Port Hole?

My wife found this artifact while we were out walking along the local river bed on Friday evening. I think we would need scuba gear to do the same route today!
Her plan is to create a picture at the back and incorporate the old wood as if it were foreground rocks. It sounds like a good plan, but I will no doubt have to come up with a way of fixing it to the wall!
I'm not sure what it was, but the diameter and remaining wood makes me think it was fixed to the end of a telegraph pole?