Some like to measure the size of a river is by the size of its watershed, which is the area drained by a river and its tributaries. The Mississippi River drains an area of about 3.2 million square kilometers (1.2 million square miles) including all or parts of 32 states and two Canadian provinces, about 40% of the continental United States. The Mississippi River watershed is the fourth largest in the world, extending from the Allegheny Mountains in the east to the Rocky Mountains in the west. The Amazon for comparison drains about 7.1 million square kilometers (2.7 million square miles).
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