I'm not entirely sure about 'dragon placement' though. If the dragon is above the gorge, then he'll just fall away again - which would be sub-optimal as we want to take it out.
... maybe there's a way to specify that it should be (and stay) in range (somehow)?
Depending on its angle of approach, it should fall either onto land on next to the river. That said, we let it float away last time; even if it falls into the river again, we'll be able to concentrate fire (the casters/archers especially).