What is the term for the area downstream of an obstacle in water that creates a whirlpool effect?

The term that describes the area downstream of an obstacle in water that creates a whirlpool effect is known as a hydraulic. Hydraulics occur when water flows over an obstruction, such as a rock or ledge, and this flow creates a circular motion that pulls water back toward the obstacle. This phenomenon can pose hazards for water rescues and boaters alike, as it can trap or disorient individuals in the water.

The other terms, while related to water dynamics, do not specifically denote the whirlpool effect resulting from an obstacle. Backflow refers to the water that moves back upstream due to obstructions, while the swirl zone and current pool are more generalized descriptions of areas with circular water movement or eddies but do not specifically capture the characteristics of a hydraulic. Hence, hydraulic is the precise terminology used in swiftwater rescue contexts to describe this dangerous and complex flow pattern.