northeast (although heavy snowstorms can occur almost anywhere).
The West Coast of the continental United States and areas of Alaska (including the Aleutian Islands, the Alaskan Peninsula and southern Alaskan coast) make up part of the Pacific Ring of Fire, an area of heavy tectonic and volcanic activity that is the source of 90% of the world's earthquakes. The American Northwest sees the highest concentration of active volcanoes in the United States, in Washington, Oregon and northern California along the Cascade Mountains. There are several active volcanoes located in the islands of Hawaii, including Kilauea in ongoing eruption since 1983, but they do not typically adversely affect the inhabitants of the islands. There has not been a major life-threatening eruption on the Hawaiian islands since the 17th century. Volcanic eruptions can occasionally be devastating, such as in the 1980 eruption of Mount St. Helens in Washington.
The Ring of Fire makes California and southern Alaska particularly vulnerable to earthquakes. Earthquakes can cause extensive damage, such as the 1906 San Francisco Earthquake or the 1964 Good Friday Earthquake near Anchorage, Alaska. California is well known for seismic activity, and requires large structures to be earthquake resistant to minimize loss of life and property. The San Francisco earthquake of 1906 and resulting fire is remembered as one of the worst natural disasters in the history of the United States. The death toll from the earthquake and resulting fire, estimated to be above 3,000, is the greatest loss of life from a natural disaster in California's history. Two of the most destructive earthquake in recent California history were the 1989 Loma Prieta earthquake, which affected the San Francisco bay area, and the 1994 Northridge earthquake, which hit the Greater Los Angeles area. Both quakes caused widespread damage and deaths in their respective regions. Outside of devastating earthquakes, California experiences minor earthquakes on a regular basis.
Other natural hazards include tsunamis around Pacific Basin, mud slides in California, forest fires in the west, and permafrost in northern Alaska, a major impediment to development. For example, the October 2007 California wildfires destroyed at least 1,500 homes and over 500,000 acres (2,000 square kilometers, or about 770 square miles) of land burned from Santa Barbara County to the U.S.-Mexico border. Nine people died as a direct result of the fires; 85 others were injured, including at least 61 firefighters.
There is an international interest in issues surrounding climate change in the United States due to its dominance in world affairs and its high level of greenhouse gas emissions. It is one of the few countries that has not ratified the Kyoto Protocol.
The soil of a place owes its characteristics to such things as the parent rock material, climate, topography, and decaying plants and animals. Hundreds of different types of soil result from the interaction of these elements. Any particular soil is unique because of its mix of properties (such as color and texture) and composition (including organic content and the action of soil colloids).
Colloids are small soil particles. Their properties and influences on soil are complex and often important. Soil acidity (or alkalinity), for example, is a result of the alteration and integration of soil colloids. Acid soils are characteristic of cold, moist climates; alkaline soils typically are found in dry areas. Most soils of the major agricultural zones of the eastern United States are moderately to strongly acidic. Lime must be added periodically to neutralize that acidity before these soils can be used to produce most row crops.
Color is perhaps the most obvious soil property. A dark color usually indicates an abundance of organic materials, and red, the presence of iron compounds. Generally, however, color is a result of the soil-forming processes. For example, the pale-gray soil of the northern needleleaf forest results from the leaching of organic matter and minerals from the soil's surface layer.
Soil texture, which determines a soil's ability to retain and transmit water, refers to the proportion of particles of different size in the soil. Sand is the coarsest measure of soil texture, silt is intermediate, and clay is the finest. Soils called "loams" contain substantial proportions of each of the three particle grades and are considered best. They are fine enough to hold moisture yet are not so fine that they cannot easily take up water.
The U.S. Department of Agriculture has developed a soil classification system that indicates the most important soil types for an area of the country. Aridisols, found mostly in the Southwest, gain their name from arid. These soils of dry climates are low in organic content and have little agricultural value. Spodosols generally develop in cool, moist climates, although they are found in northern Florida. They are quite acidic and low in nutrients, and are of agricultural value only for acid-loving crops. Tundra soils, which also have little agricultural value, are associated with a cold, moist climate such as Alaska. The soil is shallow, frequently water saturated, and with a subsurface of perennially frozen ground. Highland soils, found in West Virginia, Utah, and Alaska, are little developed and agriculturally worthless.
