All through the night I watch the journey of the red threadlike electronic line on the computer screen. A trend line that traces the progress of the “invisible strangler” at work on our water-intake pipe, a mile out into Lake Michigan under 50 feet of water. The strangler visits every year about this time, when arctic fronts scour the already frigid water with screaming blasts of polar air.

I work at a water plant on the lakefront just north of Chicago. My partner and I pump and purify water for a quarter million people and their pets and plants. People don’t use much water in winter. No car washing, lawn watering, pool filling. Just the basic things. So winter pumping is usually easy. I go through a shift watching the progress of Ursa Major cartwheeling around Polaris, the star that doesn’t move. Then one night I’ll notice the trend line on the computer screen signaling that the water-inlet pipe is choking. And I know there’ll be no more stargazing that night. That’s what happened on my midnight shift in the middle of February.

Someone had plopped a trade-journal reprint on the desk. A 1977 article by D.M. Foulds and T.E. Wigle titled “Frazil–the Invisible Strangler.” The authors explain that there are two kinds of ice that concern water-plant operators: static or surface ice, the kind that forms on the top layer of quiescent pools, puddles, ponds, and stream edges; and frazil ice, the dynamic type formed in the turbulence created when arctic winds supercool open waters. When that happens, the genesis of ice nuclei begins–tiny disks and spheroids careen through the water column, frantically journeying up and down like the confused rush of snow in a winter blizzard.

Foulds and Wigle have photographed these icy beads as they sprout appendages, reach out, metamorphose into irregularly shaped crystals that grow in size as they collide and cluster together. The distinctive feature of frazil ice is its stickiness. It solders itself to objects on the lake bottom–to chains and anchors, rocks and boulders, and the gratings that cover the intake ducts of water utilities. The swirling crystals stick, stack up, and bridge over the duct openings, slowly strangling the flow of water and shutting the spigot on the fresh water supply we take so for granted.

Clustered frazil ice is called anchor ice. Because ice is more buoyant than water, the clumps of anchor ice that stick to lake-bottom rocks, stones, and beer bottles sometimes overcome the objects’ weight. Then the entire mass is lifted into the free-for-all of the churning waters.

Frazil and anchor ice form under a specific coincidence of conditions. They need an offshore arctic wind that drives the shoreline ice cover eastward to the Michigan side of the lake, exposing the open waters, already hovering at 32 degrees Fahrenheit, to cooling by evaporation and to vigorous mixing. They need the sky to be clear, allowing for the efficient dissipation of the day’s stored sunlight energy and of the latent heat generated when water transforms itself into ice.

Water-plant operators monitor the whole distribution system. All sorts of information flashes into the control room–raw and finished water-quality data, chemical usage rates, and pressures, levels, and temperatures at outlying stations and standpipes. But on nights when anchor-ice conditions converge, the water-plant operator pays special attention to the suction wells. These are the sunken shoreline silos attached to the intake pipes out in the lake, out of which we pump raw water for treatment. If we weren’t pumping, the water would sit in the wells at lake level. When we pump out of a well the level drops as the intake water unsuccessfully tries to maintain lake level. Within a few minutes a new equilibrium is reached. The amount of lowering is called the drawdown.

We usually pump at a steady rate for hours at a time, so as long as the water flows unhampered into the well the drawdown should stay about the same. But as anchor ice slowly builds at the intake crib and bridges over the gratings, the flow of water is pinched and doesn’t reach the well as quickly. The deficit shows up as a steady lowering of the drawdown, the measure indicated by the red trend line on our computer screen.

As the night and the supercooling progress, the gaps on the inlet gratings are narrowed and the red line drops farther. It steps down at every update in the information system. Experience over the past 60 years leads us to believe that higher flow rates accelerate the buildup of anchor ice, and therefore hasten the lowering of the well level to the point at which the raw water pumps will start to gulp down dollops of air from the emptied well. That makes the pumps spin too fast, causing the motors to overheat and shut down.

