Serpentine road on part of the site of the serpentine lake is just visible top left of the picture. The part where the onion fair was held was later the car park/asda space behind the sports centre next to the Church. i.e. all on the same side of the railway as the church & hall are, immediately behind the embankement of the Railway.
The river runs the other side of the embankment - it was diverted to flow along it. The ground probably flooded often - it still happens occasionally now on the outer circle, by the end of Tame Road, Electric Avenue, but not so much since they dredged, and built the retaining walls. The artist has probably called the flloded are the sepentine, without realising the actual lake was beyond the railway.
It's still a very good picture, and shows the rural nature of the area!