Gallatin River, river rising in the Gallatin Range in the northwestern corner of Yellowstone National Park, Wyoming, U.S., and flowing 120 miles (193 km) north to Three Forks, in southwestern Montana. There it joins with its tributary, the East Gallatin (which rises near Mount Blackmore), and the Madison and Jefferson rivers to form the Missouri River. Named for Albert Gallatin, the early 19th-century statesman, it forms a deep canyon about 70 miles (110 km) long between the Madison and Gallatin ranges. The river is used for irrigation.