Mollisols are grassland soils of the semiarid and subhumid climates of the Central, North Central, and Pacific Northwest sections of the country. They are thick dark brown to black and have a loose texture and high-nutrient content. They are among the most naturally fertile soils in the world and produce most of America's cereals.
Alfisols are second only to mollisols in agricultural value. They are soils of the mid-latitude forest and the forest-grassland boundaries. They are very much "middle" soils in a climatic sense. They are located in areas moist enough to allow for the accumulation of clay particles but not so moist as to create a heavily leached or weathered soil.
Alfisols are divided into three categories, each with its own characteristic climatic association. Udalfs are soils of the deciduous forests of the Middle West. Somewhat acidic, they are nevertheless highly productive when lime is used to reduce the acidity. Ustalfs, found in warmer areas with a strong seasonal variation in precipitation, are most common in Texas and Oklahoma. They are highly productive if irrigated. Xeralfs are soils of cool, moist winters and hot, dry summers. Found in central and southern California, they too are highly productive.
Ultisols represent the ultimate stage of weathering and soil formation in the United States. They develop in areas with abundant precipitation and a long frost-free period, such as the South. Particle size is small, and much of the soluble material and clay has been carried downward. These soils can be productive, but high acidity, leaching, and erosion are often problems.
Entisols are recent soils, too young to show the modifying effects of their surroundings. They are widely scattered and of many types, from the Sand Hills of Nebraska to the alluvial floodplains of the Mississippi River Valley. The agricultural potential of entisols varies, but the alluvial floodplain soils, drawn from the rich upper layers of upstream soils, are among America's most productive.
A coniferous forest of white and red pine, hemlock, spruce, jack pine, and balsam fir extends interruptedly in a narrow strip near the Canadian border from Maine to Minnesota and southward along the Appalachian Mountains. There may be found smaller stands of tamarack, spruce, paper birch, willow, alder, and aspen or poplar. Southward, a transition zone of mixed conifers and deciduous trees gives way to a hardwood forest of broad-leaved trees. This forest, with varying mixtures of maple, oak, ash, locust, linden, sweet gum, walnut, hickory, sycamore, beech, and the more southerly tulip tree, once extended uninterruptedly from New England to Missouri and eastern Texas. Pines are prominent on the Atlantic and Gulf coastal plain and adjacent uplands, often occurring in nearly pure stands called pine barrens. Pitch, longleaf, slash, shortleaf, Virginia, and loblolly pines are commonest. Hickory and various oaks combine to form a significant part of this forest, with magnolia, white cedar, and ash often seen. In the frequent swamps, bald cypress, tupelo, and white cedar predominate. Pines, palmettos, and live oaks are replaced at the southern tip of Florida by the more tropical royal and thatch palms, figs, satinwood, and mangrove.
The grasslands occur principally in the Great Plains area and extend westward into the intermontane basins and benchlands of the Rocky Mountains. Numerous grasses such as buffalo, grama, side oat, bunch, needle, and wheat grass, together with many kinds of herbs, make up the plant cover. Coniferous forests cover the lesser mountains and high plateaus of the Rockies, Cascades, and Sierra Nevada. Ponderosa (yellow) pine, Douglas fir, western red cedar, western larch, white pine, lodgepole pine, several spruces, western hemlock, grand fir, red fir, and the lofty redwood are the principal trees of these forests. The densest growth occurs west of the Cascade and Coast ranges in Washington, Oregon, and northern California, where the trees are often 100 feet or more in height. There the forest floor is so dark that only ferns, mosses, and a few shade-loving shrubs and herbs may be found.
The alpine tundra, located in the coterminous United States only in the mountains above the limit of trees, consists principally of small plants that bloom brilliantly for a short season. Sagebrush is the most common plant of the arid basins and semideserts west of the Rocky Mountains, but juniper, nut pine, and mountain mahogany are often found on the slopes and low ridges. The desert, extending from southeastern California to Texas, is noted for the many species of cactus, some of which grow to the height of trees, and for the Joshua tree and other yuccas, creosote bush, mesquite, and acacias.
The United States is rich in the variety of its native forest trees, some of which, as the species of sequoia, are the most massive known. More than 1,000 species and varieties have been described, of which almost 200 are of economic value, either because of the timber and other useful products that they yield or by reason of their importance in forestry.
Besides the native flowering plants, estimated at between 20,000 to 25,000 species, many hundreds of species introduced from other regions - chiefly Europe, Asia, and tropical America - have become naturalized. A large proportion of these are common annual weeds of fields, pastures, and roadsides. In some districts these naturalized "aliens" constitute 50 percent or more of the total plant population.
