If a city (ancient/medieval level technology, though I am not concerned about absolute realism in that regard) along a river, which in that particular tract happen to have a rather tumultuous course, would want to have a calmer artificial port, could it obtain that by channeling waters to an artificial pond/small lake? Perhaps with a mobile dams system? or is that not even necessary (i.e. simply channelling waters to an artificial pond would create a less agitated environment?
If that could be done, what would be the best way (especially in terms of shapes of the channel/port system, which is what would affect the actual map)?
I don't know where to start with this, and I would like some suggestions. Many thanks.