The Yukon River is a major river in western Canada and Alaska. It is 1, 980 miles long and is the third longest river in the United States. The river originates in British Columbia and flows through the Yukon Territory before emptying into the Bering Sea in Alaska. The river is known for its role in the Klondike Gold Rush of the late 19th century, when prospectors from around the world traveled to the Yukon in search of gold. The river was explored by John Ross in 1829 and later by Robert Campbell and John Bell in 1840. The Liard River is a tributary of the Yukon River, joining it near Fort Simpson in the Northwest Territories.