With most of North America, the United States lies in the Nearctic faunistic realm, a region containing an assemblage of species similar to Eurasia and North Africa but sharply different from the tropical and subtropical zones to the south. Main regional differences correspond roughly with primary climatic and vegetal patterns. Thus, for example, the animal communities of the Dry West differ sharply from those of the Humid East and from those of the Pacific Coast. Because animals tend to range over wider areas than plants, faunal regions are generally coarser than vegetal regions and harder to delineate sharply.
The animal geography of the United States, however, is far from a natural pattern, for European settlement produced a series of environmental changes that grossly altered the distribution of animal communities. First, many species were hunted to extinction or near extinction, most conspicuously, perhaps, the American bison, which ranged by the millions nearly from coast to coast but now rarely lives outside of zoos and wildlife preserves. Second, habitats were upset or destroyed throughout most of the country - forests cut, grasslands plowed and overgrazed, and migration paths interrupted by fences, railroads, and highways. Third, certain introduced species found hospitable niches and, like the English sparrow, spread over huge areas, often preempting the habitats of native animals. Fourth, though their effects are not fully understood, chemical biocides such as DDT were used for so long and in such volume that they are believed at least partly responsible for catastrophic mortality rates among large mammals and birds, especially predators high on the food chain. Fifth, there has been a gradual northward migration of certain tropical and subtropical insects, birds, and mammals, perhaps encouraged by gradual climatic warming. In consequence, many native animals have been reduced to tiny fractions of their former ranges or exterminated completely, while other animals, both native and introduced, have found the new anthropocentric environment well suited to their needs, with explosive effects on their populations. The coyote, opossum, armadillo, and several species of deer are among the animals that now occupy much larger ranges than they once did.
Arrangement of the account of the distribution of the fauna according to the climatic and vegetal regions has the merit that it can be compared further with the distribution of insects and of other invertebrates, some of which may be expected to fall into the same patterns as the vertebrates, while others, with different modes or different ages of dispersal, have geographic patterns of their own.
The transcontinental zone of coniferous forest at the north, the taiga, and the tundra zone into which it merges at the northern limit of tree growth are strikingly paralleled by similar vertical zones in the Rockies, and on Mount Washington in the east, where the area above the timberline and below the snow line is often inhabited with tundra animals like the ptarmigan and the white Parnassius butterflies, while the spruce and other conifers below the timberline form a belt sharply set off from the grassland or hardwood forest or desert at still lower altitudes.
A whole series of important types of animals spread beyond the limits of such regions or zones, sometimes over most of the continent. Aquatic animals, in particular, may live equally in forest and plains, in the Gulf states, and at the Canadian border. Such widespread animals include the white-tailed (Virginia) deer and black bear, the puma (though only in the remotest parts of its former range) and bobcat, the river otter (though now rare in inland areas south of the Great Lakes) and mink, and the beaver and muskrat. The distinctive coyote ranges over all of western North America and eastward as far as Maine. The snapping turtle ranges from the Atlantic coast to the Rocky Mountains.
In the northern coniferous forest zone, or taiga, the relations of animals with European or Eurasian representatives are numerous, and this zone is also essentially circumpolar. The relations are less close than in the Arctic forms, but the moose, beaver, hare, red fox, otter, wolverine, and wolf are recognizably related to Eurasian animals. Even some fishes, like the whitefishes (Coregonidae), the yellow perch, and the pike, exhibit this kind of Old World-New World relation. A distinctively North American animal in this taiga assemblage is the Canadian porcupine.
The hardwood forest area of the eastern and the southeastern pinelands compose the most important of the faunal regions within the United States. A great variety of fishes, amphibians, and reptiles of this region have related forms in East Asia, and this pattern of representation is likewise found in the flora. This area is rich in catfishes, minnows, and suckers. The curious ganoid fishes, the bowfin and the gar, are ancient types. The spoonbill cat, a remarkable type of sturgeon in the lower Mississippi, is represented elsewhere in the world only in the Yangtze in China. The Appalachian region is headquarters for the salamanders of the world, with no less than seven of the eight families of this large group of amphibians represented; no other continent has more than three of the eight families together. The eellike sirens and amphiumas (congo snakes) are confined to the southeastern states. The lungless salamanders of the family Plethodontidae exhibit a remarkable variety of genera and a number of species centring in the Appalachians. There is a great variety of frogs, and these include tree frogs whose main development is South American and Australian. The emydid freshwater turtles of the southeast parallel those of East Asia to a remarkable degree, though the genus Clemmys is the only one represented in both regions. Much the same is true of the water snakes, pit vipers, rat snakes, and green snakes, though still others are peculiarly American. The familiar alligator is a form with an Asiatic relative, the only other living true alligator being a species in central China.
