The dyeing of the Chicago River is kind of an odd tradition but it certainly attracts a crowd. 

I used to work downtown and would walk over the river everyday on my route between the train station and the radio station. I'll say this about the dyed river. It does NOT stay this color. After two days it's just a greener-gray than it usually is and after 4 days there's not much left.

That being said it does look pretty cool for about 4 hours. So they have that going for them which is nice.

Also here's the obligatory scene from The Fugitive that you MUST quote anytime someone mentions dyeing the Chicago River.

Such a great movie. YOU SWITCHED THE SAMPLES!!