I was just about to say that possibly the terrain you have in Iceland, with massive run-off from ice sheets over relatively flat areas, results in forking rivers. Likewise I'm personally very familiar with terrain that is probably ridiculously mountainous to someone from, say, the eastern two thirds of the USA and don't think much of sticking buildings on 45° slopes .

Anyway - the way rivers behave on Earth doesn't matter at all, as long as you have a *reason* for it (it's a fictional world after all) ... although it's good to have a better reason than "MAGICK" (little stars and unicorns fly around singing).