Final answer:
Many Americans migrated to the West due to demographic pressures from a rapidly growing population needing more land and economic opportunities, such as the California Gold Rush and trade prospects with Asia facilitated by the Transcontinental Railroad.
Step-by-step explanation:
In the mid-nineteenth century, many Americans migrated to the West, propelled by a combination of demographic pressures and the lure of economic opportunities. As the United States population surged from about five million to over twenty million between 1800 and 1850, there was a marked increase in the demand for land. This was due to the declining soil fertility of overworked farms in the east, making the inexpensive and fertile lands of the West increasingly attractive.
Moreover, the political anxiety that the British might occupy the Western territories if Americans did not drove the migration further. Additionally, eastern merchants aimed to control Western ports to facilitate trade with Asia, which linked the notion of westward migration to economic drive as well. The discovery of gold in California and the completion of the Transcontinental Railroad also significantly contributed to the migration westward, sparking the California Gold Rush and facilitating easier movement and the promise of prosperity in the West.