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