An ultrasonic open channel flow meter uses ultrasonic reflection to determine the distance from the face of the ultrasonic transducer to the surface of the water.  The elapsed time between when the signal is transmitted and when it is received determines the water level.  The ultrasonic transducer acts as both a pulse emitter and an echo receiver.

Note that the speed of sound is varies with air temperature, so for accurate flow measurement, the transit time must be corrected for speed of sound changes caused by air-temperature changes.  Many ultrasonic transducers have temperature sensors built in, but others may require a stand-alone temperature sensor.   

It is also important that the temperature sensor / ultrasonic transducer not be dramatically hotter (cooler is not usually a problem) than the air temperature - a sensor sun shade may be necessary.

Ultrasonic flow meters are:

• Low cost
• Require little maintenance as there are no moving parts / the sensor is not directly in the flow stream
• Immune to the effects of silting, suspended solids, suspended grease
• Suitable for large pipes, irrigation channels, rivers, and large streams