In the search for a big-city refuge from climate change, Chicago looks like an excellent option. At least, it does on a map.

It stands a half-continent away from the threat of surging ocean levels. Its northern locale has protected it, to some extent, from southern heat waves. And droughts that threaten crops, forests and water supplies in so many places? Chicago hugs the shore of one of the grandest expanses of freshwater in the world.

Water is, in fact, why Chicago exists. The nation’s third-largest city grew from a remarkable geographical quirk, a small, swampy dip in a continental divide that separates two vast watersheds: the Great Lakes and the Mississippi River Basin. In the 19th century, Chicagoans dug a canal linking those two watersheds, transforming their muddy town into a metropolis of commerce by making the riches of the American Midwest accessible to the world.

But the same waters that gave life to the city threaten it today because Chicago is built on a shaky prospect: the idea that the swamp that was drained will stay tamed and that Lake Michigan’s shoreline will remain in essentially the same place it has been for the past 300 years.

The lake may have other plans.

Climate change has started pushing Lake Michigan’s water levels toward uncharted territory as patterns of rain, snowfall and evaporation are transformed by the warming world. The lake’s high-water cycles are threatening to get higher; the lows, lower. Already, the swings between the two show signs of happening faster than at any time in recorded history.

A series of ferocious storms in recent years has made it clear that the threat this poses to a metro area of 9.5 million people is not abstract.


Lake Michigan’s water level has historically risen or fallen by just a matter of inches over the course of a year, swelling in summer following the spring snowmelt and falling off in winter. Bigger oscillations, a few feet up or down from the average, also took place in slow, almost rhythmic cycles unfolding over the course of decades.

No more.

In 2013, Lake Michigan plunged to a low not seen since record-keeping began in the mid-1800s, wreaking havoc across the Midwest. Fears grew that the lake would drop so low, it would no longer be able to feed the Chicago River, the defining waterway that snakes through the heart of the city.

That fear was short-lived. Just a year later, in 2014, the lake started climbing at a stunning rate, ultimately setting a record summertime high in 2020 before drought took hold and water levels started plunging again.

In just seven years, Lake Michigan had swung more than 6 feet.

A clash between elemental forces — sun, rain, heat and ice — is what is threatening to upend centuries of relative stability along the Great Lakes’ 10,000 miles of shoreline, including the 22 miles that define Chicago’s eastern edge. And the best explanation is climate change, said Drew Gronewold, a hydrologist at the University of Michigan who has been studying lake levels for more than a decade.

The five connected Great Lakes function more like a slow-motion river flowing west to east, with each lake dumping into the next until their collective outflow is gathered in the St. Lawrence River and carried to the Atlantic Ocean.


Like any river, that outflow must be replaced by inflows. Deposits take the form of precipitation: rain and snow. Withdrawals are measured in terms of water that flows outward to the ocean, along with the water that evaporates into the sky.

Gronewold’s work is focused on what he calls an emerging tug of war between recent increases in both evaporation and precipitation, each of which can be influenced by the warming globe.

Gauges on the U.S. side of the border show the Great Lakes Basin has, since the 1990s, received far more precipitation than average. The past five years collectively have been the wettest half-decade on record.

It is likely no coincidence that the average air temperature in the same region has increased 1.2 degrees Fahrenheit since 1991. Warmer air factors into wetter weather and a surging lake level, because it can hold more moisture.

But warmer air also means more evaporation.

This is where the ice comes in. Even a slight air temperature increase can dramatically reduce the lake’s winter ice cover. And because ice reflects the sun’s heat, less ice means warmer water, which accelerates evaporation.

Between 1999 and 2013, evaporation appeared to be winning the tug of war. Over that time, Lake Michigan spent a record 15 years below its average level, despite greater precipitation.


In early 2013, the lake hit a record low.

Then, yet another force of nature emerged: a weakening of the polar vortex.

