Chicago’s founding problem was plumbing. The city drew its drinking water from Lake Michigan — and emptied its sewers into the Chicago River, which flowed, with fatal obedience, straight into Lake Michigan.

Every rainstorm flushed the stockyards, the slaughterhouses and a million people’s waste out toward the water intakes. The nineteenth-century city was periodically ravaged by typhoid, cholera and dysentery, and every engineering half-measure — moving the intake cribs farther offshore, pumping schemes — bought only time. The river ended in the drinking glass. That was the whole problem.

So Chicago settled on a solution of magnificent arrogance: if the river ends in the wrong place, make it run the other way.

The ditch that beat Panama

The tool was the Chicago Sanitary and Ship Canal — 28 miles of channel cut from the river’s South Branch to Lockport, crossing the low subcontinental divide that separates the Great Lakes basin from the Mississippi’s. Dig deep enough, and gravity itself could be re-hired: lake water would pour into the Chicago River, push its whole flow backward, and carry the city’s diluted sewage away down the Des Plaines and Illinois rivers toward the Mississippi.

The digging, from 1892, was the largest earth-moving enterprise the world had then seen — tens of millions of cubic yards of rock and clay, new machines invented for the job, a workforce of thousands. The methods pioneered on what locals called the Big Ditch became known as the Chicago School of Earth Moving, and its veterans and techniques went on to dig the Panama Canal.

Then came the finish, which was less an opening ceremony than a getaway.

January 2, 1900: dig fast, litigate later

Downstream, the plan had been received with understandable horror. Missouri — whose largest city, St. Louis, drank from the Mississippi — saw Chicago preparing to point its sewers at their faucets, and prepared to ask the U.S. Supreme Court for an injunction before the canal could open.

Chicago’s sanitary trustees, hearing the lawsuit was coming, chose velocity. On the frigid morning of January 2, 1900, with almost no announcement, they went out to the last earthen dam at Kedzie Avenue and had it cut; one account has trustees attacking the final barrier themselves as the water broke through. By January 17 the controlling works at Lockport were opened and the reversal was fact. Missouri filed within weeks — but there was no longer a construction project to enjoin, only a river already running backward. A fisherman’s line of that season put it best: the Chicago River had become “the only river in the world that flows away from its mouth.”

Missouri pressed the case anyway, and Missouri v. Illinois became a landmark. In 1906, Justice Oliver Wendell Holmes delivered the Court’s answer: Missouri had failed to prove that Chicago’s distant, diluted sewage was what sickened St. Louis — a burden made harder to carry by the awkward fact that St. Louis and every city between were pouring their own raw sewage into the same rivers. The suit failed; the reversal stood. The case, in the process, helped found the federal common law of interstate pollution — the legal machinery later used against actual proven polluters. St. Louis, for its part, opened proper water treatment and, by the era’s rough justice, came out fine.

Still backward, 126 years on

The reversal delivered exactly what its engineers promised Chicago. With sewage no longer marinating the water intakes, the city’s typhoid and waterborne-disease death rates collapsed within a generation, and the project entered the engineering pantheon — the American Society of Civil Engineers eventually named it a monument of the millennium, alongside the Panama Canal its workers went on to dig.

And the river still obeys. A century and a quarter later, Lake Michigan water flows in at the lakefront and runs backward through downtown, the reversal maintained by locks and controlling works — with asterisks the original engineers would recognize as their own debts coming due. In the biggest storms, when even the city’s cavernous Deep Tunnel reservoirs fill, the gates must occasionally be opened and the river allowed to run the old way into the lake for a few hours. The canal that separated Chicago’s sewage from its drinking water also stitched together two watersheds nature had kept apart, which is why it is now the front line of the Asian carp fight — an electric fish barrier humming in the channel to keep the Mississippi’s invaders out of the Great Lakes. And every few years, someone seriously proposes re-reversing the river, on the grounds that the 1900 solution has become the new problem.

Perhaps it will happen someday. But for now the fact stands: a major American city, told by geography where its river went, declined — and 126 years later, visitors on the Michigan Avenue bridge watch a river flowing calmly away from its own mouth, carrying the legacy of a suit Missouri filed after the canal opened and ultimately lost.

Correction, 15 September 2026: The headline and closing sentence were revised to identify Missouri as the litigant and to make clear that the state filed its suit after the canal opened.