Who did the corps of discovery hire as a guide and interpreter when they reached fort mandan?

Sacagawea

Explanation:

The Fort Mandan refers to the name of the encampment which the Lewis and Clark Expedition built for wintering over in 1804-1805. It was located on the Missouri River and about twelve miles from the location of present day Washburn, North Dakota.

Sakagawea was the name of the person which was hired as a guide and interpreter when they reached Fort Mandan. She was 17-year-old and wife of Toussaint Charbonneau, a French Canadian trader hired by Lewis and Clark as a Hidatsa interpreter. She proved to be an invaluable member of the expedition.