Must be from the river, on that scenario no one has access to ships.
So I understand correctly, rivers must be actively blocked. Even if you controll both shores the enemy can still reinforce via rivers.

In the battle of Bull run I planned on trapping the confederates at Washington DC. I had the area surrounded to cut off supply, I had a full garrison and I believe it was winter. My question is that as I was hammering the confederates they continued to resupply men and the other bars on the stack ca...