Originally Posted by
Gumboot
There are, of course, exceptions to every rule. The phenomenon of dividing waterways is called bifurcation. Divide Creek in Canada splits in two near Kicking Horse Pass, with one branch flowing eventually into the Atlantic and another flowing eventually into the Pacific. Likewise, North Two Ocean Creek divides in two at Two Ocean Pass in Wyoming, forming two creeks (Pacific and Atlantic Creek) which flow into their respective oceans.
To quote Wikipedia:
"An interesting aspect of geology that makes Parting of the Waters possible is that the entire drainage of North Two Ocean Creek occurs within a hole in the Continental Divide. Two separate legs of the Continental Divide completely surround the drainage of North Two Ocean Creek, with one juncture of these two legs occurring at Parting of the Waters, and the other juncture occurring high up on Two Ocean Plateau approximately 2 miles (3.2 km) due North. As a consequence, a drop of rain falling anywhere within the North Two Ocean Creek drainage has an equal chance of flowing either into the Atlantic or Pacific Oceans."
It's worth noting that these divides happen in small creeks situated perfectly in the middle of a drainage divide. The same happens very rarely with lakes that are situated perfectly across a drainage divide, such as Wollaston Lake in Canada which has two outlets. Isa Lake in Yellowstone National Park is believed to be the only lake in the world that naturally drains into two different oceans. Thus, not only is this phenomenon very rare, but it's even more rare in rivers of the size that are likely to make it onto anything but the most localised, detailed maps.
River bifurcation can also occur due to mankind's influence. A good example is the Nile which had a split called the Bahr Yussef which only existed during the yearly floods, when excess water would flow into an inland sea at Fayyum. During the 12th Dynasty a canal was built to increase the flow, but the channel eventually dried up and the inland sea dried up with it, forming a depression instead. More recently, a modern canal has been cut to reopen the channel and feed water from the Nile back into Fayyum.
One important thing to bear in mind about bifurcation is that while rivers and lakes change course, position, and size over time (with news ones emerging and old ones disappearing), this is particularly true of bifurcation as any waterway with two channels is inherently unstable and one will eventually close up.
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