After you use the tinny walls for define the land near the river, DON'T delete the walls (yet).
Connect them around the river then use AutoDimensioning plug-in to generate build area around walls. This will generate a room outside the walls.
Now you could delete the tinny walls and probably the gap will disappear.
Probably will be on different level but you could cut/paste the room(s) where you need.
Remember:
Will generate build area around ALL walls at that level (connected or not) so you will have to keep only needed ones.
Or maybe could be used as tinny rooms?? I don't know! You have to test.