This is done every year right before St. Patrick’s day.
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 fluorescein dye to trace sources of illegal pollution discharges. The dyeing of the river is still sponsored by the local plumbers union.
The United States Environmental Protection Agency (EPA) outlawed the use of fluorescein for this purpose, since it was shown to be harmful to the river. The present ingredients used to dye the river green today are safe for people and wildlife. Forty pounds of powdered vegetable dye are used to color the river for the celebration.
In 2009 First Lady Michelle Obama, a Chicago native, requested that the White House fountains be dyed green to celebrate St. Patrick’s Day.