As part of a more than fifty-year-old Chicago tradition, the Chicago River is dyed green in observance of St. Patrick's Day. The actual event does not necessarily occur on St. Patrick's Day and is scheduled for the Saturday of the closest weekend. The dye takes days to dissipate.

The tradition of dyeing the river green arose by accident when some plumbers used fluorescent dye to trace sources of illegal pollution discharges. The dyeing of the river is still sponsored by the local plumbers union.