They consumed meat, cornmeal, berries, nuts, roots and whatever else they could find. Along the Missouri, they hunted game. On the Columbia, they swapped for salmon.
According to Stephen E. Ambrose, in his book Undaunted Courage, each soldier in the Lewis and Clark expedition ate up to nine pounds of meat per day. 33 people and a big Newfoundland dog gobbling seven to nine pounds of meat would require the equivalent of two buffalo calves, two elk or six deer per day.