A Lake Divided by Rail
The Great Salt Lake is bisected by a 20-mile rock causeway built for the Lucin Cutoff railway line. Construction of this solid embankment, which replaced an earlier wooden trestle, finished in 1959. It effectively partitioned the lake into a northern arm (Gunnison Bay) and a southern arm (Gilbert Bay). This division created two distinct aquatic environments. The southern arm receives nearly all of the freshwater inflow from the Bear, Weber, and Jordan rivers, which keeps its salinity relatively low. The causeway severely restricts this freshwater from reaching the northern arm. Intense evaporation in the isolated north arm concentrates its salts, making it significantly more saline than the south.
This sharp salinity gradient dictates which microorganisms can survive in each section, directly influencing the water's color. The difference is so pronounced that it is visible to astronauts in orbit. Salinity in the southern arm typically fluctuates between 8% and 17%. The northern arm, by contrast, is often near saturation at approximately 27% salinity.