In its mammals and birds the southeastern fauna is less sharply distinguished from the life to the north and west and is less directly related to that of East Asia. The forest is the home of the white-tailed deer, the black bear, the gray fox, the raccoon, and the common opossum. The wild turkey and the extinct hosts of the passenger pigeon were characteristic. There is a remarkable variety of woodpeckers. The birdlife in general tends to differ from that of Eurasia in the presence of birds, like the tanagers, American orioles, and hummingbirds, that belong to South American families. Small mammals abound with types of the worldwide rodent family Cricetidae, and with distinctive moles and shrews.
Most distinctive of the grassland animals proper is the American bison, whose nearly extinct European relative, the wisent, is a forest dweller. The most distinctive of the American hoofed animals is the pronghorn, or prongbuck, which represents a family intermediate between the deer and the true antelopes in that it sheds its horns like a deer but retains the bony horn cores. The pronghorn is perhaps primarily a desert mammal, but it formerly ranged widely into the shortgrass plains. Everywhere in open country in the West there are conspicuous and distinctive rodents. The burrowing pocket gopher is peculiarly American, rarely seen making its presence known by pushed-out mounds of earth. The ground squirrels of the genus Citellus are related to those of Central Asia, and resemble them in habit; in North America the gregarious prairie dog is a closely related form. The American badger, not especially related to the badger of Europe, has its headquarters in the grasslands. The prairie chicken is a bird distinctive of the plains region, which is invaded everywhere by birds from both the east and the west.
The Southwestern deserts are a paradise for reptiles. Distinctive lizards such as the poisonous Gila monster abound, and the rattlesnakes, of which only a few species are found elsewhere in the United States, are common there. Desert reptile species often range to the Pacific Coast and northward into the Great Basin. Noteworthy mammals are the graceful bipedal kangaroo rat (almost exclusively nocturnal), the ring-tailed cat, a relative of the raccoon, and the piglike peccary.
The Rocky Mountains and other western ranges afford distinctive habitats for rock- and cliff-dwelling hoofed animals and rodents. The small pikas, related to the rabbit, inhabit talus areas at high altitudes as they do in the mountain ranges of East Asia. Marmots live in the Rockies as in the Alps. Every western range formerly had its own race of mountain sheep. At the north the Rocky Mountain goat lives at high altitudes - it is more properly a goat antelope, related to the takin of the mountains of western China. The dipper, remarkable for its habit of feeding in swift-flowing streams, though otherwise a bird without special aquatic adaptations, is a Rocky Mountain form with relatives in Asia and Europe.
In the Pacific region the extremely distinctive primitive tailed frog Ascaphus, which inhabits icy mountain brooks, represents a family by itself, perhaps more nearly related to the frogs of New Zealand than to more familiar types. The Cascades and Sierras form centers for salamanders of the families Ambystomoidae and Plethodontidae second only to the Appalachians, and there are also distinctive newts. The burrowing lizards, of the well-defined family Anniellidae, are found only in a limited area in coastal California. The only family of birds distinctive of North America, that of the wren-tits, Chamaeidae, is found in the chaparral of California. The mountain beaver, or sewellel (which is not at all beaverlike), is likewise a type peculiar to North America, confined to the Cascades and Sierras, and there are distinct kinds of moles in the Pacific area.
The mammals of the two coasts are strikingly different, though true seals (the harbor seal and the harp seal) are found on both. The sea lions, with longer necks and with projecting ears, are found only in the Pacific--the California sea lion, the more northern Steller's sea lion, and the fur seal. On the East Coast the larger rivers of Florida are inhabited by the Florida manatee, or sea cow, a close relative of the more widespread and more distinctively marine West Indian species.
