Chicago dyes the river green as a St. Patrick’s Day tradition that grew out of a 1960s effort to track pollution and celebrate the city’s Irish heritage.

Quick Scoop

  • City workers originally used a green tracing dye in the early 1960s to find illegal sewage discharges in the river.
  • When officials saw how striking the color looked, they turned it into a St. Patrick’s Day spectacle to honor Chicago’s large Irish-American community.
  • Today, a plumbers’ union oversees the event, using about 40–60 pounds of a plant-based, food-grade dye that appears orange in powder form but turns the water bright green.
  • The formula is kept secret, but local and federal agencies have said the mixture used for the official event is considered non-toxic and not harmful to river life in the small quantities applied.

How it works now

  1. The dyeing typically happens on the Saturday around St. Patrick’s Day, in the downtown stretch of the river.
  1. Boats go up and down the river, sprinkling the orange powder and churning the water so it turns vivid emerald within minutes.
  1. The intense green color lasts several hours before fading back toward the river’s usual tone.

A bit of background flavor

  • The tradition formally dates to around 1961–1962 and has since become one of Chicago’s most recognizable holiday rituals, drawing large crowds and copycat events in other cities.
  • Environmental groups still debate it symbolically, arguing that even if the official dye is mild, it can send the message that it is acceptable to treat the river as a prop.

TL;DR: Chicago dyes the river green each March to turn a once-practical pollution-tracing dye into a flashy St. Patrick’s Day celebration of its Irish roots, using a closely guarded, plant-based dye that’s designed to be short- lived and non-toxic.

Information gathered from public forums or data available on the internet and portrayed here.