Final answer:
Lewis and Clark established Fort Clatsop when they reached the Pacific Ocean, where they overwintered before returning to St. Louis.
Step-by-step explanation:
The fort that Lewis and Clark established when they reached the Pacific Ocean was named Fort Clatsop. After a challenging journey, including nearly starving in the Bitterroot Mountains, the Corps of Discovery reached the Pacific Ocean in 1805 and spent the winter of 1805-1806 in this fort located in Oregon.
Despite the hardships and the failure to discover a Northwest Passage, they succeeded in mapping the territory, establishing relationships with Native American tribes, and claiming the land for the United States.