From the MWRD:
"The MWRD’s Aquatic Ecology & Water Quality and Industrial Waste Division testing looked at temperature, dissolved oxygen, conductivity, and pH (acidity) readings today along the Chicago River. The water that comes in from Lake Michigan has a greenish blue color, is colder and has a higher dissolved oxygen level compared to the water in both the north and south branches of the river. Because lake water is colder than river water, it is more dense and therefore, it is pushing the warmer river water along with sediments to the surface in some locations. This is what we are seeing as two different colors in the river. Additionally, the north branch of the river and the Chicago Sanitary and Ship Canal are much more turbid, and suspended solids are more prominent in high flow situations when it rains; on the other hand, the main stem downtown Chicago is more channelized, surrounded by concrete, and thus, not as turbid. There still is no indication of illegal discharges or contaminants in the waterway system. At this time we believe these reasons account for the difference in visible water color."
“Sadly this is not entirely a new situation,” said Margaret Frisbie, Friends’ executive director. “There were massive combined sewer overflows (CSOs) over the weekend which would certainly contribute to the discoloration. We will be interested to see if the MWRD team discovers anything else from the sampling that they are doing today. No matter what the source of the discoloration, we know we need to stop CSOs, implement a regional green infrastructure plan, and take a personal stake in reducing the amount of water that goes down the pipes and into the sewer and ultimately the river.