A Canal narrow boat on a Viaduct

A picture of a boy walking along a viaduct next to the canal with a narrow boat approaching. I believe this is in Wales, because it was with another couple of slides of the Ffestiniog railway, but I don't think it is the famous Pontcysyllte Aqueduct because the path which runs next to that canal is raised above the water.

This was probably taken in 1978.

If anyone recognises this I would be pleased to know where it is so I can add a map reference - let me know via the comments form below.