I've always found the dying of the Chicago River to be an interesting St. Patrick's Day tradition. I looked up the story of how this cultural ritual started. Here is what I found out. Hopefully, it's not blarney!
The Plumber's Union, under the leadership of Stephen Bailey, was in charge of the St. Patrick's Day Festival in Chicago. In the year 1961, a plumber came into Bailey's office wearing white coveralls which had bright green stains on them. Bailey asked him how he got the stains and the plumber replied that he was putting green dye into the waste system in order to see if any of the pipes leaked into the Chicago River. After the plumber left his office, Bailey realized that the union could make the St. Patrick's Day festival special that year by coloring the entire river green. One hundred pounds of dye was dumped into the river that first year and turned the Chicago River into the Shannon River for an entire week. Today, only forty pounds of a more environmentally friendly dye is used. This amount keeps the river green for about five hours.