People and wagons have no trouble passing even closer to the lake, so why can’t I build a road ?
If a wagon passes, a road should be possible to build.
I have the same issue in my latest town. I can see both sides of this. A road cannot be built over a creek without a bridge, which is not available; but a person or a oxen driven cart could certainly ford a shallow creek. I resolve this issue in the manner you did, and simply restart the road on the other side.
If you cannot build a road, you cannot build a wall either. If you build a wall to the marsh the raiders can simply go around the wall… that has happened to me. While if I build a wall to a lake side, the raiders don’t come around it. Aggravating when you think you have them walled out and realize there is no way to wall them out at certain spots.