Dyeing of the Chicago River
Time Out says
A tradition since 1962, representatives of the Journeymen Plumbers Local Union 130 pour dye into the Chicago River on the morning of the annual St. Patrick's Day Parade, turning the water a bright shade of green. The dye is vegetable-based, so the river typically reverts to its usual murky shade by the end of the weekend. The dyeing takes place between Columbus Drive and State Street, so head to the Michigan Avenue bridge or snag a spot along the Riverwalk for the best views of the action.