: the
Rocky Mountains (16–19), the
Intermontane Plateaus (20–22), and the
Pacific Mountains (23–25) Along the
Pacific Ocean coast lie the
Coast Ranges, which, while not approaching the scale of the
Rocky Mountains, are formidable nevertheless. They collect a large part of the airborne moisture moving in from the ocean. East of the Coast Ranges lie several cultivated fertile
valleys, notably the
San Joaquin and
Sacramento valleys of
California and the
Willamette Valley of
Oregon. in southern
Utah is one of five national parks in the state. ,
California covers much of the
Southwestern United States. ,
Arizona Beyond the valleys lie the
Sierra Nevada in the south and the
Cascade Range in the north.
Mount Whitney, at the tallest peak in the contiguous 48 states, is in the Sierra Nevada. The Cascades are also volcanic.
Mount Rainier, a volcano in
Washington, is also over .
Mount St. Helens, a volcano in the Cascades,
erupted explosively in 1980. A major volcanic eruption at
Mount Mazama around 4860 BC formed
Crater Lake. These mountain ranges see heavy precipitation, capturing most of the moisture that remains after the Coast Ranges, and creating a
rain shadow to the east forming vast stretches of arid land. These dry areas encompass much of
Nevada,
Utah, and
Arizona. The
Mojave Desert and
Sonoran Desert along with other deserts are found here. in the
High Desert region of
California region of
Oregon of Southeast
Montana ,
Colorado Beyond the deserts lie the
Rocky Mountains. In the north, they run almost immediately east of the
Cascade Range, so that the desert region is only a few miles wide by the time one reaches the Canada–US border. The Rockies are hundreds of miles wide and run uninterrupted from
New Mexico to
Alaska. The Rocky Mountain Region is the highest overall area of the United States, with an average elevation of above . The tallest peaks of the Rockies, 54 of which are over , are found in central and western
Colorado. East of the Rocky Mountains is the Great Plains, the western portions (for example, the eastern half of Colorado) of which are generally considered to be part of the western United States. The West has several long rivers that empty into the
Pacific Ocean, while the eastern rivers run into the
Gulf of Mexico. The
Mississippi River forms the easternmost possible boundary for the West today. The
Missouri River, a tributary of the Mississippi, flows from its headwaters in the
Rocky Mountains eastward across the
Great Plains, a vast
grassy plateau, before sloping gradually down to the forests and hence to the Mississippi. The
Colorado River snakes through the
Mountain states, at one point forming the
Grand Canyon. The
Colorado River is a major source of water in the Southwest and many dams, such as the
Hoover Dam, form reservoirs along it. So much water is drawn for drinking water throughout the West and irrigation in
California that in most years, water from the Colorado River no longer reaches the
Gulf of California. The
Columbia River, the largest river in volume flowing into the
Pacific Ocean from
North America, and its tributary, the
Snake River, water the Pacific Northwest. The
Platte runs through
Nebraska and was known for being a mile (2 km) wide but only a half-inch (1 cm) deep. The
Rio Grande forms the border between
Texas and
Mexico before turning due north and splitting
New Mexico in half. According to the
United States Coast Guard, "The Western Rivers System consists of the
Mississippi,
Ohio,
Missouri,
Illinois,
Tennessee,
Cumberland,
Arkansas, and
White Rivers and their tributaries, and certain other rivers that flow towards the
Gulf of Mexico." The Ohio River portion of the system includes parts of several Atlantic coastal states, from Georgia to New York.
Climate and agriculture Most of the public land held by the
U.S. National Forest Service and Bureau of Land Management is in the Western states. Public lands account for 25 to 75 percent of the total land area in these states. The climate of the West is
semi-arid, yet parts of the region get high amounts of rain or snow. Other parts are true desert which receive less than of rain per year. The climate is increasingly unstable, and subject to periods of severe drought. The seasonal temperatures vary greatly throughout the West. Low elevations on the
West Coast have warm summers and mild winters with little to no snow. The
desert southwest has very hot summers and mild winters. While the mountains in the southwest receive generally large amounts of snow. The
Inland Northwest has a
continental climate of warm to hot summers and cold to bitterly cold winters. Annual rainfall is greater in the eastern portions, gradually tapering off until reaching the Pacific Coast where it increases again. In fact, the greatest annual rainfall in the United States falls in the coastal regions of the
Pacific Northwest. Drought is much more common in the West than the rest of the United States. The driest place recorded in the United States is
Death Valley, California. In Western states, drought is closely associated with fire risk, and there have been a number of notable wildfires causing extensive property damage and wildlife
habitat destruction. The Western United States is predicted to experience drought-like conditions for much of the 21st century. Many varieties of
chile peppers are grown in the valleys of
New Mexico. Starting in 1902, Congress passed a series of acts authorizing the establishment of the
United States Bureau of Reclamation to oversee water development projects in seventeen western states. During the first half of the 20th century, dams and irrigation projects provided water for rapid agricultural growth throughout the West and brought prosperity for several states, where agriculture had previously only been subsistence level. Following
World War II, the West's cities experienced an economic and population boom. The population growth, mostly in the
Southwest states of
New Mexico,
Utah,
Colorado,
Arizona, and
Nevada, has strained water and power resources, with water diverted from agricultural uses to major population centers, such as the
Las Vegas Valley and
Los Angeles.
Geology Plains make up much of the eastern portion of the West, underlain with sedimentary rock from the Upper
Paleozoic,
Mesozoic, and
Cenozoic eras. The
Rocky Mountains expose igneous and metamorphic rock both from the
Precambrian and from the
Phanerozoic eon. The Intermountain States and
Pacific Northwest have huge expanses of volcanic rock from the Cenozoic era.
Salt flats and salt lakes reveal a time when the great inland seas covered much of what is now the West. The Pacific states are the most geologically active areas in the United States.
Earthquakes cause damage every few to several years in
California. While the
Pacific states are the most volcanically active areas, extinct
volcanoes and lava flows are found throughout most of the West. == Wildlife ==