Abstract: In a method and apparatus for measuring the depth of a flowing fluid, and controlling the flow rate of the fluid based upon the depth measurement, the fluid is caused to flow through a conduit, where it is irradiated with a known intensity of light, and the fluid is caused to fall from a horizontally directed exit terminus into a reservoir having a reflective bottom. The fluid serves as a wave guide for the light it received within the conduit, and releases the light within the reservoir. The intensity of light reflected from the bottom of the reservoir and emergent from the fluid is directly proportional to the depth of the fluid in the reservoir, and is measured by a photodetector which transmits a control signal to a valve in the conduit.