Rapids

Rapids are stretches of a river where a relatively steep bed gradient increases the water velocity and turbulence, typically forming where the streambed is unusually resistant to erosion compared with the channel downstream. Four conditions are needed to produce them: sufficient flow, a steep gradient, a constriction, and obstacles in the channel. The shallow, rock-strewn water mixes in air as it churns past and over these obstacles, producing the white, aerated appearance known as whitewater, and the turbulence itself helps aerate and improve the quality of the water passing through.