The correct answer to this open question is the following.
Although the question is incomplete because it does not mention which expedition is referring, we can assume that it is talking about the Lewis & Clark Expedition.
So being that the case, our recommendation would be to visit the library of the Congress website to get more interesting information about the expedition. Another good site to visit is "history net."
When US President Thomas Jefferson ordered the exploration of the territories west the Mississippi River in 1804, he picked Meriwether Lewis to led the expedition. The US had both the Lousiana territory that belonged to the French, and Jefferson wanted to first send an expedition to later authorized the settling of that vast territory. William Clark became the right hand of Lewis, and both started the expedition that ended in 1806.