The US West starts within the tier of Great Plains states west of the Mississippi—Texas, Oklahoma, Kansas, Nebraska and the Dakotas—and includes all of the continent beyond to the Pacific Ocean, as well as the isolated states of Alaska and Hawaii.
The easternmost part of the West is a broad swath of plains that rise gradually from the Gulf of Mexico and Mississippi Valley; the western two-thirds is a vast, corrugated expanse of mountains and plateaus interposed with canyons, valleys and basins of varying size. Although landforms have been more than 2 billion years in the making, the current uplift began 130 million years ago, after the Pacific Plate subducted the North American Plate. The tectonic collision brought island masses crashing into the continent and slowly raised vast uplands from what previously had been a shallow sea. Magma intruded through weakened parts of the earth’s crust, welling over as volcanoes and volcanic plateaus. Faults (most aligned north-south) thrust mountain ranges sharply upward, creating abrupt escarpments; or dropped blocks of land to form grabens, typical of the Great Basin region. Streams, rushing from the rising highlands, cut deep canyons. Sediments flowing to lowlands deposited valley soils and built the Great Plains on the eastern side of the Rocky Mountains.
The subduction zone where the Pacific and North American Plates meet is part of the Ring of Fire, the geologically unstable zone that circles the Pacific Ocean. In the American West, its most volatile indicators are California’s earthquake-prone San Andreas Fault, the volcanic Cascade Range and Alaska’s Aleutian volcanoes; a bit farther east, Yellowstone National Park is the world’s largest geothermal area.
During the early Pleistocene Epoch some 2 million years ago, alpine glaciers covered high-mountain expanses of the Rockies, Cascades, Sierra Nevada and Alaskan coastal ranges, sculpting glacial troughs, hanging valleys, cirques and other features, including Alaska’s deep Pacific fjords. Enormous pluvial lakes covered thousands of square miles in the Great Basin. Cataclysmic floods periodically scoured the inland Pacific Northwest when glacial dams melted and broke. The continental ice sheets diminished the water level of the oceans, exposing a land bridge across the Bering Strait and spurring migration of animals and humans between Asia and North America.
Before about 5000 BC, cool, wet conditions prevailed in what is now the West. A drier, hotter climate subsequently began to dominate. Except for areas of high rainfall along Pacific Coast ranges and the Gulf Coast of Texas, the West today is characterized by aridity, a fact that has colored Western life in innumerable ways. An entirely new legal system called appropriation doctrine was created to manage water resources. The 100th meridian, which runs through the heart of the Great Plains, marks the approximate division between traditional farming and dryland ranching. East of the meridian, annual precipitation averages more than 20in per year; west, rainfall rapidly diminishes, making agriculture impractical without irrigation. Throughout the West today, farming and urban development depend on groundwater withdrawals or massive water-transfer schemes such as the aqueduct that brings Sierra Nevada snowmelt to Los Angeles.
Regions and Climates
Coastal Pacific Northwest
West of the coastal ranges of Oregon and Washington mild summers and wet, cool winters encourage the prolific growth of Douglas fir, spruce, hemlock and other evergreens. Broken only by the Columbia River and Strait of Juan de Fuca between California and Canada, the ranges are drained by short, swift streams. The western slope of the Olympic Peninsula receives more than 150in of annual rainfall, creating rain forests in the canyons beneath 7,965ft Mt. Olympus. East of the Olympics, the Strait of Juan de Fuca opens into the many-isled harbor of Puget Sound. On its eastern shore is Seattle, largest city of the region. The fertile and populous lowland that extends south between the Coast Ranges and Cascades encompasses the city of Portland and, below it, the pastoral Willamette River Valley.
This barrier of volcanic peaks, stretching over 600mi from Canada to California’s Lassen Peak, is breached by the Columbia—largest river in the West and a natural highway between the dry Columbia Plateau and the maritime regions to the west. Two dozen distinct peaks present a line of majestic domes, many capped by brilliant glaciers. Highest are Washington’s Mt. Rainier (14,410ft) and California’s Mt. Shasta (14,162ft). Snowy winters and mild summers, often doused with showers, keep slopes lush with evergreen forests, a boon to timber and recreation industries. The volcanoes are largely dormant, but scientists monitor signs of life that may escalate to explosive eruptions, as at Lassen in 1914 and Mt. St. Helens in 1980. Fewer than 8,000 years ago, the mere blink of an eye in geologic time, massive Mt. Mazama exploded, leaving a gaping crater that filled with snowmelt and rain to form Oregon’s Crater Lake.
