The Great Basin and the Sierra Nevada mountains through which the trail passed were first explored by British and American fur trappers.
 U.S. trapper, explorer and fur trader Jedediah Smith led two expeditions into California and over the Sierra Nevada mountains and back from 1826 to 1829.
 It is believed that on his first trip he used the Mojave River route (later part of the Old Spanish Trail) to get into California and 8,730 feet (2,660 m) Ebbetts Pass when leaving California in the spring 1827.
On Smith's second trip he entered California the same way and left through Oregon.
Smith was killed in 1831 before he could publish his explorations, which were only known by word of mouth.

In 1828–1829, Peter Skene Ogden, leading expeditions for the British Hudson's Bay Company, explored much of the Humboldt River area—named by him the Mary's River.
The results of these explorations were held as proprietary secrets for many decades by the Hudson's Bay Company.
In 1834 Benjamin Bonneville, a United States Army officer on leave to pursue an expedition to the west financed by John Jacob Astor, sent Joseph R. Walker and a small horse mounted party westward from the Green River in present-day Wyoming.
 They were charged with the mission of finding a route to California.
Walker confirmed that the Humboldt River furnished a natural artery across the Great Basin to the Sierra Nevada mountains.
He eventually got across the Sierra Nevada mountains in southern California over Walker Pass.
 Bonneville had the account of his and Walker's explorations in the west written up by Washington Irving in 1838. (
See: "The Adventures of Captain Bonneville").

A few hundred mountain men and their families had been filtering into California for several decades prior to 1841 over various paths from Oregon and Santa Fe.
The first known emigrants to use parts of the California Trail was the 1841 Bartleson–Bidwell Party.
 They followed the Humboldt River across Nevada and eventually made it into northern California.
Other parts of this party split off and were one of the first sets of emigrants to use the Oregon Trail to get to Oregon.
The California-bound travelers, striking out from the Snake River and passing into Nevada, missed the head of the Humboldt River there.
They abandoned their wagons in eastern Nevada and finished the trip by pack train.
After an arduous transit of the Sierras (its believed over Ebbetts Pass), members of this group later founded Chico, California in the Sacramento Valley.
In 1842 (a year without any known California Trail emigration), Joseph Chiles, a member of the Bartleson–Bidwell Party of 1841, returned with several others back east.
In 1843, Chiles led a party (of seven he eventually would lead) back to California.
 At Fort Hall he met Joseph Reddeford Walker who he convinced to lead half the settlers with him traveling in wagons back to California down the Humboldt.
Chiles led the rest in a pack train party down the Malheur River to California.
Walker's party in 1843 also abandoned their wagons and finished getting to California by pack train.

In 1844, Caleb Greenwood and the Stephens-Townsend-Murphy Party became the first settlers to take wagons over the Sierra Nevada and into California over what became the Truckee Trail.
They abandoned their wagons in the early snow in the winter of 1844/1845 and finished retrieving their wagons from the mountains in the spring of 1845.
In 1845, John C. Frémont and Lansford Hastings guided parties totaling several hundred settlers along the Humboldt River portion of the California Trail to California.
They were the first to make the entire trip by wagon in one traveling season.
In 1846 it is believed that about 1,500 settlers made their way to California over the Truckee branch of the California Trail—just in time to join the war for independence there.
Many of the 1845 and 1846 emigrants were recruited into the California Battalion to assist the U.S. Navy's Pacific Squadron with its sailors and marines in the fight for California's independence from Mexico.

The last immigrant party
The Great Basin and the Sierra Nevada mountains through which the trail passed were first explored by British and American fur trappers.  U.S. trapper, explorer and fur trader Jedediah Smith led two expeditions into California and over the Sierra Nevada mountains and back from 1826 to 1829.  It is believed that on his first trip he used the Mojave River route (later part of the Old Spanish Trail) to get into California and 8,730 feet (2,660 m) Ebbetts Pass when leaving California in the spring 1827. On Smith's second trip he entered California the same way and left through Oregon. Smith was killed in 1831 before he could publish his explorations, which were only known by word of mouth.
In 1828–1829, Peter Skene Ogden, leading expeditions for the British Hudson's Bay Company, explored much of the Humboldt River area—named by him the Mary's River. The results of these explorations were held as proprietary secrets for many decades by the Hudson's Bay Company. In 1834 Benjamin Bonneville, a United States Army officer on leave to pursue an expedition to the west financed by John Jacob Astor, sent Joseph R. Walker and a small horse mounted party westward from the Green River in present-day Wyoming.  They were charged with the mission of finding a route to California. Walker confirmed that the Humboldt River furnished a natural artery across the Great Basin to the Sierra Nevada mountains. He eventually got across the Sierra Nevada mountains in southern California over Walker Pass.  Bonneville had the account of his and Walker's explorations in the west written up by Washington Irving in 1838. (See: "The Adventures of Captain Bonneville").
A few hundred mountain men and their families had been filtering into California for several decades prior to 1841 over various paths from Oregon and Santa Fe. The first known emigrants to use parts of the California Trail was the 1841 Bartleson–Bidwell Party.  They followed the Humboldt River across Nevada and eventually made it into northern California. Other parts of this party split off and were one of the first sets of emigrants to use the Oregon Trail to get to Oregon. The California-bound travelers, striking out from the Snake River and passing into Nevada, missed the head of the Humboldt River there. They abandoned their wagons in eastern Nevada and finished the trip by pack train. After an arduous transit of the Sierras (its believed over Ebbetts Pass), members of this group later founded Chico, California in the Sacramento Valley. In 1842 (a year without any known California Trail emigration), Joseph Chiles, a member of the Bartleson–Bidwell Party of 1841, returned with several others back east. In 1843, Chiles led a party (of seven he eventually would lead) back to California.  At Fort Hall he met Joseph Reddeford Walker who he convinced to lead half the settlers with him traveling in wagons back to California down the Humboldt. Chiles led the rest in a pack train party down the Malheur River to California. Walker's party in 1843 also abandoned their wagons and finished getting to California by pack train.
In 1844, Caleb Greenwood and the Stephens-Townsend-Murphy Party became the first settlers to take wagons over the Sierra Nevada and into California over what became the Truckee Trail. They abandoned their wagons in the early snow in the winter of 1844/1845 and finished retrieving their wagons from the mountains in the spring of 1845. In 1845, John C. Frémont and Lansford Hastings guided parties totaling several hundred settlers along the Humboldt River portion of the California Trail to California. They were the first to make the entire trip by wagon in one traveling season. In 1846 it is believed that about 1,500 settlers made their way to California over the Truckee branch of the California Trail—just in time to join the war for independence there. Many of the 1845 and 1846 emigrants were recruited into the California Battalion to assist the U.S. Navy's Pacific Squadron with its sailors and marines in the fight for California's independence from Mexico.
The last immigrant party 
California Trail