I’m clearly not up to speed on traditions, so this came as a neat surprise to me. Chicago’s been dyeing the Chicago River green for St. Patrick’s Day, a tradition since the 1960s!
It started as a dye to trace the pollution in the river. It was the St. Patrick’s Day Parade Chairman, Stephen Bailey, who saw the potential of this green dye. He talked the powers that be into dying the river green and they said ‘Okay’ (paraphrasing).
The history is more detailed here in Chicago Magazine
This is how they dye the river: