The Missouri River is the longest river entirely within the United States, stretching approximately 2,341 miles (3,767 kilometers) from its source in the Rocky Mountains of western Montana to its confluence with the Mississippi River in St. Louis, Missouri.

When combined with the Mississippi River, which it feeds into, the Missouri-Mississippi river system becomes the longest river system in North America, extending about 3,902 miles (6,275 kilometers). This combined system plays a crucial role in the ecology, economy, and transportation infrastructure of the United States.

Key Facts:

• Missouri River Length: ~2,341 miles (3,767 km)
• Mississippi River Length: ~2,320 miles (3,730 km)
• Combined Missouri-Mississippi System: ~3,902 miles (6,275 km)

The Missouri River is a major waterway that traverses seven U.S. states: Montana, North Dakota, South Dakota, Nebraska, Iowa, Kansas, and Missouri.