Plumbers in the state of Chicago have dyed the river green as part of a longstanding tradition to celebrate St Patrick's Day - using vegetables to pull off the show stopping scene.
The practice, which has been going on for more than 60 years, was started completely by accident.
A former Chicago Mayor wanted to clean up the city's riverfront area, to do this he got plumbers to use an orange dye into people's water systems.
When the solution came into contact with sewage, it turned a bright green.
The bright green wash gave one member of the Chicago Journeymen Plumbers Local union an idea.
Today, the descendants of just two families from that plumbers union are still the ones responsible for dyeing the river each year.
Typically the river is dyed every Saturday before St Patrick's Day, with this year thousands of braving the cold last weekend to celebrate and the green river, NPR reports.
The families say they now use a more environmentally-friendly solution made from vegetables (and has been since 1966).
However the actual recipe for the dye is a closely guarded secret.
The dye is known by the two families as 'Leprechaun Dust' and is orange before coming into contact with water.
Margaret Frisbie, executive director of Friends of the Chicago River, has long campaigned to end the practice of dyeing the river.
She argues that it perpetuates the idea that humans can do whatever they like to the environment.
Frisbee told Euronews: "Illinois EPA has never required a National Pollutant Discharge Elimination System (NPDES) permit so there is no way to know what the dye is or whether it is harmless".
"Imagine fish, beavers or otters swimming through that dyed-green water and you might think a new celebration is the right idea.
"While we don't know the full impact of the dye on the ecosystems of the Chicago river, we do know that dyeing the river has encouraged copycats and not just in other cities".
Only a certain section of the Chicago river is allowed to be dyed each year.
However illegal dyeing occurred in both 2020 and 2021, during the Covid-19 pandemic.
The Chicago TV news station WTTW reports that police officers will be on the lookout for any illegal dyeing taking place this year.
