Thousands looked on as the Chicago Journeymen Plumbers Union poured in the dye to turn the river green on Saturday morning.
"One boat drops the dye into the water and it's an orange powdery substance and it's completely environmentally safe. Once the orange powder hits the water, it turns a vibrant green and the second boat follows and stirs it up to make sure that that dye spreads out," said St. Patrick's Day Parade coordinator Pat McCarthy.
River dyeing timelapse
The tradition started back in 1961.
"The dye that we use is a leak detection dye that we use in plumbing pipes, and apparently there were a couple guys doing it a lot one day. They came to the union hall and the business manager saw them covered in green, and he got the idea," McCarthy added.
PHOTOS: 2017 St. Patrick's Day Parade in Chicago
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The river dyeing was followed by the Chicago St. Patrick's Day Parade, which stepped off at noon and included more than 100 different local organizations.
WLS-TV contributed to this report.