Hi Megg82,
Oops! It seems your post passed under the radar for me too. Welcome to the forum.
I haven't tried this option either, but I would do it as follows:.
* Regarding the River, I think an option worth trying is to build the River starting from the mouth back to the source. I would place the mouth on the border and would build it backwards trying not to get into trouble (discarding any tiles trying to get outside outside the map or obscuring a pre-printed city tile.) This does not guarantee that the River can be placed uneventfully either: it may be ending abruptly next to a city tile, for example. In any case, you might have to skip a number of tiles and even you might be unable to place the source tile either. In this case, I would consider the River closes the city on that edge.
* Besides the River layout, I would place at least one start tile, for example, on the farthest start town from the river, so you offer an additional starting for players.
Cheers!