A method and apparatus for an improved dewatering tank system that allows for safely controlling a water stream exiting the dewatering tank system. The apparatus can include a sound velocity detector, a control system, and a control element. The sound velocity detector can include a transducer, a detector, and a transmitter. The control system can include a computer and a program product. The apparatus can optionally include a dewatering tank, a drain line, and a controllable valve. The apparatus allows for transmitting sound energy through the water stream flowing in the drain lined that is connected to the dewatering tank, calculating the velocity of the sound energy as the sound energy travels through the water stream, monitoring the velocity of the sound energy for a period of time, and controlling the position of the controllable valve depending on the calculated velocity of the sound energy.