structural:expansion, structural:scholar_edition, stylistic:tone, linguistic:paraphrasing
Natural geography

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.

Beyond the valleys lie the Sierra Nevada in the south and the Cascade Range in the north. Mount Whitney, at 14,505 feet (4,421 m) 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 14,000 feet (4,300 m). 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.

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 4,000 feet (1,200 m). The tallest peaks of the Rockies, 54 of which are over 14,000 feet (4,300 m), 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.