The Chicago River is 251 kilometers long and flows through the main city of Chicago. As part of conserving the environment, the Chicago River was diverted to flow to Chicago Portage into the Michigan and Illinois canal. The river has a history of being dyed on St. Patrick's Day every year. This started as a fluke when some plumbers started applying dye to seek out contraband material that went on to pollute the river. That was considered as a pollutant, but today, safe dye is used to avoid harming organisms found in the river.

http://en.wikipedia.org/wiki/Chicago_river

Turning the river green

http://www.fairfieldmirror.com/2.4873/other-cities-dye-ing-to-know-what-turns-chicago-river-green-1.482160

This has been a special moment for the past 40 years in the city. The original color of the die is normally orange and one would think that the moment it mixes with the water, it turns orange. This is becomes quite contrary to what is expected and with the river turning green at the immersion of the orange die. Mike Butler, the great mind behind this idea, claims that other cities have tried this kind of thing on their rivers but they fail to manage. This phenomenon makes Chicago travel amazing.

The secret behind this is that it takes 40 pounds of vegetable dye to create the green carpet along the river waters that last for four to five hours.

On March 14th every year, during St. Patrick's parade, hundreds of people turn out to see the Chicago River turn Murky Irish green. The site can best be viewed from the eastern side of the bridge at Columbus Drive or at the Waker Drive. Anyone planning a Chicago travel can not miss to make a date at Chicago River.

http://www.greenchicagoriver.com/

This website offers other useful details of Chicago River.