The land area of the state of Hawaii consists of the tops of a chain of emerged volcanic mountains that form eight major islands and 124 islets, stretching in a 1,500-mile crescent from Kure Island in the west to the island of Hawaii in the east, with a combined land area of 6,471 square miles (16,759 square kilometers). With the exception of Midway, a U.S. naval reservation near the western end of the archipelago, the leeward coral atolls and central lava islets - forming a total of only 3 1/4 square miles - are in the Hawaiian Islands National Wildlife Refuge. The eight major islands at the eastern end of the chain are, from west to east, Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe, Maui, and Hawaii.
Volcanic activity has become dormant, with the exception of the volcanoes of Mauna Loa and Kilauea on the easternmost and largest island, Hawaii, where spectacular eruptions and lava flows take place from time to time. The highest Hawaiian mountains are Mauna Kea and Mauna Loa, reaching 13,796 feet (4,205 meters) and 13,678 feet (4,169 meters) above sea level, respectively.
There is little erosion in the geologically young areas, where the terrain is domelike and the volcanic craters are clearly defined. In the older areas the mountains have been shaped and eroded by the action of sea, rain, and wind. Their aspects thus include sharp and craggy silhouettes; abrupt, vertically grooved cliffs pocked with caves; deep valleys; collapsed craters (calderas); and coastal plains. The powerful Pacific surf, churning and crashing against the fringing coral shelves and the lava shorelines, has carried minute shells onto the shore and reduced coral and large shells to sand, creating the state's famous expanses of beach.
Volcanic ash, gravel, rotted vegetation, crumbling lava, and windblown sand and dust all help to make up the alluvial, residual, and organic soils found in various depths and densities in valley floors, the regions between mountain ranges, and along the shores. Oxidation of iron causes a ubiquitous bright red soil and rock strata. The iron content is, however, insufficient for smelting, and there are no coal or petroleum deposits.
Because the topography is generally abruptly descending or sloping, there are few surface collecting basins or lakes. Excess rainfall seeps through porous mountain areas to collect in subterranean chambers and layers retained by less permeable lava and ash beds, or it is prevented by underlying salt water from seeping to the sea. The resultant artesian water supply is tapped for use in irrigation and also for human consumption.
Heavy rainfall in mountainous areas produces an extremely voluminous runoff, which is responsible for the erosion that forms the numerous grooves, ridges, and V-shaped valleys characteristic of the older volcanic islands such as Kauai and Oahu. The action of rain combined with waves has had a particularly dramatic effect on the more exposed windward sections of the islands.
Hawaii lies just below the tropic of Cancer, and its mild tropical climate is considered by many people to be the world's ideal. Although often humid by U.S. mainland standards, temperatures are conditioned by the northeast trade winds, which prevail most of the year. Blowing for many miles over the open Pacific, the trades pass along the great reservoir's stabilizing influence, to make living on the islands delightfully comfortable.
The average temperature in downtown Honolulu is 72°F (22°C) in the coolest month and 78°F (26°C) in the warmest, with extremes from 57°F (14°C) to 88°F (31°C) having been recorded there. The average water temperatures off Waikiki Beach, near Honolulu, range from 75°F (24°C) in late February to 79°F (26°C) in late September. Mountainous regions are considerably cooler, especially during the winter months, when there can be frost; a temperature of 1.4°F (-17°C) has been recorded on the summit of Mauna Kea, and winter snows frequently blanket the crests of Mauna Kea and Mauna Loa.
Rainfall variations throughout the state are dramatic. Mount Waialeale, on the island of Kauai, is often called the wettest spot on Earth, with an annual average rainfall of 444 inches (11,280 millimeters) over a 60-year period, the highest long-term median on record. The driest area is at Kawaihae, on the island of Hawaii, where the average annual rainfall is only 8.7 inches (220 millimeters). The average yearly rainfall in Honolulu is 23 inches, and in Hilo it is 129 inches.
As moisture-laden air is carried over the islands, most frequently by the trade winds, it is apt to condense, form cap clouds, and dissipate against the shores and mountains of the windward coasts, which are therefore more lush in foliage than the leeward coasts.
The seeds of endemic plant species were carried to Hawaii by birds, winds, or currents and tides, bringing about extensive forestation, shrubbery, and grasslands, where soil and precipitation were favorable. Since the first Polynesian settlement a tremendous variety of food and ornamental plant life from many parts of the world has been introduced. Food plants grown commercially or in backyards for home consumption include sugarcane, pineapples, papayas, bananas, mangoes, guavas, lichee, coconuts, avocados, breadfruit, macadamia nuts, limes, passion fruit, taros, and tamarinds. Nearly all varieties of common garden vegetables are raised in the islands, and flowers abound all year.
