How big and heavy? Inner tubes came to mind, say off of a tracter or truck. Depending on the size of the culvert I wonder if you could get them to waters edge and put 2-3 tubes around the culvert then air them up and float them to the area you are wanting them to be planted, then just deflate the tubes, Not sure how you could recover the tubes though, or how much stress the rubber will hold up to but I think it would be fun to try out
