The Chicago River flows through downtown after it was dyed green in celebration of St. Patrick's Day on March 13, 2021 in Chicago, Illinois.

Chicago has long been a champion of celebrating St. Patrick’s Day – and the dying of the Chicago River running through town is a perfect example of that.

According to Enjoy Illinois, the river being turned green has gone on for five decades. It’s a tradition that always draws some interesting attention.

Much of the attention each year is about whether the dye is environmentally friendly or not. Officials say the 40 pounds of dye in the Chicago River is harmless.

The tradition traces back to 1962 when the Plumbers Local Union decided to dye the river using a dye plumbers commonly used to detect leaks in pipes. The problem back then was that it kept the river green for a WEEK.

Enjoy Illinois says the Plumbers Local Union has kept up the tradition, while figuring out the right amount and type of dye to use to make it last just a few hours.

Check out this cool video and some photos from the Windy City of Chicago as they prepare for St. Patrick’s Day.