Why was William Clark chosen to accompany Lewis on an official exploratory mission?

Former United Stated President Thomas Jefferson wanted to find a practical route across the western half of the continent, and expanding the American presence before the Britain and other European powers will claim it, so he ask the congress to fund the Corps of Discovery which was headed by U. S Army Captain Meriwether Lewis, together with his close friend, second lieutenant William Clark. Jefferson heard a lot about William Clark, so when he knew Lewis recruited Clark to formed of Corps of Discovery, the Lewis and Clark Expedition then formed.