David Thompson, an English-born surveyor and fur trader, was the first known explorer to reach Sanders County. Sent at age 14 to Hudson Bay, Canada, to begin seven years of indentured servitude as a clerk for the Hudson's Bay Company, he learned French and native languages, exploration, navigation, mapmaking, building trading posts, and wilderness survival, often with critical assistance and cooperation from North American natives. Between 1807 and 1812, he established trade among tribes west of the Rocky Mountains and built the region's first four trading posts, including Saleesh House near Thompson Falls. In 1811 he located and mapped the entire course of the Columbia River, finding its source and following it to the Pacific Ocean in a large cedar plank canoe of his own invention, thereby solving the mystery of the Northwest Passage as a coast-to-coast route long sought by explorers. By the end of his career, he had mapped nearly 1.5 million square miles of previously uncharted territory, about one-fifth of North America, helped establish the first cross-continental network of remote wilderness forts and trading posts, and traveled more than 65,000 miles by foot, horse, and canoe.