From a recent email:

My understanding of river names is that the name of the larger, longer, segment of a river system, should apply after two segments join. What you have here called the lower Mississippi (referring to our new Lower Mississippi River Valley map) should rightfully be, imho, the lower Missouri. What do you think??

Thanks for the comment and enthusiasm! 

As for the names— geographic names are, in practice, historical accidents. Usually the name follows the larger stream, since the folks who first named them, Native American or European, had little if any idea where these streams rose. At St. Louis the Mississippi is unquestionably larger than the Missouri, hence the name. (As for the Missouri itself, should its name to be applied only below the confluence of the Madison, Jefferson, & Gallatin? But, it is. Again, historical usage). 

Raven does emphasize the upstream sources on Raven Maps, in line with your comment. So, for example, we give the Pit in NE California a greater line weight than its flow would warrant, since it’s much longer than the (much larger) Sacramento. The Pit commonly shows up as a major river on European maps, since it is not only long, but also appears to come out of an apparently large lake (Goose Lake, on the California - Oregon border). In fact Goose Lake is periodically a dry lake, and would only spill over into the Pit after a very wet decade, but Europeans generally don’t get that.

A further entertaining wrinkle: Spanish tradition, also many tribes, applied descriptive names
to local reaches of rivers. What we regard as a single named stream can just as well be thought of as a string of distinct names. Each system has its advantages.