Rapids occur where hard rock layers or debris such as boulders cause the gradient to locally increase downstream. Rapids are similar to waterfalls except the gradient is not large enough to cause plunging. The turbulence through the rapids causes the erosion.
Water approaching the rapids becomes smooth and less turbulent; this condition is called shooting flow. At some point, the thickness of the water increases and the zone of great turbulence begins; the beginning of this zone is called the hydraulic jump. Boulders, cobble, and pebbles swirling in the turbulent water often erode cylindrical hollows which are called potholes or scour pockets.