That’s true - you make another good point. Yeah, I don’t see why that wouldn’t work with the rules as I had envisioned them. You can pre-generate hexes with rivers when you want in a certain area, and you can generate on the fly at a different session if you wish. I mean, as the game starts, it makes sense for the immediate area around the starting town to be known (read: pre-generated). Then, once you go a bit further out and you reach uncharted territory, then you can decide on the spot if you want to pre-generate an area in that direction or if you want to do it on the fly hex by hex.
Yeah, I like this! Thanks for your input!