I have literally no idea how to go about this. The CK doesnt appear to have any sort of estimation tool included so my guess is that they used an external tool.
As far as I know, water flow maps are generated for each cell for every worldspace using this format
Here is an example of riverwood's cell
and the one just north
It seems that the channels do different things as the texture is probably just an easy way to store data.
Alpha: Speed or Viscosity?
Blue: I don't think this is used, except in the noise.
I came about this because I wanted to extend water flows to falskaar and I was greeted with a very strange "bug".
Here is an example from a custom worldspace.
I have no idea how to go about generating this sort of thing and I know it will not be an easy task.
The only way in my grasp is to probably handpaint which is what SP suggested. This is possible for a handful of cells but definitely not all.
I really hope I am just being an idiot and missing something but I have no idea where to look for this information as water flows are relatively new.