Bubbler systems are used to measure water level by detecting the pressure required to force air through a submerged tube. The tube is mounted with the end of the tube below the water surface being measured, and the air emerges from the bottom of the tube as a stream of bubbles. The air flow rate is relatively small--just enough to prevent water from backing up into the tube--so the pressure required to push air through the tube is equal to the pressure at the tube's outlet. This pressure is proportional to the water depth above the bottom of the tube.
A big advantage to bubblers is the location of the pressure sensor. The sensor, or transducer, is at the air source on the canal bank instead of being under water. Submerged transducers can have problems with corrosion, clogging, freezing, and physical damage from debris, animals, or people. The only submerged part of a bubbler system is the air tube, which is inexpensive to replace if it becomes damaged.