I don't think there's too much terraforming visible on those medieval Dutch maps, I believe that really started much later. They're probably more spread out than your average medieval city though (notice all those gardens and even farmland inside the city walls?)
True, Enheim is well above sea level, but for such a sizeable lake I would expect the terrain to be reasonably flat anyway. (The swamp to the south is another good indication.) You can't really deduce the speed of the water just from the width of the river anyway. And you probably need some kind of canyon or seasonal meltwater to create rapids of the kind you're describing. All in all, the current map looks like a rather placid river to me...
True, Enheim is well above sea level, but for such a sizeable lake I would expect the terrain to be reasonably flat anyway. (The swamp to the south is another good indication.) You can't really deduce the speed of the water just from the width of the river anyway. And you probably need some kind of canyon or seasonal meltwater to create rapids of the kind you're describing. All in all, the current map looks like a rather placid river to me...