Up to this point, most of the immigration came from Mexico, but as the US grew, a flux of immigrants started to trickle in from the East.
Around 1840, the governor of California granted a Swiss immigrant, John Sutter, a large piece of land near today's Sacramento, to build a fort and "prevent the robberies committed by adventurers from the United States, the invasion of savage Indians, and the hunting and trading by companies from the Columbia river".
Sutter had fled debts to find fortune in America, abandoning his wife and 5 children behind, and named his settlement New Helvetia after his homeland (Helvetia = Switzerland).
Far from "keeping the Americans out" however, Sutter provided them shelter as they came in through the California trail.
His fort and the area became a major trade and agricultural hub, and can still be visited today.
The California Trail was part of a series of mythical East-West trails.
The Oregon Trail connected Missouri to Oregon, and was used by about 400,000 settlers, including farmers, miners, ranchers, business owners and their families. Modern highways like the Interstates 80 and 84 follow parts of it.
The California Trail also began in Missouri. It was used by a trickle of travellers before the Gold Rush. One of the tough parts was crossing the Sierra Nevada, as proved by the Donner Party, who got stuck in winter and resorted to eating each other.
Sutter's Fort provided shelter, supplies and guidance to these new settlers.