The Lava Plateaus
Extensive lava plateaus spread eastward in the rain shadow of the Cascades at 2,000-3,000ft elevation. The Columbia Plateau covers most of eastern Washington and parts of Oregon and Idaho. The Modoc Lava Plateau covers the northeastern corner of California and part of Oregon. Farther east rise several small ranges, including the Wallowas, which form the western wall of enormous, 8,000ft-deep Hells Canyon of the Snake River. Upstream, the Snake River Plain of Idaho and northern Nevada form yet a third extensive lava plateau, tracing its origins not to the Cascades but to clusters of spatter cones and volcanoes south of the Idaho Rockies. The Columbia Plateau and Snake River Plain have proven very fertile under irrigation from the Columbia and Snake Rivers.
Coastal Northern California
The 600mi shoreline of northern and central California embraces a climate that varies from moist and mild (near Oregon) to semiarid Mediterranean. Washed by the Alaska Current, the rough, cold Pacific waters are rich in sea life but dangerous for shipping and swimming. The rugged Coast Ranges are breached only at the Golden Gate, entrance to San Francisco Bay. At several points, the coastal mountains yield to narrow strips of fertile lowlands—the agriculturally rich Napa, Sonoma and Salinas Valleys. Redwood forests grow profusely in the north and intermittently as far south as Big Sur. Drier chaparral, grasses and oaks predominate inland and to the south.
The San Andreas Fault parallels the coastline from Point Reyes (north of San Francisco) to Point Concepcion (northwest of Los Angeles), where it cuts inland.
Coastal Southern California
Shielded from the cold waters of the Alaska Current, the southern California coast is relatively warm and hospitable. Rainfall seldom exceeds 15in per year, giving Santa Barbara, Los Angeles and San Diego an enviable Mediterranean climate, free of winter snows except in nearby mountains. The Los Angeles Basin, California’s largest and most heavily populated coastal plain, is hemmed on the north by the San Gabriel Mountains. These are a part of the Transverse Ranges that follow the San Andreas Fault eastward from the coast, rendering southern California one of the most seismically active regions of the US. East of the basin are the lower Santa Ana Mountains, part of the Peninsular Ranges that run south through Mexico’s Baja Peninsula.
The Sierra Nevada
Running southeasterly almost 400mi from the Cascades, 50-80mi wide, the fault-block Sierra Nevada rises in an abrupt escarpment on the east more than 2mi above the Owens Valley at Mt. Whitney (14,494ft)— highest peak in the contiguous US. The lofty range hinders weather systems, creating a rain shadow to its east. Westward slopes descend gradually through alpine high country, evergreen forests and rugged foothills. Streams and rivers run through great canyons to feed the Central Valley, a fecund plain with the richest agricultural land in the US. Remarkable Yosemite Valley is the best place to see the Sierra’s sculpted peaks and U-shaped glacial valleys. Although it has prodigious winter snowfalls, providing excellent skiing, the Sierra also enjoys plenty of summer sun. Lake Tahoe is a year-round recreation center.
East of the Sierra Nevada and west of the Rocky Mountains, the sagebrush-cloaked Great Basin is a high desert of hot, dry summer days, cool nights and cold winters. It is corrugated with parallel fault-block mountain ranges, some above 13,000ft, divided by valleys known as grabens. Escarpments of 5,000-6,000ft are common; below the 11,200ft Panamint Range, Death Valley falls to 282ft below sea level, lowest point in the Western Hemisphere. No streams that flow into the Great Basin drain to the sea; they evaporate or disappear into lakes or marshy sinks. Utah’s Great Salt Lake is a remnant of prehistoric Lake Bonneville, which once covered some 20,000sq mi. For 150 years, mining towns have boomed and busted in this resource-rich, water-poor region. Except for cities at the foot of well-watered mountains—Reno in the west, Salt Lake City in the east—population density is the lowest of any region of comparable size in the contiguous US.
The nation’s highest plateau region covers 130,000sq mi of Utah, Colorado, New Mexico and Arizona at a mile above sea level. Scattered mountain ranges reach as high as 11,000ft, but the most remarkable features are the myriad canyons carved by the Colorado River and its tributaries—thousands of feet deep, through eons-old rock strata. More than 25 national parks and monuments—including Grand Canyon, Zion, Bryce Canyon and Canyonlands—preserve arches, eroded pinnacles, natural bridges and immense gorges in rainbow hues, all carved by wind and water. With an arid climate and a dearth of fruitful soil, the area is home to such hardy plant species as sagebrush, juniper and piñon pine. The ruins of ancient Puebloan cliff villages may still be seen at Mesa Verde in Colorado, Chaco Canyon in New Mexico, and at the Betatakin and Canyon de Chelly ruins on the Navajo Indian Reservation.
