I always thought the US Continental Divide would be an imaginary path connecting all the highest peaks in the Rockies to assure rivers to the east of this line all flow eastward, and rivers to the west of this line all flow westward. However, this is not the case. The Continental Divide does not align with the highest peaks. Why not? And, how then did they identify the path of the Continental Divide? (BTW, if one was ever curious, there is an Eastern equivalent where rivers either flow east to the Atlantic or west to the Mississippi.)
Plus there’s the north-south continental divide, where waters flow north ultimately to the Arctic Ocean and Hudson’s Bay. The Souris River and the Red River both drain to Hudson Bay.
Because the Rocky Mountains aren’t a single unified ridgeline.
Not to mention triple points, such as one near the western NYS-PA border where water on the three sides goes either to the Mississippi or St Lawrence or Chesapeake.
There’s nothing that says the tallest peak of a mountain chain can’t be all draining to one direction. It depends which way the valleys around it drain. It’s not about peak heights, it’s more about high points of valleys from one peak to another. And generally, any ambiguous result became a mountain lake, and whichever way happened to be the lowest drain, erosion over time would emphasize that advantage - turning a stream outlet into a canyon, while the “almost” going the other way never eroded.
You can also end up with situations like the Columbia river. Some of the mountains in British Columbia, the east side drains east (as would the east side of all mountains) - until they hit a valley which drains south, then turns west.
Or places like the Great Divide Basin that don’t drain to any ocean.
A really cool example of the divide not being at a highest point is Two Ocean Pass in Wyoming. North Two Ocean Creek splits there into Atlantic Creek and Pacific Creek.
Suppose you have a line of higher and lower mountains separated by valleys:
m_m_M_m_m_M
But in the real world, the peaks and valleys look more like this:
M-m–m___M-mmm_mM-M-m
The watershed (which is what the Continental Divide is) will not be between the two highest peaks (M-M) but will actually follow the lowest valleys (m___M and m_m)
Here’s a map of North American watersheds. Note especially the Great Basin (where there’s no outflow.) Also note that the Great Lakes watershed is relatively narrow on the U.S. side. In Chicago, essentially only the area east of the Des Plaines River drains to Lake Michigan.
Dividing continents by river drainage basins can get weird. The highest mountain ridge in the world drains to the West into the Arabian Sea via tributaries of the Indus, and to the South into the Bay of Bengal via tributaries of the Ganges. To the north of these high mountains, water drains into the Tsangpo-Brahmaputra which flows east then south, eventually splitting into two distributaries which are themselves … tributaries of the Ganges! (The Ganges then changes names, first to Padma after it merges with one Brahmaputra distributary, then to Meghna after it merges with the other. In full flood the Meghna Estuary discharges more water than any other river in the world except the Amazon.)
While most of the circumference of the Himalayas thus drains into the Meghna Estuary, just to the north of Tsangpo-Brahmaputra, the Salween River flows, squeezed between the headwaters of the Mekong to its North and the Tsangpo and eventually Irawaddy to its south, proudly tributary to no other river (and, so far, proudly undammed), draining a very long yet very narrow region and eventually emptying, still alone, through the Gulf of Martaban into the Andaman Sea.
And north draining rivers can be a real problem. The mouth doesn’t thaw till a month after the sources, which can then back up and cause major flooding in the south. The Red River and its tributaries are especially prone to this.
If anyone is having trouble visualizing things, here’s a simple case:
ridge is divide
|
V
|
|
\
(X) |
Mt. Big Peak |
/
\
\
|
|
/
/ (X)
/ Mt. Other Side
/
|
|
\
|
\
|
The ground here is sloping down and away from both the left and right sides of the ridge (that’s what makes it a ridge), so water on either side of the ridge is stuck on that side (assuming the ridge in this simple example runs forever up and down). However, there can easily be peaks (as shown) or entire other mountain ranges (not shown) on either side of the ridge that are themselves taller than the ridge. This just means that any water flowing away from the ridge has to go around those peaks or ranges to get where it’s going. And, naturally, any water flowing from the top of the high peaks is still trapped on its respective side of the ridge.
I heard a discussion of Norilsk in Siberia - during the spring thaw, the river that drains into the Arctic floods regularly and anything in town has to be pulled 1/2 km back from the river bank.
By approaching it from the other direction, simulating a rise in the water level of both oceans, inch by inch, pixel by pixel. As soon as the two bodies of water meet at any point, that point is marked as a part of the divide. Note how isolated mountain peaks on either side become islands within the drainage basin of one ocean, even if they’re at a higher altitude than the eventual divide.
In addition, thanks to the many tunnels and water projects the actual Continental Divide is often many miles West of where the tourist signs say in Colorado. The water is collected off the West Slope and sent to the Front Range.
And there’s always the movie: Continental Divide (film) - Wikipedia
I’ll have to ponder this further. Obviously, my mental picture is oversimplifed. Maybe the Wiki movie will help. I’ll go make the popcorn…
An interesting point (kinda obvious when you think about it) is that drainage divides not at the highest points along a mountain ridge or chain, but at the lowest: the passes.
A really interesting case is Isa Lake (which is more like a pond than a lake) in Yellowstone Park. It straddles the Continental Divide (in Craig Pass) and thus has 2 outlets, one of which carries water to the Atlantic, the other to the Pacific. To add to the curiosity, it’s the water that departs to the west that ends up in the Atlantic; the water that starts off headed east is destined for the Pacific.
I think the OP makes the assumption that a high-point or ridge means water drains into different drainages. This is not a good assumption, as has been pointed-out above - tall mountains can be entirely within one river drainage.
One way to think about the Continental Divide is not based on height, but on the watersheds. Waters flowing east to the Missouri and Mississippi on one side of the divide, and waters flowing to the Colorado and Columbia on the other side of the divide (U.S.-centric).
There is also Triple Divide Peak in Montana - waters flowing off it end up in the Pacific, Atlantic, and Arctic oceans.
Here is a map showing the North American drainage basins with the “divides” identified.
#/media/File:NorthAmerica-WaterDivides.png){kind=link}
Aw, I was just about to mention Isa Lake. But Crotalus’ mention of Two Ocean Pass just might have it beat.
As an aside, this means that it’s impossible for any South American vampire to reach Canada, because it must always cross at least one natural flow of running water to get there.
If we’re counting that, then all the Great Lakes and their streams except Erie and Ontario would partly drain into the Gulf due to the reversal of the Chicago River.