Why is the Rio Grand either the fourth or fifth longest river system in America? Why can't we say with certainly that it is only the fourth longest and not fifth longest, for example?
The Rio Grande river is one of the major rivers in the Southwestern United States. It starts from Colorado and ends in New Mexico, with a majority of it's course running through New Mexico.
Officially, it is recognized as the 4th longest river in the United States. It's length is considered to be around 1800 miles. In the 1980s, the International Water and Boundary Commission stated that the length is 1,896 miles. At this measurement, it surpasses the length of the fifth longest river definitively. Recognized as the fifth longest river, the Colorado River runs 1450 miles.
However, the reason why there are discrepancies and disputes on whether it is the 4th or the 5th longest river is because it's length changes. This happens due to two very related reasons - the way it is measured changes, and this is because it changes course. It has two streams, and it dwindles to nearly nothing towards the Presidio end. Hence, depending on which course it takes, or how it is measured, it can be the 4th or the 5th longest river. Nonetheless, it appears that measurement by the IBWC would secure it's 4th position.
The Rio Grande is sometimes the 4th and sometimes the 5th longest river depending on how it is measured. The measurement may change because its course varies. It is officially measured to be around 1800 miles (the fifth longest river is at 1450 miles).