Who was the first American who enter California by crossing the rocky mountains?

The first American to enter California by crossing the Rocky Mountains was Jedediah Smith. In 1826, Smith, a fur trapper and explorer, led an expedition that traversed the Sierra Nevada and entered California through the southern route. His journey was significant for opening up the region to further exploration and settlement. Smith's expeditions contributed to the westward expansion of the United States during the 19th century.