Hmm... flowing different directions seasonally would be perfect after all. It would truly make the river an important economic resource, as well as create a pulsating kind of life for the fictional settlements along it.
Your reference to the Gulf stream is pretty fascinating, now that I've looked it up. I'm also reminded of the African rift lakes... Id love to implement some similar biology to the river, as Im a fish enthusiast and keep a large tank of lake malawi cichlids.
I could go with the magic route, definitely... but I actually like to keep the basic elements of the world mundane. The seam or scar or whatever may have been created by divine means, but Id like to find a realistic solution to why it hasnt eroded away afterword. So now that this has all come up, maybe some of you geography experts might be able to help me make it work.
First, the 'seam' left behind wouldve been extremely deep and of approximately uniform depth. Given that the surrounding rock would be extremely hard and slow to erode, could this formation continue to reach the ocean at both ends for centuries? Perhaps the river IS eroding to a point where it will split into two, but hasnt reached that point YET? Possible?
Second, is there any way that the inhabitants of the continent couldve extended this state of affairs, possibly even indefinitely? There have been thousands of years of civilization. Is there any system of damming or digging that could produce this result?
What about tectonic activity? This would obviously be an unnatural fracture in the existing tectonic plates... could it be that the river has split in the past, but tectonic activity continually widens the river at random intervals, interrupting that normal erosion process? Maybe throughout history the river has split and rejoined several times. This option might be the best, as it could resemble the situation with the Great Rift in Africa, giving me a point of research to start with.
Any of these things sound possible? Thanks for your help in advance.