A rapi is a section of a river where the river bed has a relatively steep gradient causing an increase in water flow and turbulence. A rapid is a hydrological feature between a run (a smoothly flowing part of a stream) and a cascade. A rapid is characterised by the river becoming shallower and having some rocks exposed above the flow surface. As flowing water splashes over and around the rocks, air bubbles become mixed in with it and portions of the surface acquire a white colour, forming what is called "whitewater". Rapids occur where the bed material is highly resistant to the erosive power of the stream in comparison with the bed downstream of the rapids. Very young streams flowing across solid rock may be rapids for much of their length.