Betsy Mason, for National Geographic:
The Mississippi River is impressive on its own, but when you consider all the other rivers that feed it on its way to the Gulf of Mexico, it seems all the mightier. The Mississippi’s massive watershed stretches from the Rocky Mountains in the west to the Appalachian Mountains in the east. It covers all or part of 31 states and reaches into two Canadian provinces. It drains nearly 40 percent of the contiguous United States—1.2 million square miles in total.
The article contains an excellent animation showing how the rivers feed into the Mississippi, and examples of these 19th century comparative charts: