What geographic feature divides East from West?

The Mississippi River and the Rocky Mountains are the two major geographic features that traditionally divide the eastern United States from the West, with the Mississippi forming a common shorthand divide and the Rockies serving as the more dramatic continental separation. The Mississippi River runs about 2,320 miles from northern Minnesota to the Gulf of Mexico, forming or running through the center of ten states (Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana). For much of American history the Mississippi was treated as the dividing line between East and West. Before the Louisiana Purchase of April 30, 1803, the Mississippi was the western boundary of the United States. After the purchase, the river marked the boundary between the older organized states and the trans-Mississippi territories.

The Old West before the Civil War referred to the region just west of the Appalachians and east of the Mississippi (Kentucky, Ohio, Indiana, Illinois). The trans-Mississippi West referred to everything beyond. Frontier studies traditionally use the Mississippi as a key reference.

The Rocky Mountains form the more dramatic division between the eastern half of the country and the West proper. The Rockies extend about 3,000 miles from northern New Mexico through Colorado, Wyoming, Idaho, Montana, and into Canada. The Continental Divide, running along the crest of the Rockies, separates waters that flow east toward the Atlantic and Gulf of Mexico from waters that flow west toward the Pacific. East of the Continental Divide, the Great Plains gradually slope down to the Mississippi system. West of the Divide, the Colorado Plateau, Great Basin, and Sierra Nevada/Cascade ranges drop to the Pacific coast.

The four Census regions of the United States divide the country at the Mississippi roughly: the Northeast and South lie east of the Mississippi (with some overlap), and the Midwest and West lie west, although the Census Midwest extends from Ohio west to the Dakotas and Kansas. Beyond formal divides, the East and West differ culturally, economically, and historically. The East was settled first, has older cities and infrastructure, and historically dominated American politics, finance, and culture. The West was settled later, includes more federal public land, has more sparsely populated areas, and developed distinctive economies based on mining, ranching, irrigated agriculture, and more recently technology and entertainment.

The United States is sometimes described as having four time zones excluding Alaska and Hawaii (Eastern, Central, Mountain, Pacific), with the Mountain time zone roughly bracketed by the Rockies. Other major dividing lines include the 100th meridian (the historical boundary between humid and arid lands), the Mason-Dixon line (between North and South), and the Continental Divide.