The temperature plummeted earlier this week, and next week's projections are looking even colder. As temperatures below freezing hit more consistently, the Chicago River continues to flow, which begs the question, how cold does it need to be for the river to freeze?
Former Fox Chicago and current WABC-TV
"Water has its smallest volume and greatest density at 39 degrees [farenheit] (approximately 4 degrees celcius) ... so all the water needs to be cooled to this temperature--it would happen most readily at the surface of the river--close to cold Chicago winter air masses." Freeze told RedEye.
Professor Douglas MacYeal teaches in the University of Chicago's geophysical sciences department and also "studies the behavior of ice in the natural environment of Earth," according to his biography.
In an email, MacYeal said Lake Michigan feeding the river makes extensive freezes difficult. Once the water at the surface of the lake is cooled to a low enough temperature, it is continually diplaced by the leftover warmer water underneath "until the entire lake is one massive iso-thermal body of water that is uniformly 4 degrees [celcius]... then the surface water can cool below 4 degrees all the way to zero [32 degrees fahrenheit] without sinking, and thus form surface ice."
According to a chart from the Great Lakes Environmental Research Lab, in 2005, the previous warmest year before 2012, it took until mid-January for the shallower portions of Lake Michigan near Chicago to accumulate ice cover greater than 50 percent.
So there might be some river ice, but "the river is being fed by waters that are still above 4 degrees [celcius] ... so that water doesn't really cool down enough by the time it's already past the city and into the southern reaches of the Chicago Sanitary and Ship canal," MacYeal wrote.