It's a tradition in Chicago, if you hadn't seen this before. Quoted as "It's a little bit of normalcy"
https://www.chicagotribune.com/coronavirus/ct-chicago-river-green-st-patricks-day-20210313-m3wygh6si5cfhiups4meykvhu4-story.html
Chicago River dyed green for St. Patrick’s Day - Chicago Tribune
The Chicago River glowed Kelly green downtown this St. Patrick’s Day weekend after all.
https://www.chicagotribune.com/coronavirus/ct-chicago-river-green-st-patricks-day-20210313-m3wygh6si5cfhiups4meykvhu4-story.html