Bartleson–Bidwell Party - Wikipedia

On November 4, 1841, the first wagon train arrived in California. From the article:

In 1841, the Bartleson–Bidwell Party, led by Captain John Bartleson and John Bidwell, became the first American emigrants to attempt a wagon crossing from Missouri to California.

The trail to California
The trail to California had been established not by the government, but by members of the "Emigrant Societies" formed in the 1840s. The efforts of three parties had established a passable wagon road over the two main obstacles: the Great Salt Lake Desert in Utah, and the Sierra Nevada Mountains in California. The result was a journey of 2,008 miles in a single summer and fall, by oxen or horse or mules at 15 miles a day, which meant a voyage of about five months.

The party
In May 1841, the party assembled at Sapling Grove, near Westport, Missouri, with twenty-one-year-old John Bidwell a member of the party. A man named Bartleson was also in this party; he would eventually take a route into California which diverged from the group Bidwell traveled within the final stages of the journey.

Numbering more than sixty, the group decided to travel together to California. While they did eventually end up at John Marsh's Ranch, that was not their original destination. Their intent had been to cross the Sierra Mountains into California with the intent of arriving near what was Sutter's Fort (now Sacramento). Due to a series of unfortunate circumstances, they would travel along the north side of the Great Salt Lake and then cross the desert. One man (Reuben Cole Shaw) crossed that region would say "Humboldt is not good for man nor beast... its waters contain the alkali to make soap for a nation." After crossing what would become the state of Utah, the traveling party missed the head of the Humboldt River. They abandoned their wagons in what is now Nevada, near Big Spring in the Pequop Mountains. Using their pack animals they crossed to what is now Lovelock, Nevada. Crossing the Forty Mile Desert on the east side of the Sierras, they turned south until they reached what is now called the Walker River. They followed the river westward and then ascended the Sierra Nevada Mountains, somewhat along the same trail which had been taken by Jedediah Smith in 1828. Along the trail they had to kill many of their animals to make food for themselves. All of the party survived the journey, but it was a severe trial.

Arriving in California, their first stop was at the ranch owned by John Marsh. After a short rest, Bidwell went north to Sutter's Fort. Eventually, Bidwell would be the founder of what is now Chico in Butte County, California.

Moving west, the immigrants traveled over the Oregon Trail with Father Pierre-Jean De Smet and a Jesuit party guided by mountain man Thomas "Broken Hand" Fitzpatrick.

Included in this party, traveling with Bartleson, was Josiah Belden, later to be the first mayor of San Jose, California.

The trip
At Sheep Rock, in present-day Caribou County, Idaho, about half of the original party changed their plans and decided to take the easier road into Oregon. The remainder of the Bartleson-Bidwell party split off from the trappers' trail to Oregon and headed south along the Bear River and then west along the north shore of the Great Salt Lake. Crossing the desert west of the lake, they were forced to abandon their wagons. Accompanied by their surviving animals, they eventually found the Mary's River (now the Humboldt) and followed it to its sink (near present Lovelock, Nevada). Crossing the desert to the south, they reached the Walker River, which they ascended over the Sierra Nevadas in the same region crossed by Jedediah Smith in 1827.