There is an invisible wall inside one of the tunnels on more-or-less the right side (IIRC) of the damn (when looked at from the Valley).

I don't have a vid, as Kjerstin is not a map I play often, but, run through the various tunnels and you will encounter it eventually.

Edit: I thought it was near the AT-08 junction, but today (as I was trying to get a vid) I could walk straight past it.