A turbidity sensor assembly for a dishwasher appliance is provided. The turbidity sensor assembly measures the turbidity of wash fluid in a manner similar to conventional turbidity sensors, but also includes an integral water level indicator. The water level indicator is an opaque buoyant ball that rises and falls with the water level to provide a water level indication that can be used to shut off the drain pump when the sump is empty. More specifically, when the water level drops below a predetermined level, the opaque ball blocks the transmission of light between the emitter and receiver of the turbidity sensor, thus indicating that the sump is almost empty. As a result, the pump operating during a drain cycle need only operate long enough to drain the sump, and this may be achieved without an additional measuring device or complicated control system.