The historical event that significantly impacted the development of California is The Gold Rush.

The California Gold Rush of 1848–1855 was a transformative event that led to rapid population growth and economic development in the region. It began when gold was discovered at Sutter’s Mill, prompting an influx of hundreds of thousands of people from across the U.S. and around the world. This mass migration, known as the "Forty-Niners," drastically changed California’s social and economic landscape, fueling the establishment of towns, infrastructure, and industries to support the booming population. The Gold Rush ultimately paved the way for California’s statehood in 1850 and established it as a pivotal economic and cultural hub in the United States.