Ah... so that's two rivers, that are unnaturally merged.. but then the question is: since rivers form from rain runoff (or springs), are these mountains representational of a fairly large mountain chain enough create a deep enough channel for navigation? ie, is there enough water flowing into the newly formed channel to keep it filled from natural sources (or unnatural for that matter, such as a bound water elemental) Also, how do they deal with the currents(ie, going against them), especially if part of the route is underground(rowing, steam power, etc)?

Or, perhaps I am missing it since your description seems to indicate an unseen source of water above the map where at least one of the rivers originate.

As for the waterfall, once you get an idea for placement(you need either a cliff or two mountains joined together to form the "lip" as well as a mountain behind to form the water source), let us know... I see a few places that might fit the second approach with a small bit of moving around.

Oh yea, there is a "river" that splits from the top river, goes through forests and then joins another river near the bottom that empties into the sea.. Again, not a likely natural occurring formation.