My pigs like to rub on the big trees and may knock a little bark loose but they are not killing them for sure. They will uproot small trees and knock over dead ones, and may cause damage to shallow roots. Not sure about the maples but your oaks should be fine.
Are you trying to make a pond for drinking water or for mud wallows? They'll make their own wallows with any amount of water on the ground. If you have a Trackhoe and some clay content in your soil you can easily make a small pond. Even if it doesn't seal well at first it probably will after they get in there pigging it up a bit. I wouldn't use the road as a dam personally, especially if it's your main road. Too much risk of undermining the soil structure under the road. I'd make a small dam with an overflow that will flow into your road pipe.
They'll make a bit of a mess, especially if left there without rotation, but having the pigs in the woods is a good way to keep them cool and happy in the summer and fat on acorns in the fall.