Riffle

They then proceeded below the milldam, where there was a strong riffle, with likely looking pools and eddies

The big trout feed aggressively and tend to lose their caution in the rifles.

[...] Brook Drain has now been diverted into a newly dug replacement river section [...]. It has been designed in conjunction with the Environment Agency to hold a much larger volume of water, in order to help with local flooding issues. It also features riffles (shallow sections of water) and side pools where aquatic wildlife can take shelter during storms.

Near-synonym: rifle