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Sometimes on a river, it picks up speed and creates rapids. These happen as a result of the natural deposits of rock and sediment as the water channels downstream.
In the graphic link above, the water churns and increases in velocity and doubles back on itself. If someone gets into it, they may become trapped and have to wait until it kicks them out. They can try to make for the edges or to shore, but they can exhaust themselves as they fight against the rapid.
Worse, if the formation of rock creates a cave or the flipside of the graphic above, then the churning creates a “washing machine” effect and they swirl in a submerged state. Other detritus has been caught by it in the past, maybe branches and their clothing can get hung up on it. If so, unless the hydraulic spits them out, then they can drown.
In rapids, you're moving from one dangerous spot to another in very brief seconds and then maneuvering the next one. Your shoulders and arms will burn from the exertion. Few people can do it for two days if Class 5 or more.
No one should attempt whitewater unless they're experienced, know the river, can read the signs, is in a good mental state, paying attention, blah blah blah.