Final answer:
The trading post built by the Pacific Fur Company at the mouth of the Columbia River was named Fort Astoria.
Step-by-step explanation:
The trading post built by the Pacific Fur Company at the mouth of the Columbia River was named Fort Astoria. This fort was an important site in the history of American expansion and the fur trade in the Pacific Northwest. Establishing such trading posts was a common practice during the period of Westward expansion and played a significant role in the economic and cultural interactions between European settlers and Native Americans.
Fort Astoria was founded by the American Fur Company, led by John Jacob Astor, and was later sold to the British during the War of 1812 and renamed Fort George. It is a symbol of the early American presence in the Oregon Country and was central to the economic endeavors of the Pacific Fur Company in the region.