In November 1804 Charbonneau and one of his two squaws were engaged by Lewis & Clark to accompany them and act as interpreter among the Indians. Sacajawea and Charbonneau represented vital links in an involved chain of interpretive measures that would be required to communicate with Indians on the westward journey. The interpretive process was complicated because of the limited language knowledge of the parties involved. The Frenchman Charbonneau was conversant in French and Hidatsa, but spoke no English. Sacajawea spoke both Hidatsa and Shoshoni, but neither French nor English. This was resolved through a third person, Private Francois Lebiche, a member of the expedition, of French and Omaha Indian extraction, who spoke French and some English. The process went as follows: "I spoke . . . to Labiche in English -- he translated it to Charboneau in French -- he to his wife in Minnetaree -- she in Shoshoni to the Indians." Sacajawea gave birth to a son on February 11, 1806 and Charbonneau named him Jean Baptiste, but Sacajawea called him Pomp or Pompy. The infant member of their expedition was a delight to his exploring companions and held an affectionate place in all their hearts.

Sacajawea became very ill at the Great Falls of the Missouri and wasn't expected to live. There was great concern among the men as to the fate of the baby. The Captains also were very concerned because she was their vital link to the Shoshoni Indians. Captain Clark bled her three different times and Captain Lewis spent much time treating her with salves and ointments. It wasn't until they gave her mineral water from a spring close by, that she began to get better. The spring today bears the name "Sacagawea Spring".