To see some really great stuff on this, checking out the forum called "low-magic dnd" on this site. That's got some cool stuff on it. It's really long, though, so if you want to jump in, go to the bottom. Near there, there's a couple of posts summarizing all the posts ...
• Use a rope and tie it to a tree jumper – with the aid of a jump spell it should be simple, of course with a jump spell, the river itself might be able to be crossed. • Drop a tree across the river, yes the trees are that tall, but then a wet tree is not easy to...