According to Chicago's WUSN-FM radio station:
Each year members of the plumbers’ union dye the Chicago River green for St. Patrick’s Day on March 17, 2012. The River was first dyed green in 1962 and has become a St. Patrick’s Day tradition in Chicago.
A time-lapse video was released, meaning you can see the whole thing turn green under two minutes! According to PEOPLE, the dyeing takes five hours and uses 40 pounds of powdered green vegetable dye.