I have never forgotten my viewing of this huge spring back in 1959! The coloring and the steam is sublime. It has a temperature of 147-188°F. It is the third largest hot spring in the world. The Hayden Expedition in 1871 named this spring because of its beautiful coloration, and artist Thomas Moran made water-color sketches depicting its rainbow-like colors. Grand Prismatic discharges well over 500 gallons per minute.