The rapid westward migration caused by the discovery of gold in california in 1848 led directly to:

This discovery of gold lead directly to the California Gold Rush.

Thanks to the discovery of gold at Sutter's Mill by James Marshall, California became flooded with individuals looking to make it rich by finding gold. Roughly 300,000 citizens from across the country came to settle in California. This greatly increased the size of California's population, eventually allowing it to join the US as a state rather than a territory.