Okay, the suspense is over - no shock attack was triggered. The 21st Division arrived without any fireworks. Apparently, since I owned the hexside, the 1/3 AV requirement didn't matter.
So, as long as you control the hexside, you can cross a river without worry.
Now, what I want to know is why the word "enemy" was shown in red next to the word "river". When I see something in red, whistles and bells go off telling me something really bad is about to happen. In this case, I guess red just happened to be someone's favorite color.
I think the red 'Enemy' text is always there when entering a hex containing enemy units, regardless of hexside control. It just so happened to be placed next to the blue 'river' text, but no linkage between the two.