CHICAGO (AP) — The Chicago River is once again glowing kelly green as the city opens its annual St. Patrick’s Day celebrations.

Thousands lined the river and packed bridges 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 — which falls on Monday this year — celebrates Irish culture. St. Patrick is the patron saint of Ireland and is credited with having brought Christianity there in the fifth century.

The dye is nontoxic. While the river stays bright for several hours, some trace of color can remain for days.

A second St. Patrick’s Day parade was scheduled for Sunday on Chicago’s South Side.