[Disclaimer: I am not a member, so I wasn't able to read more than the first few paragraphs. If my comments are totally off the mark, I apologize profusely in advance; and no offense intended.]

Grant, I love your history articles (even though I cannot read the major part of some of them, they allow me to Google your chosen subject and learn something new each time).

But...

In this article you stray from history into hydrology and are no longer on solid ground. The entire notion of a river's "source" is an arbitary (and delusional) human construct. Any point on the rim of a river basin has equal claim to being its "source". By way of example -- perhaps the most obvious one -- if the Missouri River had been "discovered" before the Mississippi, the "source" might have been considered somewhere in Yellowstone.

Hydrologists do identify a basin's main river system by looking at each place water flows converge and following the one that drains the largest area upstream. This is meaningful when areas and the corresponding water flows are large, it gets a little pedantic as one gets higher up in the headwaters. (A landslide can alter the ridge between two streams and thereby switch which is the one to follow.)

This approach is useful and practical for hydrological purposes. (The distance from "source" to mouth of a river is less useful, while more fun for comparisons.) But for adventurer/explorers its more exciting and exotic to lay claim to having "discovered the source" of the Nile or whatever.