CHICAGO, Il. – When the Chicago River turns green, you know it’s time to celebrate St. 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 water, 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 green. And that’s when the city got the idea to dye the river.

The green color lasts about five hours.

PHOTOS: 2016 Atlanta St. Patrick's Day Parade