The three longest rivers in the world are outside of North America: the Nile in Africa (4,132 miles), the Amazon in South America (4,000 miles) and the Yangtze in Asia (3,915 miles). But the United States still boasts some truly impressive river systems, including several iconic waterways that it shares with neighboring Canada and Mexico.
Measuring the length of a river isn’t as straightforward as it seems, says Eric Taylor, a zoologist at the University of British Columbia and author of Rivers Run Through Us: A Natural and Human History of Great Rivers of North America. A river’s length is defined as the distance from its source (sometimes called the “headwaters”) to its mouth (where it empties into the ocean or another body of water). But even scientists don’t always agree where a river begins and ends.
“That's part of the ‘fun’ of all this,” says Taylor. “People will argue and advocate for their particular perception because rivers mean so much to them.”
For example, some sources still claim that the Mississippi is the longest river in America, and some lists don’t include the St. Lawrence River at all. Below are the longest U.S. rivers, according to the most recent data published by the US Geological Survey (USGS).