I had that very thought.
I might be wrong- but I think he’s killed himself. He went on an adrenaline pumping killing spree, drove his car to the river and left it there. Did he have another vehicle there that he left in? Or walk into woods/river and shoot himself? Or- he’s alive and going to hide as long as possible. If he was psychotic during shooting, he may have “normalized” and is now in a vague panic. So much depends on his mental state.
Shooter Card is an avid fisherman with military surival skills. Left his car near a boat launch. Could have taken the boat across the river to another mode of transport. Apparently, the river prevents going any distance upriver (rapids) or downriver (dam). I’m guessing he is still on the run.