You can't on that map...you have to move to another map. You can than move back to the 'river' map if you need to, once you 'group' your guys to stay on the same side of the river when they return to it.
I've walked 4 maps with no way to cross. I'm on the HQ mission. I tried following the river to a fifth map, but it won't let me leave the map in that direction.
Hmmm...it has been a while since I've played. Does the river angle some to one side? If so, you might try to exit to the next map with all your guys at both banks of the river and see if they would then show up on the same side on the next map. I'm sure it can be done, because I do remember having that problem and getting it solved. Just can't remember the exact details of what I did.
Figures...Can't find the CD...got the case, but ain't no CD in it. We moved a few months ago, and my 'computer room' is smaller than what I had, so a lot is still packed until I need to go looking. I'll find it sooner or later, but for now, I've struck out. Sorry about that...