UPDATE
The fire was a work site accident, likely started by blowtorches, the Tribune reports:
[I]t's possible it started Thursday evening, as contractors with blowtorches cut into a metal ring around the piling, said Chicago Department of Transportation spokeswoman Maria Castaneda. Then, nestled in tar-soaked wooden pilings beneath a concrete can and heavy steel band, "we're assuming a spark got in there and smoldered for 10-plus hours," she said.
Contractors from Rausch Construction are expanding the riverwalk beneath the southern foot of the State Street bridge, across the river from the IBM Plaza and Trump Tower.
Pilings in the Chicago River near State Street and Wacker Drive on the southeast side of the State Street bridge were burning this morning, sending heavy gray smoke over the area," the Tribune reports.
Though it's unclear what caused the fire, CBS 2 noted "an oil slick on the river nearby suggested the presence of a flammable liquid."
Watch footage of the blaze: