Discovering the source of a river, and continental divide

How did explorers discover the source of a river? As you follow a river upstream, it keeps branching off again and again and again. And at each branching, it’s not always obvious which is the main river and which is a tributary. So how did they know which to follow, to find the true source of the river? Or was it simply a matter of very time-consuming trial and error?

And discovering the “continental divide” presents an even greater problem. Don’t you have to create a whole lot of maps, in order to say “the rivers east of here wind up in a different ocean than the rivers west of here”?

In many cases, area natives could give guidance. “This branch peters out a few days’ travel upstream, but that branch flows from yonder distant mountains.” If not, then . . .

Sometimes. Sometimes they got it wrong, since the Missouri should really be the Mississippi. Just six years ago, National Geographic people with GPS devices pinpointed the exact source of the Amazon River, based on the convention that the branch which flows the furthest is the main branch.

Sometimes, it’s sort of obvious, like when you run up against the Front Range of the Rockies and no river breaches it. But yeah, to have an accurate picture of the Continental Divide over its entire length, you need to do a lot of mapping!

I asked this question before (it may have been sci.geo.hydrology)
Thses day the “main branch” is defined by the one that drains a larger area.
Historically, it was chosen by flow and geomorphology.
As pointed out buy the modern definition the Missouri is the main branch of the Mississippi, but too late to change it now.
Continenatl divide: Don’t forget there is the Pacific / Arctic and Atlantic / Actic divides in N America, not to mention “mini divides:” like the Gulf of Mexico / Atlantic
divide (part of which goes through Wisconsin)


Just go up on top of the mountain, turn on the hose, and follow it down.

Ever notice how the border between Montana and Idaho is all wiggly for a while, but then goes straight north? It was originally supposed to be the Continental Divide the whole way, but about halfway up the wiggly part, the surveyors ended up following the wrong ridge. At the top end of the wiggly part, a river crossed their path, thereby proving that they weren’t following the Divide any more. At which point they threw up their hands, said “to heck with it”, and just went straight north from there.

[divide trivia] At a point 50-60 mi WNW of Rawlins, WY the continental divide splits with one branch going toward the east and one toward the south. The two branches rejoin about 10 miles south of Rawlins. The bowl that is thus formed is The Great Basin of The Continental DIvide, or more compactly The Great Divide Basin. Drainage from snow melt and rainfall is to the interior of the basin and it just evaporates.[/divide trivia]

One thing to recall is that it is all pretty arbitrary, so the rules conform to what humans what to identify, not to some objective natural reality.

For example, as noted, the current rules state that the “real” river is the one with the greatest drainage basin while the older rules stated that it was the branch with the greatest flow. Originally, the Mississippi, wandering through swampy, rainy Minnesota and Wisconsin got the nod over the Missouri wandering across the arid West (and surrendering some part of its flow to evaporation along its length). So, from the perspective of the Earth, what is the objective reality that is being decided by these rules?

Even the Missouri has a clouded provenance, given that its “headwaters” are the conjunction of the Jefferson, Madison, and Gallatin rivers. There is nothing (besides tradition) stopping us from renaming one of those three “Missouri” and extending the length of the Missouri for a few hundred extra miles (with an attendant increase in basin size).

We discussed river selection from a slightly different perspective the first summer this board was up in The Mississippi or Missouri.

Not only that, but doesn’t the flow of the Ohio outmatch the flow of the Mississippi where those two rivers meet?

If it doesn’t, it sure looks like it does. I visited the confluence of the rivers once, just south of Cairo, and my first thought was, “If you didn’t know, you’d swear the Ohio was the main branch.” Of course from the surface you can only judge width, not depth, and I imagine there can be seasonal variation.

Question: If basin size or flow at the confluence are the determining factor, why did the stories on the Amazon expedition emphasize distance?

It sounds like they’re just making it up as they go along.

A less argument prone convention would be that whenever two or more rivers of roughly equal size joined, so that you couldn’t agree that one was clearly a tributary, you simply give neither of them the name of the merged river. As with the Missouri headwaters or the Monongahela and Allegheny joining to form the Ohio. Disappoints people wanting to establish “longest river” statistics though.

There’s even more debate about that. The three rivers don’t actually all join at exactly the same point: There’s a stretch of about 100 meters between where the Madison and Jefferson join, and where the Gallatin joins. Does the Missouri start at the first joining, or the second? It may seem irrelevant, but there’s actually been a mildly simmering political debate about it for several years, involving federal and state funding for Headwaters State Park.

You could, if you chose, make the question even more complicated. For a few miles upstream of the Headwaters, all three rivers are a mishmashed network of channels splitting and re-joining. Where, exactly, do the rivers officially join? Fortunately, the politicians seem to all agree on this point, but whenever we go tubing through there, I always seem to end up on a different sub-channel than the beverage cooler.