As Chicago’s St Patrick’s Day parade makes it way through the city today, it will be met with a slightly odd sight: a stretch of the Chicago River glowing a bright, chemical green.
For long time Chicago residents, however, the sight is now pretty commonplace. For over 50 years now, the city has dyed the river green to mark the Irish holiday and decorate the route of its annual parade. This image shows the river on the big day in 2009.
The tradition actually started as an accident: plumbers used a type of flourescent dye to trace the source of pollution in the river, but then realised that the bright green colour could be used to mark St Patrick’s Day. The dyeing is sponsored by the Plumber’s Association to this day, and every year two boats set out to distribute the dye an hour before the parade starts.