Why did Lewis and Clark travel mostly by water and not by land?

Lewis and Clark traveled mostly by water to take advantage of the rivers that provided a more efficient and faster means of transportation compared to the challenging and often rugged terrain of the land. Rivers like the Missouri and Columbia allowed them to navigate vast distances with less physical exertion and facilitated the transport of their supplies and equipment. Additionally, waterways offered a relatively direct route to their destination, making it practical for their exploration objectives.