boil
An upwelling of water on the river's surface, caused by deep current hitting a submerged obstacle and being forced upward. Boils appear as smooth, rounded swells that rise and collapse, as if the river is breathing from below. Large boils in big water can destabilize boats.
Etymology
From the resemblance to water boiling in a pot — the upwelling looks like bubbling, though the cause is hydraulic rather than thermal.
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