Dyeing Chicago River

Since more than 50 years, each year on St. Patrick's Day, the Chicago River is dyed green. The tradition arose from an accident, when plumbers used fluorescein dye to trace illegal discharges of polluted water. The event is still sponsored by the local plumbers union, but they are using environmentally safe vegetable dye.

Another interesting fact about the Chicago River is that the direction of its flow was reversed in 1900, using a series of canal locks, so that sewage and other pollution no longer flowed into Lake Michigan, the city's drinking water source.