They're sure to have sentries on the bridge so you'll have to float the stuff down from upriver which means building a raft.
9 seconds sound clip from the The Bridge on the River Kwai movie soundboard.
You can hear this line at 02:06:03.875 in the Blu-ray version of the movie.
Quote context
[...]
- Shears, you'll have to pick a spot on our side of the bank where you can cover Joyce.
- Right.
- Perhaps I might even take a few potshots at the train.
- All clear? Any questions? Right, we'll start as soon as it's dark.
- Yai will take three women to help you build the raft. One will stay here with me.
- All right, now, get cracking.
- You know, if it wasn't for my ankle, I'd take Joyce's assignment.
- You think he'll be all right?
- I think so. Want me to handle it?
- I'd let you stay up here with the mortar if I could. I'm sorry.
[...]