It is generally considered fact that the largest river in the United States is the Mississippi River (in terms of length); however, this is due to the title and not necessarily the geographic definition of river – depending upon the definition used. River length is generally defined as the distance between the source to the mouth (end) of the river; however, even within this definition their is debate; as determining the source of a river with many larger tributaries (rivers feeding into other rivers) is not always straightforward. Determining the end of a river is debate as well doe to the fact that when rivers near the mouth there can be a swampy mix of salt/fresh water and there are distributary channels that carry water in multiple directions. That being established:
The United States Geological Survey (USGS) regards the Missouri River to be the largest river in the United States, as it does not include the Arachfalaya River, a distributary of the Mississippi River, as part of the Mississippi River. The USGS records the length of the largest river, the Missouri River, to be 2,540 miles long and the length of the second largest river, the Mississippi, to be 2,340 miles long. If the Mississippi River’s length included that of the Arachfalaya River, the combined length would be 3,760 miles long; making it the largest river in the United States. Further complicating the matter is that the Missouri River part of the Mississippi River System and if and if the judging criteria for the largest river was the distance between the mouth a river to the source of the farthest tributary, the Mississippi River System would be longest via the Missouri River tributary (the length of the Missouri River plus the length of the Arachfalaya River).
Put simply, the largest river in the United States is either the Missouri River or the Mississippi River depending upon the criteria used for determination.
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Also See The Largest