This looks like the water running over the weir is flowing uphill! A deliberate trick of perspective as I was standing over the weir on a bridge rather than the more common downstream perspective. The object on the left is a fish trap - not in use here.
I have photographed this weir on the river Meu dozens of times and subsequently deleted all the images as I hadn't managed to capture on the sensor what had captured my imagination. Today I finally got the image that I have been carrying in my mind's eye... box ticked - moving on!