Why is most of the land surrounding the Mississippi River prone to flooding

The Mississippi River takes on water from a vast area. Its watershed is the fourth largest on the planet, covering a 1.2 million square mile (3.1 million square kilometers) swath across 31 states. In fact, rain and water from as far away as the Rocky Mountains in the west and the Allegheny Mountains in the east make their way to the Mississippi through a massive network of 250 tributaries.