Endemic birds, long isolated from others of their kind, have taken on certain characteristics of their own. These include the nene (Hawaiian goose), the Hawaiian stilt, and a variety of small forest birds. Some species have become extremely rare, but as the result of an increased environmental awareness, great strides have been taken to preclude their extinction. Seabirds nest in profusion on the western islands of the archipelago and to a far lesser extent among the major eastern islands. There has been considerable importation of birdlife. Quantities of mynas, sparrows, cardinals, and doves live in the trees in both urban and country areas. Every fall the small golden plover make an awe-inspiring, nonstop 3,000-mile (4,800-kilometer) flight from Alaska to Hawaii, where they spend the winter, together with ducks from Alaska, Canada, and the northwestern United States.
Wild animal life includes mongooses, rats, frogs, toads, and, in the more remote regions of some of the islands, deer, sheep, pigs, and goats. The insect population is multitudinous, and marine life abounds in Hawaiian waters. The green sea turtle (Chelonia mydas), locally known as Honu, is one of the symbols of Hawaii.
The immense area of Alaska has a great variety of physical characteristics. Nearly one-third of the state lies within the Arctic Circle and has perennially frozen ground (permafrost) and treeless tundra. The southern coast and the panhandle at sea level are fully temperate regions. In these latter and in the adjoining Canadian areas, however, lies the world's largest expanse of glacial ice outside Greenland and Antarctica. Off the extreme western end of the Seward Peninsula, Little Diomede Island, part of Alaska, lies in the Bering Strait only 2.5 miles (four kilometers) from Soviet-owned Big Diomede Island; both countries have shown a tacit tolerance of unintentional airspace violations, which are common in bad weather.
Alaska is composed of nine distinct physiographic and environmental regions. Much of the mainland panhandle region, a narrow strip of land 25 to 50 miles wide lying east and south of the St. Elias Mountains, is composed of the Boundary Ranges. There are several large icefields, and the peaks include Mount St. Elias (18,009 feet), from whose summit the Alaska-Yukon border swings due north. The western extension of this mountain chain is the Chugach Range, a giant arc at the northernmost edge of the Gulf of Alaska. Many remote valleys and high ridges are still unexplored, and the relief and glaciation inhibit exploitation. The coast is characterized by frequent and intense oceanic storm systems that have produced dense rain forests on the coastal mountain flanks. In the valleys rivers produce devastating annual floods often associated with excessive snowmelt and glacial meltwaters.
The region of the south coastal archipelago and the Gulf of Alaska islands includes the Alexander Archipelago in the panhandle region, with 11,000 islands, plus Kodiak Island and its satellites south of Cook Inlet. These islands, extensions of the southern region, are lower, less rugged, and less glaciated. All receive heavy rain and are affected by waters warmed by the Kuroshio Current.
The Aleutian region includes the narrow Alaska Peninsula, which forms the south shoreline of Bristol Bay, and the 1,100-mile-long Aleutian chain that separates the North Pacific from the Bering Sea. The chain includes 14 large islands, 55 significant but smaller ones, and thousands of islets. The largest are Unimak, Unalaska, and Umnak. On the occasionally clear summer days, active volcanoes and such glacier-covered peaks as symmetrical Shishaldin Volcano (9,372 feet [2,857 meters]) on Unimak can be seen. Such magnificent views represent the Aleutians at their scenic best. Usually, however, the weather is wet and stormy, the winds horizontal and cutting, and the fog all-pervading.
The broad Alaska Range region connects the Aleutian Range across the southern third of mainland Alaska to the Wrangell Mountains, which abut against the vast complex of the St. Elias Mountains. The Wrangell Mountains have large active volcanoes and high valley glaciers. The flanks of this subarctic range are largely tundra-covered.
The low-lying interior basin region between the Alaska Range in the north and the Chugach-Wrangell-St. Elias mountains to the south and east enjoys a relatively temperate climate. The lower valleys contain good farmlands, and it is there that most of the people of Alaska live.
The central plains and tablelands of interior Alaska constitute a vast region west and north of the Alaska Range; they reach as far north as the Brooks Range. The area is rolling and dissected by numerous streams tributary to the Yukon and Kuskokwim rivers. The plains extend from the Canadian border to Norton Sound, the Seward Peninsula, the Yukon delta, and south to the northern rim of Bristol Bay on the Bering Sea. The region is characterized by river flats and truncated upland tablelands. With abundant game, it is an important nesting ground for waterfowl, including great numbers of migrating birds.
