As far as humans interfering with the natural course of a river...sure it can be done. It requires enormous amounts of time and labor. And that means it also requires a vast amount of either slaves or money in a pre-technological culture. The thing from a mapping standpoint that you need to concern yourself with is how this alteration will affect where the river goes from there. If it's rather close to a coast then it's most likely no big deal but if it's way inland then you should plan to maybe make the diverted river rejoin the main/previous river channel or all heck could break loose downstream.