hole

A river feature where water pours over a submerged rock or ledge, drops, and recirculates back upstream, creating a churning, foamy pocket. Small holes are playgrounds for kayakers who surf them for sport. Large holes can flip rafts and hold swimmers underwater. Also called a hydraulic.

Etymology

Paddling slang. The name comes from the visible depression in the water's surface downstream of the obstacle — the river literally dips into a hole before climbing back out.