Thousands lined the river and packed bridges on Saturday and erupted in cheers as members of the Chicago Journeymen Plumbers Union Local 130 sprayed dye into the water from boats, carrying on a tradition they began some 63 years ago.
The dyeing immediately precedes the annual downtown St Patrick's Day parade.
The day honouring the patron saint of Ireland is a global celebration of Irish heritage. And nowhere is that more so than in the United States, where parades take place in cities around the country and all kinds of foods and drinks are given an emerald hue.
In fact, it was among Irish American communities that the day became the celebration it is, from its roots as a more solemn day with a religious observance in Ireland.
But even in America, it was about more than a chance to dye a river green or just bust out a favourite piece of green clothing, it was about putting down roots and claiming a piece of the country's calendar.
The day — which falls on Monday this year — celebrates Irish culture and St Patrick who is credited with having brought Christianity there in the fifth century.
The dye is non-toxic. While the river stays bright for several hours, some trace of colour can remain for days.
A second St Patrick's Day parade was scheduled for Sunday on Chicago's South Side.
15°C