Video: Watch the Chicago River turn St. Patrick’s Day green

If you’ve ever wondered exactly how Chicago turns its river bright green each year for St. Patrick’s Day, you can watch the entire process unfold in a matter of seconds in this high-definition, time-lapse video.

Shot by videographer Chris Biela from the rooftop terrace of the LondonHouse hotel at the corner of Michigan Avenue and Wacker Drive, the clip provides a unique perspective of the holiday tradition that draws more than 400,000 spectators to the river’s edge each year.

The dyeing the river traces its roots to 1961 when a group of plumbers noticed that a fluorescein chemical used to detect pollution could turn large swaths of the river a bright green hue. In 1966, the celebration switched to a more eco-friendly vegetable-based dye—its exact formula is still a closely guarded secret held by the Chicago Journeymen Plumbers Local 130.

The current process involves the use of multiple boats—some to deposit the dye, others to snake around and spread the mixture out. It typically takes about 45 minutes and the color lasts about 5 hours. The best place to catch a glimpse of the action is along the river’s main branch, between the Columbus Drive and Wabash Avenue bridges. If you can’t sneak inside a tall building, Pioneer Court, AMA Plaza, and the Chicago Riverwalk make excellent vantages points.

Although the St. Patrick’s Day dyeing of the river is a once-a-year occurrence, the city broke from tradition in the Fall of 2016 to color the river blue in recognition of the Cubs World Series victory. At the time, the Friends of the Chicago River advocacy group cautioned against setting a future precedent where the city “adds chemicals to the river for every celebration.”