A hydraulic jump is a sudden rise in water level due to decreasing velocity. In flowing water, the water before the jump is moving much faster than the water after the jump. When the velocity of the water decreases to critical flow speed a jump occurs. Above this velocity, the water is moving too quickly to allow gravity waves to move upstream. At the point where the water reaches critical flow speed, the gravity waves become unable to move upstream, and build up at the jump, forming the sudden increase in water level, called a hydraulic jump.
