Why is the Missouri River considered as a tributary of the Mississippi River, despite the Missouri being longer?
There is no universally agreed upon definition such that any stream’s source can be systematically determined. The United States Geological Survey (USGS) states that a river’s “length may be considered to be the distance from the mouth to the most distant headwater source (irrespective of stream name) or from the mouth to the headwaters of the stream commonly identified as the source stream”. Thus, for example, the USGS considered the Missouri River a tributary of the Mississippi River, despite the Missouri being longer.