Several brutally cold winters settled over the Great Lakes starting in 2014, driven in part by the destabilization of the famous swirl of frigid air around the North Pole. The cold blasts caused Lake Michigan’s ice cover to surge for several winters. That lowered water temperatures and slowed evaporation — and helped drive the lake level to the record summertime high in 2020.

Chicagoans paid a heavy price.

Beloved sandy beaches disappeared. Wind-riled waters shattered living room glass and flooded apartment basements. Extreme storms turned city streets into rivers.

Since last fall, the lake has fallen about 1 foot because of a relatively mild winter and a continuing drought. But nobody knows where this is headed. If warmer winters persist, the increased evaporation could help to shrink the lake back into record-low territory.

When Lake Michigan hit its low in 2013, conservationists warned it was very likely only a matter of time until the lake dropped so far in relation to the Chicago River that the river, which flows out of the lake and carries Chicago’s treated wastewater south toward the Gulf of Mexico, might actually reverse course and begin flowing into the lake — the city’s drinking-water source.

The estimate then was that the river could potentially reverse itself if the lake level dipped a mere 6 inches.


But then, just seven years later, high water was the problem.

A Metropolis on Stilts

In 1673, Jesuit missionary Jacques Marquette and fellow explorer Louis Joliet, a philosophy student turned fur trader, became the first known Europeans to set eyes on what is today Chicago.

The two men were returning from a voyage down the Mississippi River. On their outbound trip, the expedition had to carry its canoes overland in Wisconsin. But on the return trip, Native Americans steered the explorers toward a shortcut back to the Great Lakes — a swamp now called Chicago.

In their natural state, the Mississippi River and Great Lakes basins were separated by a ridge in the landscape that kept the two basins’ waters from mingling.

In Chicago, the divide is, or was, a wetlands surrounding a shallow lake whose indolent outflows could, in periods of high water, drift in both directions: eastward toward Lake Michigan and westward into the Mississippi Basin. The explorers found that crossing between the two basins at this divide required only a relatively brief slog through the mud.

Joliet saw the swamp as a gateway into the heart of America, opened simply by digging a roughly 1.5-mile channel across it so that vessels could float between the Mississippi Basin and the Great Lakes.


Construction of such a canal had to wait a century and a half, until 1836. But by 1870, the canal had helped propel Chicago from a mangy frontier outpost of less than 5,000 into a metropolis of 300,000.

But even as a metropolis rose from the mud, the flat landscape never went away. Storm and wastewater drainage in the young city was next to impossible, leaving streets smothered in a septic goo.

Chicago could not fix this problem the way other cities did — by laying sloped sewers. The land was so low, it was impossible to place sewers below the streets and still have enough tilt to carry wastewater into the Chicago River.

So instead of putting sewers under the streets, Chicago’s leaders put sewers on top of the streets, then built new roads atop the old ones. They effectively hoisted the city out of the swamp.

While jacking up Chicago to make room for sewers may have solved one predicament — the filthy, impassable streets — it caused another. All the sewage still flowed into the Chicago River. And the river still flowed into the lake, the city’s drinking-water source.

Desperate to protect residents from waterborne scourges like cholera, city leaders at the end of the 19th century hatched another audacious plan: reverse the direction of the river so it flowed away from Lake Michigan instead of into it.


They achieved this by dynamiting a 28-mile-long canal connecting the Chicago River to the Des Plaines River, which flows toward the Mississippi.

The Chicago Sanitary and Ship Canal opened in 1900, a feat of engineering 160 feet wide and 25 feet deep and, importantly, lower than Lake Michigan. So gravity dictated that the Chicago River would henceforth flow in the opposite direction.

Today on the Chicago waterfront stands the Harbor Lock, a set of mammoth steel gates separating lake water from river water. It marks the spot where boats pass between the Great Lakes Basin and the Mississippi Basin.

For most of the 121 years since it opened, the river and canal, the centerpiece of the city’s huge human-made waterway system, functioned just as its designers had hoped. It reversed the city’s namesake river, sending wastewater toward the Gulf of Mexico and away from the city’s drinking-water intake pipes on Lake Michigan.

