A railroad causeway built across Utah’s Great Salt Lake nearly divided it into two separate bodies of water with radically different salinity — today the north side can appear pink-purple while the south remains blue-green

Fly over northern Utah on a clear afternoon and the water below looks like two paint tins spilled side by side and never stirred. Blue-green on one side of a dead straight line, pink to violet on the other. That line is a railway.

Union Pacific owns the crossing, a rock-filled causeway running about 20 miles across the Great Salt Lake, and the railroad’s own site treats the split as a curiosity: ordinary from the shore, genuinely strange from above.

A shortcut with consequences

Southern Pacific first crossed the lake in the early 1900s on a wooden trestle called the Lucin Cutoff. Crews swapped the timber for rock fill between 1957 and 1959, leaving two culverts so water could slosh through. Both eventually sank into soft lakebed mud and were plugged in 2012 and 2013.

Rivers were the real issue. Roughly 95 per cent of the fresh water entering the lake arrives on the southern side, delivered by the Bear, Weber, Ogden and Jordan. A paper published in PLOS One put the long-run averages since 1966 at about 317 grams of salt per litre in the north arm and 142 in the south. Seawater manages roughly 35.

One lake, two chemistries, held apart by a wall of rock.

Why one half went pink

So what turns brine pink? Salt-loving microbes, mostly. Archaea living in the hypersaline north produce carotenoid pigments and light-harvesting proteins in shades of red and violet. They share the water with Dunaliella salina, an alga that stuffs itself with beta-carotene, the pigment that colours carrots. David Parrott of the Great Salt Lake Institute told Fox 13 that these archaea use “pink or purple protein” to capture light and convert it into energy.

Those pigments are not decoration. They work as sunscreen, shielding cells that sit all day in fierce high-desert light with nowhere shady to hide.

South of the tracks, salinity never climbed high enough for that crowd to take over. Green algae, brine shrimp and brine flies still make a living there. That is why the southern water reads blue-green from a plane, and why the arm feeds millions of migratory birds every year.

Pretty water, dead water

Tourists drive out past Robert Smithson’s Spiral Jetty to photograph the pink and occasionally wade into it, making the north arm one of the few ecological cautionary tales with its own queue of visitors. Parrott described it as a preview of the worst case: no brine shrimp, no brine flies, no green algae, just saturated water and the microbes tough enough to tint it. Mineral companies are less troubled by that. Consistently high salinity suits evaporation ponds. Compass Minerals runs the largest mineral extraction operation on the lake, drawing brine from the north arm to produce potash, salt and magnesium chloride, per FRIENDS of Great Salt Lake. Birds get nothing out of it.

Rocks as a control knob

In 1984, after two unusually wet years, the southern half sat 3.7 feet higher than the northern half, the widest gap ever recorded. Crews cut a 300-foot section out of the causeway that August and dropped in a low bridge. Within two months the gap had narrowed to 0.75 feet, per the US Geological Survey’s record of the crossing.

That lever pulls both ways. It mattered most in 2022. Salinity in the south spiked to about 185 grams per litre as the lake bottomed out, past the point where brine shrimp can cope. Engineers raised a rock berm in the causeway breach by four feet to stop dense northern water sliding back underneath, a fix modelled in advance by the Utah Water Research Laboratory at Utah State University.

It worked, and the heavy machinery involved was a backhoe.

Rock is a blunt instrument, though. By autumn 2024 the berm had eroded into a V shape and its low point had dropped roughly three feet, sending state land managers hunting for an adjustable mechanical replacement, as FRIENDS of Great Salt Lake describes it. A structure that has to be rebuilt with a backhoe every time the weather changes is not much of a valve.

Still going down

Legislators returned to that berm in October 2025. Both chambers voted unanimously for a bill allowing the state to build it as high as 4,192 feet, high enough to cut the north arm off from fresh water altogether, as reported by Utah News Dispatch. By mid-August 2026 the surface sat at 4,190.40 feet, about two feet above the record low of 4,188.5 feet set in November 2022. The Salt Lake Tribune reported that state officials did not expect a new record this year.

Drying out is the part nobody can fix with rocks. Exposed lakebed turns into airborne dust over the Wasatch Front, and no amount of berm management adds water that never arrived.

Colour was the accident. Keeping the southern half blue has become a permanent job, and the main tool is still a pile of rocks.