>>714003476
You asked where those two rivers come from and I told you where they flow
They come from Mt. Gelmir and Mountaintops
On Mountaintops the water freezes but flows under to Rauh
Rauh has a giant crumbling artificial acqueduct that brought water the Helphen Keep and the region of Gravesite plain and the Norther region of the Realm of Shadow.
Rot doesn't spread because it is cut off exactly by the flow of the Siofra river

So Miyazaki actually wrote the map taking care of every single detail