Sacagawea was the most important member of The Corps and the reason The Corps succeeded in their mission. Sacagawea was originally hired just as a useful figure to help get horses to cross the Bitterrroot Mountains but she turned out to be of higher value than her husband. Sacagawea was also hired as an interpreter along with her husband and Francois Labiche. On the journey when The Corps was either running out of food or did not want the dehydrated soup, Sacagawea was sent out to gather roots, berries, and other edible plants for The Corps' dinner. A few of the plants that Sacagawea brought back were even able to be used for medicinal purposes.
With out Sacagawea we would be missing some of Lewis and Clark's journals. About 2200 miles from St. Louis the boat that Sacagawea was on, being steered by Charbonneau, almost capsized. Without a backward glance, Sacagawea jumped into the river with Pomp strapped to her back to rescue the important parcels. Meanwhile, Charbonneau let go of the rudder and was praying to God to save him. Sacagawea saved most of the parcels including the journals, a compass that belonged to Clark, and medicines. She received honors and thanks from Lewis and Clark for remaining calm in times of danger.
When The Corps reached a place in the river Sacagawea started to reconize landmarks from her childhood. She was able to direct The Corps to the general area where her people should be. After searching near and far, they finally found the Shoshone. Halfway through interpreting Sacagawea got up and walked over to the cheif and hugged him and started to cry. The cheif was Camaiwait, her brother. They had finally been reunited after years of being apart. The Corps was able to trade for the horses necessary to cross the Bitterroot Mountains.
Sacagawea would also serve a purpose unexpectedly. During the entire length of the journey, The Expedition was protected from unfriendly Indians by Sacagawea. The Indian tribes recognized that no war party travels with a woman; especially a woman with an infant. Sacagawea and Pomp were "living, walking peace flags".
On the return trip, Lewis went north to explore for a water route for trade while Clark went south discovering Yellowstone. Sacagawea knew many different trails and saved much time for the party. The most recognized path is present day Bozeman Pass to the Yellowstone River. Clark referred to Sacagawea as his "pilot".
Photo from http://www.mariananderson.com/sacagawea.html