This work investigates experimentally the characteristics of the thermal area of pollution resulting from surface discharge outfalls of the steam electrical power plant cooling system. The experimental program was carried out in a rectangular flume inside the northern experimental hall of the Hydraulics Research Institute (HRI) in Egypt. The flume has dimensions of 17.8 m length, 2.5 m width, and 0.5 m deep. Formulae and curves to identify the mixing zone center-line length for different outfall angles were developed. The predicted formulae could be used in the cooling system surface discharge design phase to predict the minimum dilution of the thermal plume at different discharge rates in Egypt. The predicted formulae could also help in updating the environmental and water legislations in Egypt.