mike23 Nov 6, 2021:

A boil describes the action of water rising quickly to the surface and creating the appearance of boiling water. Boils are usually caused when too much water is being forced into one location. They'll often appear at the bottom of rapids, where the fast moving water runs into the slower moving water that is pooled
https://paddling.com/learn/managing-boils

Frank Szmulowicz, Ph. D. Nov 5, 2021:

Czy rzeka opływa skały, czy prosty grunt?