A condition in a stream where, at a certain point, the water depth passes from above critical depth to below critical depth. A change in flow conditions downstream of the point of critical depth is not readily transmitted above the control. Thus the relation between stage and discharge above the point of critical depth is fixed (or controlled) by the hydraulic characteristics of the channel at the section of critical depth. A critical depth control is usually found at a point where the channel slope steepens considerably, as above a rapids or waterfall.