On Saturday morning, the Chicago Journeymen Plumbers will turn the river from a gross shade of green to a bright, emerald shade of green in celebration of St. Patrick's Day. The dyeing has been a Chicago staple for more than 40 years, and stems back to the practices used to trace sources of illegal pollution in the waterway.
Interestingly enough, the powdered gunk that they dump into the river to produce that glowing color is not green at all, but rather orange. The concoction is vegetable-based, which replaced the toxic fluorescein dye that the plumbers were using until 1966. According to the St. Patrick's Day Parade's website, the dye's exact recipe is "a closely guarded secret," but we're pretty sure Hi-C's Ecto Cooler mix powder did the same thing in our childhood.
Whatever chemistry is at play in the orange to green transformation is pretty glorious, and if you really want to be obnoxious you can say that the "true" Chicago St. Patrick's Day color is orange.