This is of the same general design as the tsunami-evacuation-route signs along the coast, and it is indeed for a water hazard, but not the same kind as out in Seaside, Ilwaco, etc. We're downstream from the Detroit Lake dam complex, and should there ever be a catastrophic failure up there, the valley would be inundated. These signs point to roads up into the hills that surround the Santiam River valley.
And speaking of getting outta here, it's time to pack up the computers and my luggage and get ready to head to Portland within the next hour or so.