How does rain water from as far away as Montana and Pennsylvania eventually end up flowing into the Mississippi River?

Even though the states of Montana and Pennsylvania are far away from the Mississippi River, the water from the rains in this states still manages to rich the Mississippi River. This is because the Mississippi River has tributary rivers that are spread across big portion of the United States, and through the states of Montana and Pennsylvania run some of the biggest tributaries to Mississippi, like the Platte River that flows through Montana, and the Ohio River that flows through Pennsylvania. These two rivers gather in them most of the water of these states and eventually it reaches the Mississippi River.