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Today’s Question:
Sept. 12, 1980
— Updated Jan. 22, 2009

Dear Cecil:

I was on the boat tour going through the locks between the Chicago River and the lake the other night and realized I don't know how the whole system works. I had assumed the river level was the higher one and that the locks were there to prevent all the river sewage from flowing into the lake. But the lake is higher. So how could the river flow into it? Is this what makes the river flow backward? In your infinite wisdom, please explain.

— M.B., W. Roscoe

Cecil replies:

They don't cover this very well on those tours, which shows the skewed sense of priorities people have these days. Sure, it's great having all this beautiful architecture and whatnot. But give me a town where the sewers work.

The locks do two things: they keep river sewage from flowing into the lake, which is obviously pretty important, and they keep too much of the lake from flowing into the river, which it turns out is important too. They don't have anything to do with making the river flow backward — they weren't constructed till 1936, 36 years after the river was reversed. The river's direction was changed not by raising the lake end, as your question implies, but by lowering the other end. OK, it's glorified plumbing, not an inherently fascinating topic. But it's cool just the same.

Chicago in the old days drew its fresh water from Lake Michigan and emptied its sewage into the Chicago River. The river then flowed into the lake. You can see where this might lead to trouble. Indeed, for years Chicagoans believed that a massive rainstorm in 1885 had flushed vast amounts of sewage out of the river into the lake, fouling the water intakes and leading to a cholera epidemic in which 90,000 died. As has by now been pretty well established, this story is complete hogwash. However, the possibility that something like this might actually occur someday moved Chicago officials to think: we need a better plan.  

Happily, one readily presented itself. Chicago is close to the continental divide separating the Great Lakes from the Mississippi River basin. The Illinois & Michigan Canal, a somewhat dinky affair, had been dug to connect the two waterways for navigational purposes in 1848. Forty years later sanitary engineers realized that if they dug a much deeper canal (in the event, 21 feet deep) and made it slope slightly west, the Chicago River would empty into the Des Plaines River and thence the Illinois and the Mississippi, keeping the city's sewage out of Lake Michigan. Work on the Chicago Sanitary and Ship Canal began in 1889 and was completed in 1900. The New York Times greeted the latter event with the headline, "The Water in the Chicago River Now Resembles Liquid" —  excessively modest praise, perhaps. The main thing was, the scheme worked.  

The S&S Canal was, and still is, considered a masterpiece of civil engineering. It had just two defects in the eyes of its opponents: (1) it sent all of Chicago's crap sluicing down to St. Louis, and (2) it threatened to drain the lake, since there was no control device at the river mouth — the lake simply poured in at the brisk rate of 10,000 cubic feet per second.

The battle on issue #1 was joined first. On Jan. 17, 1900, officials in St. Louis, which drew its drinking water from the Mississippi, filed for an injunction to halt the canal. Unfortunately for them, they did this a little late. The trustees of the Sanitary District of Chicago (now the Metropolitan Water Reclamation District) had opened the canal a few minutes earlier, and kept it open during subsequent legal wrangling. In 1906 the U.S. Supreme Court decided the issue in Chicago's favor, and St. Louis had to build a filtration plant.

Later several Great Lakes states filed suit on issue #2, fearing that the river's reversal would lead to disastrously lower lake levels. This suit had a different outcome. Lawyers for the sanitary district argued that Chicago outflow lowered Lake Michigan by a couple inches at most, nothing compared to natural variation (a foot or so annually, six feet or more over the longer term). The Supreme Court didn't buy it. In 1930 it ordered Chicago to reduce its water diversion from 10,000 cubic feet per second to 1,500. This kept enough water in the S&S Canal for navigation, but not enough to adequately dilute the city's sewage and carry it away.

This prompted the sanitary district to do two things, both of which it should arguably have done to start with. First, it built a sewage treatment plant, which cleansed the city's effluent to the point that it was recognizably water. Second, it installed locks at the Chicago River mouth and the North Shore Channel entrance in Wilmette. Under ordinary circumstances the lake at these locations is four or five feet higher than the river, but during heavy rainfall the river can become so swollen with runoff that it's higher than the lake — on rare occasion so high that the locks must be opened to prevent flooding. This allows raw sewage to pour into the lake and usually leads to several days of beach closures. Most of the time, however, the locks do what they're supposed to do: keep too much of the lake from getting into the river, and too much of the river from getting into the lake.  

— Cecil Adams


Hill, Libby, The Chicago River: A Natural and Unnatural History (2000)

Solzman, David, The Chicago River: An Illustrated History and Guide to the River and Its Waterways (1998)

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