While working in a water channel under a mill, foreman James Marshall discovered gold. The date was January 24th, 1848, also known as the beginning of the California Gold Rush.
The mill owner, Johann Sutter, was worried of what would happen to his property and the area at large if word got out; he was right and by 1855, an estimated total of 300,000 people from all over the world flocked to California in the hopes of finding wealth by collecting gold.
At the time, California was not part of the United States, and belonged to Mexico. Following the Mexican-American War and the formal treaty ending the war on February 2, 1848, California became a possession of the US, though it didn't become an official territory until 1850.
The area underwent a complete transformation: the local Native American population was attacked and displaced, means of transportation were extensively developed, agriculture and ranching made their way West, commerce to California greatly increased to meet the high demands, and eventually a formal government was established.