For 40+ years it has been a tradition to dye the Chicago River green on St. Patrick's day (or the Saturday before). The dye is injected on both sides of the Du Sable Bridge at Michigan Avenue. River current and weather conditions determine the rate and reach of the dye on Saturday. It is not unusual to see remnants of the dye extending down the Main Stem for three or four days after its injection. These photos capture the dyeing on March 16 and the green river the day after. The dye extended from Lake Shore Dr. to LaSalle St. on March 17th.
For more information on the organization responsible for this event, follow this link.