A flow monitoring device for monitoring mud flow in a return line of a bore hole drilling operation, including a mounting bracket mounted to the return line with a viewing port for access to an interior of the return line. A light source illuminates, through the viewing port, a portion of the mud flow in an observation zone within the return line. A camera creates a plurality of images of the mud flow in the observation zone. A controller is configured to analyze the plurality images to: determine a level of the mud flow in the return line and the presence of at least one irregularity, track a movement of the irregularity across the plurality of images to determine a rate of movement of the irregularity, and calculate a flow rate of the mud flow through the return line based on the movement rate and the determined level.