How long does the green dye last in the Chicago River? Here’s what to know

The Chicago River dyeing has taken place, kicking off a weekend of St. Patrick’s Day celebrations in an annual tradition that is recognized across the world.

While those in downtown Chicago Saturday may notice a brilliant shamrock green in the immediate aftermath of the dyeing, some may be wondering exactly how long the dye lasts.

The bright green color will only persist for a few hours, according to Choose Chicago, though the river will retain a green hue for a few days.

In 2024, the Chicago River remained green for several days after the dyeing, still appearing in the middle of the following week.

Here’s more on how the process takes place:

How do they dye it green?

Each year the Chicago Plumbers union uses boats along the Chicago River, using a concoction that sprays dye out of plumping pipes and spout pumps.

But the mysterious mixture doesn’t come out green – it’s actually orange.

However, once the dye sets in, the color changes to green.

According to Local 130, other cities have attempted similar feats, but never found success.

What’s in the dye?

The answer to this has remained a mystery for years.

The plumbers union, Choose Chicago says, “still holds the river-dyeing honors today.”

But you won’t be able to find their recipe anywhere.

“Their environmentally friendly dye formula remains a closely kept secret,” Choose Chicago says.

When did the tradition begin?

The story began in 1961 when Stephen Bailey, the business manager of the Chicago Plumbers Local 130 union, was approached by a plumber whose overalls had been dyed a remarkably brilliant shade of green.

According to the story, the dye used to detect leaks in the sewer system along the Chicago River turned the water green, and the idea was born to dye the river green in honor of St. Patrick’s Day.

Each year, the Journeyman Plumbers Local 130 heads out on boats onto the Chicago River and pours an orange concoction into the water. In a seeming blink of an eye, the dye turns from orange to green, and the water maintains that brilliant hue for the iconic holiday.