Good question Phil.
Looking at this other photo from a different angle, it's technically a weir.
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It was taken on my 50th birthday back in 2003 when we met up with my son, who was at Uni in Sheffield and my parents from Cheshire.
I've got a feeling we might have walked from Monsal Head along the river to Cressbrook and Millers Dale, but I can't pinpoint that weir on any of my maps. I'm sure you'll be able to work out where it is sometime.