Chicago first dyed the Chicago River green for Saint Patrick's Day in 1962.
The idea developed after members of Chicago's plumbing inspection department noticed that a dye used to trace pollution in waterways could color the river. The city then adapted the technique into a highly visible holiday spectacle.
Today, the river is colored for a short period before Saint Patrick's Day, usually during a ceremony attended by city officials, spectators and local Irish organizations. The exact formula and quantity of dye are closely controlled, and the modern event uses an environmentally safer vegetable-based coloring rather than older chemical materials.
The river tradition is often confused with Chicago's Saint Patrick's Day parade, which is a separate event. The green river became one of the world's best-known visual symbols of the holiday.