A major mountain chain running west to east in the area north of the central plains and extending from the sea nearly to the Yukon border, the Brooks Range gradually slopes northward to a narrow linear coastal plain bordering the Arctic Ocean and westward to lower hills north of Kotzebue Sound. There are a few high Arctic glaciers, and the area is semiarid. The lower flanks and valleys are tundra-covered, with permafrost features.
The coastal lowland north of the Brooks Range, sometimes called the North Slope, is the home of great herds of caribou. The environment is truly polar, with the sea waters along the coast frozen eight months of the year and the ground permanently frozen except for a thin zone of summer melting. It is treeless, and, in summer, grasses and Arctic alpine flowers abound. The National Petroleum Reserve-Alaska is located in the western sector, while the Prudhoe Bay oil fields and part of the Arctic National Wildlife Refuge occupy the eastern sector.
The islands of the Bering Sea represent a small but unique Arctic maritime environment, typified by St. Lawrence, Nunivak, and St. Matthew islands and the Pribilof group. These tundra-covered islands are surrounded by sea ice in winter and serve as protected refuges for the world's largest herds of fur-bearing seals and sea otters, as well as sea lions and walrus. A large herd of domesticated reindeer is tended by Eskimos on Nunivak Island.
Five general climatic zones may be delineated in Alaska, excluding the great mountain ranges.
Southern coastal and southeastern Alaska, the Gulf of Alaska islands, and the Aleutians have average temperature ranges in the summer of 40°F to 60°F (4°C to 16°C) and in the winter of 40°F to 20°F (4°C to -7°C). Rainfall varies locally from 60 to 160 inches (1,525 to 4,065 millimeters), and the panhandle and southern islands are covered with Sitka spruce, hemlock, and other evergreens. The Cordova-Valdez region and parts of the west central panhandle have the state's highest precipitation, 220 inches or more. At Valdez 200 inches of snow is not uncommon. Precipitation is less in the Aleutians, but even there about 250 rainy days occur annually.
The interior basin ranges from 45°F to 75°F (7°C to 24°C) in summer and 20°F to -10°F (-7°C to -23°C) in winter. The region is drier than the coast and only slightly colder in winter, with Anchorage receiving about 15 inches (380 millimeters) of precipitation annually. The pleasant conditions and proximity to the sea have helped to make the area the center of the state's population.
The islands and coast of the Bering Sea have summer temperatures of 40°F to 60°F (4°C to 16°C) and winter temperatures of 20°F to -10°F (-7°C to -23°C). Tempering influences of the Pacific dissipate north of the Pribilof Islands, and Arctic sea ice often reaches this area.
The central plains and uplands range from 45°F to 75°F (7°C to 24°C) in the summer and -10°F to -30°F (-23°C to -34°C) in the winter. Average rainfall is 10 to 20 inches, though less than 10 inches is common.
The ameliorating effects of the Arctic Ocean keep temperatures of the North Slope at 35°F to 55°F (2°C to 13°C) in the summer and -5°F to -20°F (-21°C to -29°C) in the winter - less severe than those of the interior plains. About five inches of precipitation nonetheless remain on the ground as snow for some eight months of the year. The 24-hour sunlight of summer can produce strong buildups of radiant energy, sending temperatures to 90°F (32°C). The deep chill of winter, however, maintains the permafrost character of the High Arctic zone. Ice clogs the northern coast nine months of the year, while ice fog frequently extends southward to Fairbanks.
Forests cover about one-third of Alaska's land area. The Sitka spruce is the state tree. Some areas are almost entirely treeless, with luxuriant grasses. Flowers bloom in great variety and include the forget-me-not (the state flower), the anemone, the lupine, and the paintbrush. Parts of the Interior and of Arctic Alaska contain tundra vegetation.
Alaska is known for the richness of its wildlife, including bears (one kind of which is the Kodiak brown bear, believed to be the largest omnivorous land animal in the world), black-tailed deer, moose, mountain goats, martens, red foxes, minks, wolves, coyotes, otters, beavers, raptors, Dall sheep, and caribou. Coastal animals include sea lions, harbor seals, sea otters, porpoises, and several species of whale. Arctic animals include polar bears, hair seals, and walruses. The state bird is the willow ptarmigan.