The City of Chicago dyes its eponymous river green every year to celebrate St. Patrick's Day. This practice dates back to the early 1960s, when the city started using a green dye known as fluorescein to locate leaks in its plumbing system.
This doesn't mean that the river was dyed green by accident and happened to coincide with St. Patrick's Day. Rather, a Plumber's Union employee also happened to be a high-ranking official for the city's St. Patrick's Day parade, and he recognized a good opportunity when he saw one. The city now uses a vegetable dye out of concern for the environmental impact of using fluorescein.