Usually, but not always.

Throughout much of the 20th century, storm-loaded sewers regularly overwhelmed Chicago’s sewage treatment plants, resulting in stormwater and sewage (Chicago’s old-fashioned sewers carry both) being dumped straight into the river and canal.

But in the heaviest storms, even the river and canal system could get overwhelmed, which left two bad choices: let the river and canal overtop their banks and flood city streets with sewage, or open the lock gates so the swollen, polluted river could again, albeit temporarily, tumble into Lake Michigan.


Once more, the city was forced to try to dig itself out of a fix.

Since the 1970s, Chicago has been constructing a multibillion-dollar system of sewage storage tunnels and reservoirs. The idea is that, when rainstorms hit, the extra runoff can be safely warehoused. Once a storm subsides, all that stormwater and raw sewage can be slowly treated and released, avoiding floods and also avoiding the release of untreated filth into the lake.

The tunnels, some a yawning 33 feet in diameter and running up to 300 feet below city streets, stretch 109 miles and collectively hold 2.3 billion gallons of water. A network of reservoirs holds roughly an additional 12 billion gallons, and once the entire project is completed by decade’s end, it will have the capacity to hold more than 20 billion gallons.

While the system has dramatically increased water quality in the river and lake, it is still not big enough to handle the worst storms. This forces Chicago to continue to rely on opening the navigation lock, along with some nearby gates, as a safety valve to send pulses of storm-driven wastewater into Lake Michigan.

Then came May 17, 2020.

That afternoon, Tyrone Valley, lockmaster at Chicago Harbor, got a call. There was big trouble brewing in the river.

River managers have a trigger point for opening the lock gates — reversing the river’s flow into Lake Michigan — in order to protect downtown Chicago from disaster. That trigger is typically 3.5 feet above Chicago’s official ground level, which, in the universe of river managers, is considered zero feet.


Normally, the river, as measured on giant white rulers tiled on the lock’s walls, ranges between 2 and 3 feet below ground level.

But three days earlier, a relentless storm had dropped a record 24-hour rainfall for that date. The tunnels and reservoirs had done their job helping to contain the deluge. But then, a second storm hit while the reservoirs were still holding water from the first storm.

That meant the stormwater and sewage had to be released straight to the river. And it was too much for the river to handle.

By 5:23 p.m., the river level hit +3.5 feet, the point under normal conditions to open the lock gates and reverse the river into Lake Michigan. Messy, yes. But not as messy as letting sewage-laced water pour into downtown. However, this time conditions were not normal.

Lake Michigan’s level at that moment was at a record high for May — well above the river. So opening the lock was not an option, because that would have sent lake water pouring into the river, flooding the city.

At 6:16 p.m., the river hit +3.8 feet. Then, less than 10 minutes later, it hit +4 feet.


Valley stood along the lock wall, helpless.

Then, at 6:54 p.m., the river surged to +4.6 feet, putting it about 5 inches above the level of the lake. Finally, Valley had options again.

He gave the order, and his crew opened the immense steel lock gates. A whoosh of water carrying all manner of waste blasted into Lake Michigan.

In mere minutes, the suddenly reversed river, roaring like a freight train, dropped below lake level. This was a new problem; if the gates stayed open, lake water would slosh back into the river, further flooding the city.

There was nothing in the playbook for this scenario. Valley and the lock operators had to wing it, pinching the gates closed to let the river again rise above the lake, then swinging them open again to let the swollen river drain into the lake.

Again and again, the crew repeated these steps.

Still, it was not enough. The river kept climbing, eventually peaking at +5.12 feet a little after 7 p.m.

The resulting floodwaters not only submerged the bustling Lower Wacker Drive, one of the city’s main arteries, but also knocked out the electrical power at the nearby Willis Tower (formerly the Sears Tower) all the way up to the aircraft warning lights atop its tusklike antennas. A city hotline fielded more than 1,500 distress calls from residents whose basements were flooded.

This article originally appeared in The New York Times.