So we try to cut back pumpage as soon as anchor ice is detected. If we let the inlet ports freeze solid we’d have to shut down water service to all the people our little plant serves. If pumpage is kept to a minimum, the ice blockage will usually be melted away when the morning sun shines down through the clear water. Then the well recovers, and pumpage can be resumed. But if the conditions persist for several days the stored water capacity of the utility can be exhausted, and the silent strangler effects its sinister goal: shutting down the whole human water-sucking system.

Operators are faced with a contradiction: they have to try to rebuild water levels in the storage tanks scattered around the suburbs, yet still keep flows low enough to avoid anchor ice. As the anchor-ice function iterates itself night after frigid night, I get to thinking that nature is once again probing our weaknesses.

Only two years ago these same inlet ducts were choked by zebra mussels acting very much like frazil ice. Zebra mussel “villagers,” the larval youth, journey into the intakes, stick to the walls, and over a matter of weeks reduce the diameter of the pipe until it is effectively sealed. Water utilities all around the Great Lakes blasted the intruders with chlorine, leaving heaped piles of mussel shells under our well screens. During this bitter cold spell the piping that carried the toxic chlorine out to the intake crib has been charged with compressed air in the slim hope that the pulsing bubbles will break loose some of the ice. The air is delivered in bursts that resonate through the station like a beating heart, a drum: bum-bum-bum-bum.

My partner Raj and I wait out the night, cutting back, cutting back pumpage to reduce the anchor-ice blockage rate, only to see our stored water levels continue sinking. In the morning I lift my face in reverence to the life-giving sun, hoping that its reach will be deep and relief will come quickly. The only known solution for total blockage is to blast away at the inlet ice with explosives. Or wait for spring.

The next night I come on shift at 10 PM. We’re pumping at a high rate, trying to restore levels. “No sign of anchor ice yet,” the evening operator tells me. But as soon as he’s gone the red line nudges downward. Once up, twice downward. Once up, three down, accelerating until I decide to cut back. The drawdown recovers at the lower pumpage for about 15 minutes, then inexorably recommences–steps spiraling down. When I turn over the controls to the day-shift crew at dawn, only our smallest pumps are running, just to keep pressure up in the distribution system, just enough to balance water usage by late-night bathroom visits and early risers. The drawdown is still sinking.

You are all asleep. The whole workaday world sleeps in cozy innocence–boilers filled up and pressurized, toilet tank full and waiting, kitchen tap ready to provide water for the morning coffee. You don’t know how close we come to shutting you down. I wake from my morning sleep at noon and call the pump station. The ice has just broken loose. Pumpage is just being restored. The sun is brilliant. The weather news says more of the same tonight.

The galaxy of blinking colored lights in my control room, the beeps and clicks of warning tracking the spiraling iteration of the anchor-ice function remind me of the column of equipment by my father’s hospital bedside when he tottered at the edge of life with pneumonia. The swamping of his lungs made him weak, unable to energize his system with the oxygen it needed to power its own self-healing processes. A tube pumped 90 percent pure oxygen through his nose and into his lungs. Another sucked out accumulating fluids. The oxygen showed up at high levels in the bloodstream. The problem was that pure oxygen is toxic to the human body. High dosages could be maintained only for a short duration. After 24 hours the doctors ordered a reduction in oxygen richness. If my father was not able to keep up his blood oxygen levels on his own, he was doomed.

The doctor told us to prepare ourselves for the possibility of his death. We watched those monitors, the blips and beeps, the thump and wheeze of the heart-lung machine, as the life/death function decided whether my father would ever speak with us again. He twitched and dreamed through four days and nights, lost in a dream-limbo of his youth, journeys way far away. But he made it. When he woke, he asked if we’d brought any cherries.

I get to know the lake by measuring and monitoring. But it is a limited and narrow knowledge. I also get to know the lake as a living, breathing being. It would be easy to mistake the signals and instrumentation, the tidy displays on the computer windows, for the lake itself. They are not the lake. No more than the heartbeat and oxygen monitors were my father.