The tendency for Americans to move west has been part of the national makeup since the end of colonial times. While charters granted to early English colonies included vast western sections, often not only unexplored but simultaneously claimed by other countries, actual settlements stayed close to the ocean or at least to coastal rivers until about the time of the Revolution.
Independence released Americans from the British reluctance to expand. The territories were gradually organized, starting with the Northwest Ordinances, and plans were established to bring the new territories into the Union as states as they gained sufficient population.
The acquisition of the central part of the country through the Louisiana Purchase made still more land available, although much of it was more challenging to settlers than the nearby and relatively temperate conditions of the Old Northwest. Even before control of Oregon was established by treaties, American undertook the greatest migration of them all along the Oregon Trail, two thousand miles from St. Louis, Missouri to the Willamette Valley.
Western migration was encouraged by the Homestead Acts, which gave public land to settlers who would farm it for a specified period of time. It was also encouraged, in fits and starts, by the discovery of mineral wealth in various places and at various times, beginning with the California Gold Rush.
Migration became less arduous with the expansion of railroads. The tendency continues to this day, with the country`s population "center of gravity" moving steadily south and west each decade.