I assume impassable too, because the river tiles and the tiles within represents the complete Lake Fleesen.
But i fear, the scenario designers didn´t remember the "Short Supply" card, it´s indeed important and should be clarified better in the notes.
I´ll check it
In the Waren-Nossentin scenarios, the river tiles are actually represent the start (shoreline) of the lakes. The hexes (open hexes) behind the river tiles are still part of the lake and are also considered impassable terrain.
Answer is here and as i said, all hexes are impassable