What impact did the Lewis and Clark expedition have on developing the United States west of the Mississippi River?
Besides simply giving us a journal to follow which described what we would find out West in terms of flora and fauna (plants and animals), but helped us to know which Native tribes were friendly and which were hostile, what the safest routes were and simply the fact that it was possible to reach the West overland and live to tell the tale was something they contributed.
By exploring the Pacific Northwest, they also inadvertently claimed it for the United States, which led to increased settlement later, especially in present day Oregon through the Oregon Trail.
Prior to Lewis and Clark's journey, the American West was largely unexplored and unknown. Their encounter with Sacajewea and their findings along their route would serve as the basis for eventual westward expansion and Americans' newfound fascination with homesteading west of the Mississippi River.
Without the voyages of Lewis and Clark, the United States might have remained unaware about life and land in the western half of our country, and the displacement of Native Americans there might never have occurred.