Critical flow occurs when the flow velocity in a channel equals the wave velocity generated by a disturbance or obstruction. In this condition the Froude number (Fr) = 1. When the wave velocity exceeds the flow velocity (Fr is less than 1) waves can flow upstream, water can pond behind an obstruction, and the flow is said to be subcritical or tranquil. When Fr is greater than 1 waves cannot be generated upstream and the flow is said to be supercritical, rapid, or shooting. In this condition a standing wave is formed over obstructions in the river bed. In nature, supercritical flow is found only in rapids and waterfalls, but it is often created artificially by weirs and flumes with the aim of measuring discharge.