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…