The Desert Southwest
North America’s largest arid region spreads east from California to Texas, containing three distinct deserts with vague transition zones. The mountainous Mojave Desert, which ranges into Death Valley, is home to the Joshua tree, a yucca that may grow 50ft tall. The Mojave fades into the Great Basin north of Las Vegas and meshes with the lower-elevation Sonoran Desert through the Colorado Desert, west of the Colorado River. The Sonoran Desert, which extends through southern Arizona and northwestern Mexico, boasts a profusion of cacti—including the giant saguaro—dependent on intense monsoon cloudbursts that bring temporary relief from summer heat. Winters are mild and sunny, luring thousands of seasonal residents to Arizona. The large Chihuahuan Desert of southern New Mexico, west Texas and northeastern Mexico is a high-elevation desert of parched mountain ranges, extensive grasslands, cold winters and torrid summers. The Rio Grande flows through its heart, scribing the huge hook of Big Bend National Park.
Reaching from New Mexico to Canada, this vast mountain system comprises scores of subranges interposed with high basins, plateaus and plains. Modern resort villages, many founded as mining towns, nestle in broad valleys. A key area for timber, mining, grazing and recreation, the Rockies are perhaps most important as a source of water. Most major rivers of the western US, including the Snake, Columbia, Yellowstone, Missouri, Colorado, Rio Grande, Arkansas and Platte, originate here, flowing to the Pacific Ocean or the Gulf of Mexico from either side of the Continental Divide. The Northern Rockies are typified by the highly stratified, precipitous mountains of Glacier National Park in Montana, southern bulwark of the Canadian Rockies. Ranges like the Tetons rise above open plains or forested plateaus in the Middle Rockies of southern Montana and Wyoming. In the Southern (Colorado) Rockies are dozens of peaks above 14,000ft in elevation. The Rockies diminish in stature in New Mexico, growing generally more rounded and drier.
Built of sediment washed eastward from the Rocky Mountain slopes, the plains extend 1,000mi to the Mississippi. Semiarid high plains (the western third) naturally support short grass, ideal for bison and cattle; the tapping of aquifers permits more varied farming. Some areas are so flat that one can discern the curvature of the earth’s horizon, but rolling landscapes are more typical. South Dakota’s Black Hills and Badlands, and the Texas Hill Country, enhance an otherwise open landscape. Thunderstorms and tornadoes are frequent in summer; fierce blizzards mark the winters.
Deep, rich soils extend along the Gulf of Mexico coast of Texas to Louisiana. High humidity and rainfall, and temperatures over 90ºF, make summers muggy; winters are mild and snow-free. Numerous rivers—chief among them the Rio Grande on the US-Mexico border—water this naturally forested swath. Protecting most of the coast is a string of sandy barrier islands and peninsulas, including Padre, Matagorda and Galveston Islands, which support rich bird colonies, provide extensive recreational opportunities and help shield the mainland from hurricanes.
The largest US state contains more than 570,000sq mi of forests, mountains, glaciers and tundra. Bounded by the Pacific Ocean (south), Arctic Ocean (north) and Bering Strait (west), Alaska is a massive peninsula. The Brooks Range spans its northern tier, dividing oil-rich tundra from interior plains. The Yukon River flows through the center, bounded by the Alaska Range and North America’s highest summit, 20,320ft Mt. McKinley. Southern coastal ranges curve west as the volcanic Aleutian Islands and arc east through the Panhandle, a fjord-strewn archipelago that shelters the Inside Passage from the heavy seas of the Gulf of Alaska. Although the interior is very cold and dry in winter, summer can bring high temperatures and clouds of insects that attract enormous bird migrations. The Panhandle is cool and wet year-round.
The world’s most remote archipelago with a substantial population, Hawaii comprises 132 volcanic islands, of which the seven most southeasterly are largest. The earliest islands surfaced as volcanoes about 5 million years ago; the most recent (the “Big Island” of Hawai’i) is still growing from eruptions at Kilauea Volcano. The Big Island embraces the world’s largest volcano, 13,677ft Mauna Loa, while the huge dormant volcano of Haleakala dominates the eastern half of nearby Maui. Its tropical climate moderated by trade winds, Hawaii is diverse in weather, foliage and topography, with dramatic differences in rainfall between the wetter windward and drier leeward sides of each island. Mount Waialeale on Kaua’i receives as much as 500in of rain in a year, while the Big Island’s Ka’u Desert is exceedingly arid. Fine beaches and lush foliage contribute to the islands’ tourism fame.