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Chris Bentley/MEDILL

The story behind the nearly 50-year-old tradition of dyeing the Chicago River green. And is all that dye safe? Crew members talk about how they transform the river come St. Patrick's Day weekend.

Recipe for dyeing a river: Chicago River's annual wearin' o' the green

by Chris Bentley
March 13, 2011

In a St. Patrick’s Day tradition dating back nearly 50 years, the Chicago River Crew dumped buckets of a vegetable-based dye into the river Saturday.

The dye's exact recipe is a closely guarded secret. Some say the special sauce is fluorescein — a chemical used by Northwestern University researchers, among others, to image samples.

At any rate, crew members say the formula has not changed since 1961, when plumbers first used its bright green signature in water to trace the sources of river pollution.

Today the crew says the dye, which they add can be seen as far away as the Mississippi days later, is environmentally safe. In addition to independent approval from local and state inspectors, the dye had to meet international standards when the crew accepted an invitation to dye Dublin’s Liffey river in 1998.

This year’s event was dedicated to Mark Butler, who died of cancer this fall. Mark’s father Mike Butler has been a member of the Chicago River boat crew for more than 34 years.