The longest river in the United States is the Missouri River. It spans about 2,341 miles (3,767 kilometers), making it slightly longer than the Mississippi River. The Missouri River flows through multiple states including Montana, North Dakota, South Dakota, Nebraska, Iowa, Kansas, and Missouri, before eventually converging with the Mississippi River near St. Louis, Missouri. Along its course, the Missouri River serves as an important waterway for transportation, irrigation, and recreational activities.