Jedediah Strong Smith (1799–1831) and about fifteen trappers with the Rocky Mountain Fur Company entered California in November 1826. They were hoping to "find parts of the country as well stocked with Beaver as the waters of the Missouri." Smith and his party had crossed the Mojave Desert and the San Bernardino Mountains (a route later known as the Old Spanish Trail). They were the first Americans to enter California by land. Smith disobeyed an order from Mexican governor José María de Echeandía to turn around and return the way he had come. Instead, Smith and his men traveled north through the great San Joaquin Valley.
Smith's party trapped along the Mokelumne River in what is now northern San Joaquin County, but the Miwok-speaking Indians were less cooperative than the Yokuts-speakers to the south had been. On the Cosumnes River, near present-day Wilton, Indians stole some traps. Farther north on the American River, which is named for these trappers, Smith's party killed two Maidu Indians in a skirmish. The trappers returned to the friendly Natives on the Stanislaus River and camped for the winter near present-day Oakdale. They had accumulated more than fifteen hundred pounds of beaver pelts.
In May 1827, Jedediah Smith and two of his men crossed the Sierra Nevada near present Ebbetts Pass—the first non-Indians to cross the mountain range. They traveled east through the Great Basin Desert and attended the annual trappers' rendezvous on the west slope of the Rocky Mountains. They spoke to other trappers about the richness of the California heartland.
Smith returned in January 1828, and he and his men trapped along the lower Calaveras, Mokelumne, and Cosumnes Rivers. But Mexican authorities arrested Smith, so the American trappers left California for Oregon country.
The entry of the American trappers into this region encouraged Native freedom fighters like Estanislao and stirred old Spanish/Mexican concerns about the Russians, whose Rus colony (Americanized as "Fort Ross") had been established in 1812 on the Northern California coast. Smith's reports of the rich trapping here prompted Hudson's Bay Company at Fort Vancouver, in Oregon country, to send trapping parties south.
French Camp, about five miles south of Stockton, is the oldest non-Indian settlement in San Joaquin County. It was seasonally occupied by French-Canadian trappers working for Hudson's Bay Company from about 1829 through 1840, hence its name. Although the Mexican governor repeatedly ordered the trappers to leave, as many as four hundred trappers lived with their wives and children for part of the year at French Camp. Mexican authorities eventually allowed the company to establish a post at Yerba Buena (now San Francisco) from 1841 to 1845, but by then changes in hat styles and over-trapping had sent the fur business into steep decline.
In the fall of 1832, Hudson's Bay Company trappers unwittingly introduced malaria into the mosquito-infested wetlands of central California. The Indian inhabitants had no immunity to the disease and died in huge numbers. By the late 1830s, the Bay Miwok nations had disappeared and the Valley Miwok nations in what is now northern San Joaquin County and southern Sacramento County had lost at least 80 per cent of their people to the epidemic.