What is the longest river in the united states?

The longest river in the United States is the Missouri River. While the Mississippi River is longer if you measure from its source to its mouth in the Gulf of Mexico, the Missouri River is considered the headwaters of the Missouri-Mississippi River system. Therefore, depending on how you define "longest," the answer differs.

Here's a breakdown:

• Missouri River Length: Approximately 2,341 miles (3,767 km) long. This measurement is from its source in the Rocky Mountains of Montana to its confluence with the Mississippi River.

• Mississippi River Length (alone): Approximately 2,320 miles (3,734 km) from its source (Lake Itasca, Minnesota) to its mouth.

• Missouri-Mississippi River System Length: Approximately 3,734 miles (6,011 km) This is the combined length, measuring from the Missouri River's source to the Mississippi River's mouth. This makes it one of the longest river systems in the world.

So, while the Mississippi-Missouri system is longer overall, the Missouri River itself is considered the longest river entirely within the United States.