What river begins in Lewis and Clark County outside of Missoula, Montana?
Missoula, Montana: Missoula is a city in the U.S. state of Montana and is the county seat of Missoula County. Blackfoot River (Montana):  The Blackfoot River begins in Lewis and Clark County at the Continental Divide, 10 miles (16 km) northeast of the town of Lincoln (4536 feet, 1382 m).
Blackfoot River