13-Mar-10 – Drizzle and fog did not keep crowds from the Chicago River Saturday morning for the annual tradition of dyeing the river green for St. Patrick’s Day.

The Chicago River was turned green between the Wabash Avenue and Columbus Drive bridges.

The Chicago Police Department was expecting 50,000 people. This year, due to safety concerns, spectators were discouraged from watching the event from the bridges.

The event started promptly at 10:30 a.m., as reported by Marina City Online on Friday, based on information from GreenChicagoRiver.com. A later start time was widely reported by other news media, which may have been quoting an incorrect start time on the web site for the Saint Patrick’s Day Parade.

In 1961, it was noticed that a fluorescent dye used by plumbers to detect leaks into the Chicago River caused the water to turn bright green. The next year, when 100 pounds of the dye were used as part of Chicago’s St. Patrick’s Day celebration, the river, according to GreenChicagoRiver.com, stayed green for a week.

In 1963, 50 pounds of dye turned the river green for three days. Today, 40 pounds of a vegetable-based dye is used to create the effect for about four hours.