it is a river. PERIOD
About the last 550 miles of the Missouri River is in or along the border of Missouri. It goes through and borders several other states as well.
2,540 miles (4,087.72 kilometres), 2,341 miles (3,767.46 kilometres) and 3,767 kilometres (2,340.71 miles)
A river is a natural stream of fresh water, larger than a brook or a creek. A river flows towards another river, an ocean, a lake or an other large body of water.
A tributary.