Another stationary wave

At Scale Hill near Crummock Water there is this weir which I think is used for flow measurement. The bottom of the stream comes up, making the water shallower. The water flows at the same volume flow rate, so it must move faster to get through the decreased cross section. It then speeds up down the ramp and goes from transparent laminar flow to turbulent flow. The Reynold's number must go above 1000. A stationary wave forms at the bottom of the slope. It's amazing to think that the individual water molecules flow through it quickly but the shape stays the same for hours.