Perhaps you're right, but isn't it a bit unfair to allow the player to delay his decision indefinitely? Letting him explore the level means he gets an opportunity to score loot and experience on half an additional ID level and still recreate it if he doesn't feel quite "comfortable" with the river. And it's not like we can really blame him, because crossing a river is rarely completely risk-free - with only the Swimming skill it's still pretty dangerous!
Of course, the current rule system effectively takes out rivers as an ID dungeon feature until the Chaos Gate is open, when being unfair to the other players is encouraged That's a shame in my eyes too, but it would avoid any kind of ambiguity or unfair advantage - the river level is simply taken out of the game with the loss of maybe a few turns.
I'll gladly repeat that your authority is absolute over rule matters. I'm just, y'know, saying :P