Chicago River dyed green for St. Patrick's Day

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(Photo by Scott Olson/Getty Images)

Thousands of revelers lined the streets to watch the Chicago River be dyed green for St. Patrick's Day.

The annual tradition that dates back decades took place Saturday morning on a stretch of the river in downtown Chicago. Celebrants stood on the Michigan Avenue bridge and the city's riverwalk to watch members of Chicago Plumbers Local 130 dump and spray the dye into the river from boats.

The union says the dye formula is a "closely guarded secret" but members assure that it's been tested and is safe for the environment.

Afterward the city stepped off its annual St. Patrick's Day Parade , one of the biggest parades in the city. Participants in the 64th annual parade include Irish step dancers, bagpipers and marching bands.