March 12, 2011 – During the Irish Rebellion of 1798 against British rule, green was the official color of revolutionaries and wearing it was punishable by hanging. 213 years later, it is not quite as provocative and makes a good color for the Chicago River.

In an annual tradition going all the way back to 1962, the Chicago River was turned green Saturday morning between Wabash Avenue and Columbus Drive. Thousands of people lined both sides of the river to watch the spectacle, which started promptly at 10 a.m., but for a second year, police kept crowds off the bridges.

The effect, which lasts about four hours, is achieved by carefully dumping 40 pounds of an orange-colored vegetable-based dye. In 1961, it was noticed that a fluorescent dye used by plumbers to detect leaks into the Chicago River caused the water to turn bright green. The next year, when 100 pounds of the dye were used as part of Chicago’s St. Patrick’s Day celebration, the river stayed green for a week.

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