I saw a similar post, but they didn't mention that it can be done in the river in the middle of the map too. What you must do is, if you know you're going to lose or time has almost ran out, just jump in the river and head towards the shore where the waterfall is and drop your portal. When the game resets and you get off the boat you run to the mouth of the river and portal up and take the GYs.
IMO this is much better than placing the portal behind the gates as the other person suggested because the opposing team can see it and be ready to gank you. They most likely won't notice your portal in the river.