After you.

       No!  After you Sir, I insist.
Very courteous lot these "Boaters".  At odd intervals you'd come across a narrowing, such as this.  I'm open to correction - BUT - am happy to hazard a guess that it was a manoeuvre to reduce "raw" materials required for building bridges.
       When you see another Narrowboat approaching you both manage to decide, without a fight, or a head on collision, to decide who goes first.

Extra?  Me boasting.  What it looked like originally; before I carried off that ancient Baulk of Timber?