Mariwether Lewis was an American explorer, who with William Clark led the Lewis and Clark Expedition through the uncharted American interior to the Pacific Northwest in 1804-1806. He then served as governor of the Upper Louisiana Territory. The Lewis and Clark Expedition spanned 8,000 miles and 3 years. The expedition was known down the Ohio River, up the Missouri River, across the Continental Divide, and to the Pacific Ocean. Lewis was the field scientist (chronicling botanical, zoological, meteorological, geographic & ethnographic info.