the Great Lakes don’t drain where you think

The water levels in the Great Lakes look to remain low for the foreseeable future, according to projections by the U.S. Army Corps of Engineers.  This time, though, there’s a new suspected culprit for the prolonged declines, according to a local politician: the Chicago Sanitary and Ship Canal (CSSC).

You may not know it, but the CSSC connects Lake Michigan and the Chicago River to the Illinois River, which flows into the Mississippi River.  So besides the main outlet of the Great Lakes through the St. Lawrence River, there’s also some flow that is diverted towards the Gulf of Mexico. Originally, the city of Chicago was similar to Berlin, Germany and Las Vegas, Nevada, in that its wastewater discharged into the same body of water from which it withdraws its water supply.  But this was back in the late 1800s, when there wasn’t wastewater treatment but there was cholera.  People were rightly worried about public health.

So, the engineers came up with the easiest option: build a separate canal to connect Lake Michigan to the Mississippi River (flowing water away from the city), ostensibly for shipping, and then dump the city’s wastewater in it.  Problem solved! (Except for Milwaukee’s wastewater, but that’s a separate topic.)

With Chicago taking water from Lake Michigan and diverting its wastewater to the CSSC, a large quantity of water is diverted from the Great Lakes system, on average some 1.2 billion gallons per day, or 450 billion gallons per year.  That’s a lot of water.  But how much water is normally flowing through the Great Lakes?  Ultimately, the water in the Great Lakes goes over Niagara Falls, which has an average flow rate of approximately 4 million cubic feet per second, or roughly 944 trillion gallons per year.  So, back of the envelope says that Chicago is diverting 0.05% of the flows from Niagara to the Mississippi.  I suspect that this is not as significant as the climactic effects of record droughts and low snowpack, but that’s a bigger calculation than I’m willing to take on for today.  We’ll see if the Michigan politician mentioned in the article above ends up stirring the pot further on this question.


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