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Why is the Great Salt Lake half red and half blue?

(Al Hartmann | Tribune file photo) In this 1999 file photo, the Great Salt Lake undergoes a drastic change in color at the railway causeway. South of the causeway the lake is a bluish green and north it turns a reddish color.

A railroad causeway built in 1959 cut Utah’s Great Salt Lake into two very different lakes, ecologically speaking, as seen in this striking drone footage posted by ABC News on Wednesday. The water north of the causeway is a deep red, reflecting its highly saline chemistry.

The 20-mile rock-filed roadbed isolated the lake’s Gunnison Bay, allowing salinity levels to reach saturation levels, while the South Arm—with its regular infusion of fresh water from the Bear, Weber and Jordan rivers—has much lower salinity that rises and falls with fluctuating lake levels

ABC’s footage tracks an eastbound freight train as it approaches the causeway’s western terminal on Promontory Point, cutting a line between the contrasting waters of the lake’s North and South arms.

The red hue of the North Arm comes from a type of bacteria, called halophilic bacteria, that just flourishes when the salt level rises.

In recent years, Union Pacific Railroad breached the causeway in an effort to allow water to flow between the two arms. The breach is now spanned with an 180-foot bridge.

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