It started in 1961 with Stephen Bailey of Plumbers Local 130 and a pair of overalls. The first dyeing was in 1962. Mayor Daley wanted Lake Michigan dyed, which was never going to happen, so it became the river.
The first year took about 100 pounds of dye and the color lasted about a week. Now it's about 40 pounds of a vegetable-based powder that goes in orange and turns the river green, and the vivid color holds about four to five hours.
Catch it the Saturday before St. Patrick's Day, around 10 a.m., between Columbus and Orleans. Upper Wacker has the best view.