James W. Marshall, a carpenter, was working for John Sutter to build a sawmill at Sutter's Mill near the American River when he made a groundbreaking discovery. During the construction on January 24, 1848, Marshall noticed shiny flakes in the water while inspecting the mill's construction. He and his crew soon realized these were gold flakes. Despite their initial disbelief, they confirmed the find, and Marshall's excitement was quickly overshadowed by concerns about what this discovery would bring to their serene settlement. The news, however, would not remain a secret for long.

As word spread about the gold discovery, it created a tidal wave of interest and migration. Newspapers reported the news, and by the late summer of 1849, tens of thousands had arrived in California, seeking wealth and opportunity. This migration became known as the California Gold Rush, fundamentally changing the demographic and economic landscape of the West. Cities like San Francisco transformed from small settlements into booming hubs as fortune seekers, known as 'forty-niners,' arrived in droves.

The implications of Marshall's discovery were immense, leading to rapid economic growth and the establishment of new towns, infrastructure, and services to accommodate the influx of people. The Gold Rush played a significant role in California's admission to the Union in 1850 as the 31st state. It also had profound effects on both the local indigenous populations and the environment, as mining activities disrupted ecosystems and communities. Marshall's find at Sutter's Mill is recognized as a catalyst for one of the most significant migrations in U.S. history.