What's a riffle? Ben Gill, Ennerdale

We were on Crag Fell in Ennerdale when I found this notice at the top of a precipitous stream called Ben Gill.

 A bit of research found this https://www.therrc.co.uk/2011%20Conference/Poster%20-%20Quinlan.pdf Section 3 mentions sensors being put into a riffle. I'd never heard of one. I looked it up and found that it seems to be a shallow section in the flow of a stream, probably with a pool behind it. I'm wondering whether I found one that they had artifically created. However, this is shallow and narrow rather than shallow and wide so is perhaps a nozzle instead. So not a riffle but at least I know to look out for them now.