If James Marshall did not discover gold at John Sutter's lumber and logging mill in 1848, California would have grown slowly.
Instead, thousands of gold seekers came to California in 1849. Everything went crazy. Towns were created near the gold fields. Little towns became cities. San Francisco went from a small town to a major port city. Sutter's fort was overrun and everything was stolen. Sailors left their ships in San Francisco Bay and went to look for gold. The ships were taken apart for the wood.
Video on the movement west
3 part Gold Rush series