Open channel flow sensors measure fluid flow through open channels. Open channel flow measurement is difficult because of non-uniform channel dimensions and variations in velocities across the channel. Some open channel flow sensors measure the displacement of a vane or piston that is pushed by the fluid. A non-contacting open flow sensor avoids flow disturbances and fouling. Doppler flow meters transmit acoustic waves into the flow and use the frequency shifts of reflected waves to determine the flow velocity. Ultrasonic open channel flow sensors mount a non-contacting sensor above the flume or weir through which fluid passes. An ultrasonic flow sensor transmits an ultrasonic pulse and measures the time elapsed when an echo is received. In this way, ultrasonic flow sensors can measure the water level or "head" accurately. An open channel flow monitor can continuously monitor, display, totalize and log flow information.