Answer:
The existence of gold.
How did the Gold Rush affect Westward Expansion?
Without writing a teatise on the subject let me just present the obvious Westward Expansion in the 1840s was a given. Regardless of the circumstances, it was going to occur. The US had acquired the Louisiana Purchase in 1803 and by the late 1830s, Texas was free from Mexico and wanted to join the US. The Mexican War added the entire Southwest and California to the Uas. By 1848, nothing was going to halt westward settlement. Discovery of gold in California certainly accelerated the westward push by improving overland transportation and especially communications and by the 1860s travel to the west coast was not easy but neither was it the ordeal of the early 1940s. California was granted statehood in 1850, and by 1869 California was accessible by rail. Neither would have been likely had there not been the huge increases in population resulting from the Gold Rush.