CHICAGO (NBC) - When the Chicago River turns green, you know it's time to celebrate Saint Patrick's Day.
Turning the river Irish green takes awhile and typically attracts tens-of-thousands of people before the start of the parade.
Boats dump the orange-colored dye in the river which quickly turns the water a bright emerald green.
Back in 1961 a plumber noticed a dye used to detect leaks turned his white coveralls a green color.
And that's when they got the idea to dye the river.
The bright green color lasts for about five hours.