Watch: Chicago River Turns Green for St. Patrick's Day. Know The History

The Chicago River was dyed green last week to start the city's St. Patrick's Day celebrations. In honour of St. Patrick's Day, which is marked annually on March 17, Chicagoans have been coloring their north river for the past 60 years.

Several social media users posted the beautiful images and videos of the event when the six-member crew dyed the river with vegetable colors.

According to a CNN report, the tradition was started by plumbers. In the 1960s, Chicago city workers used a bright green dye to identify leaks in pipes, which would often stain their white coveralls, CNN reported in 2019.

In 1962, members of the Chicago Journeymen Plumbers Local Union who were feeling festive dumped 100 pounds of that dye into the Chicago River, which turned it green for an entire week, per the Illinois Office of Tourism.

The union has since perfected its viridescent formula, and these days, it only dyes Chicago's waterways for a few hours on a Saturday, on or before St. Patrick's Day, when Chicago's St. Patrick's Day Parade is held.

The people who dye the river don't use chemical-based colors; they prefer environment friendly colours. Their environmentally friendly dye formula remains a closely kept secret, according to an NBC report.