Why Half of the Great Salt Lake Is Pink — and the Railroad That Made It That Way

The Great Salt Lake is pink on its northern half because a rock-fill railroad causeway cut the lake in two in 1959, and the northern half — starved of fresh river water ever since — grew salty enough to turn a rose-to-wine pink from the salt-loving microbes that bloom in it. The southern half still gets the rivers, stays less salty, and holds a blue-green color. Same lake, one train track down the middle, two different worlds of water. The line between them is sharp enough to see from an airplane window and, on a clear day, from space.

If you have flown into Salt Lake City and looked down, you have seen it. A long straight seam runs east to west across the lake, blue-green on one side, dusty pink on the other, as if someone painted half and quit. That seam is not a trick of the light. It is a working rail line, and the color split is one of the strangest side effects any piece of Utah infrastructure has ever produced.

The causeway that split a lake in two

The line is called the Lucin Cutoff. When the transcontinental railroad first crossed northern Utah, trains looped the long way around the north end of the lake through Promontory — the same country where the Golden Spike went in. In the early 1900s the Southern Pacific decided to shortcut straight across the open water instead, first on a wooden trestle and then, decades later, on something far more permanent.

Between 1956 and 1959, crews replaced the aging trestle with a solid rock-and-earth causeway laid straight across the lake. It opened to freight in late 1959. Where the old trestle had let water slosh freely through thousands of wooden pilings, the new rock berm was a dam in everything but name. The lake could no longer mix across its own middle.

That mattered more than anyone building a shortcut expected. Almost all of the lake’s fresh water arrives from the south — the Bear, the Weber, and the Jordan rivers all empty into the southern half. The northern half, a stretch of open water called Gunnison Bay, was suddenly cut off from that supply, left with evaporation on one side and almost no rivers on the other. Salt has been building up there ever since.

The pink is alive

Here is the part most people get wrong: the pink is not a mineral, not a reflection, and not pollution. It is biology.

The northern arm now runs roughly 28 to 30 percent salt — close to the point where water simply cannot hold any more — while the southern arm floats somewhere between 7 and 15 percent depending on the year. That is a punishing difference. Almost nothing survives at north-arm salinity. What does survive owns the place.

Two microbes do the coloring. One is a single-celled algae called Dunaliella salina, which packs itself with beta-carotene — the exact orange-red pigment in carrots — as a kind of sunscreen against the brutal light and salt. The other is a group of salt-loving microbes called halophilic archaea, some of the oldest and toughest life on Earth, which carry red and pink carotenoid pigments of their own. Billions of them together tint the whole northern arm. It is the same family of chemistry that turns flamingos pink: carotenoids, concentrated in living things, at a scale you can see from a plane.

The southern arm has cousins of these microbes too, but at lower salinity a different, greener algae wins out, and grazing brine shrimp keep things in check. So the south stays blue-green. Cross the causeway line and the biology flips.

Why summer turns the color up

The pink runs strongest in the deep heat of July and August. Summer runs the lake like a slow evaporating pan: water leaves as vapor, salt and microbes stay behind, and both concentrate. The more concentrated the brine, the more the pigment-packed microbes bloom, and the deeper the color goes — from a soft rose in spring to something closer to tomato soup or wine by late summer. A hot, dry year pushes it further still.

Where to actually see it

The color line sits out in open water, so the easiest look is from the air on the way in or out of Salt Lake City — window seat, north side, clear day.

To stand next to the pink water, drive to Rozel Point on the north arm’s shore, the same remote spit of land that holds Robert Smithson’s Spiral Jetty. The water lapping the jetty is the pink water. It is a long gravel-road haul northwest of Corinne with no services at the end, so bring your own water, fuel up first, and go early before the July heat sets in. From the low rise above the jetty you get both at once: a coil of black basalt and a bay the color of a faded rose.

The honest part

The color is beautiful, and part of why it is this vivid in recent years is not good news. The lake has been trending toward historic low water levels, and the southern arm in particular has touched new lows in some recent years after weak snowpack winters. Less water means saltier water means more pigment. The pink is partly a drought reading you can see from space.

That is also why the causeway is no longer a simple wall. After the old culverts collapsed, Union Pacific opened a new breach in the berm in December 2016 to let the two arms exchange water again, and since 2022 state managers have been raising and adjusting an underwater berm in that gap to keep the south arm from turning as salty as the north — protecting the brine shrimp and the millions of migrating birds that depend on them. The color split you see from the plane is now something people actively manage, gap by gap, foot by foot.