Great Salt Lake Causeway, USA

The causeway running across the Great Salt Lake, Utah, was built in the 1950s by the Morrison-Knudsen construction company for the Southern Pacific Railroad as a replacement to a previously built wooden trestle, and is now owned and operated by Union Pacific.

About 15 trains cross the 20-mile (32km) causeway each day. Because water flow is so constrained by the causeway, it has a significant impact on various industries. The salt discrepancy is visible to anyone who flies over the causeway. The water on the north side usually has a distinct reddish tint, while on the south side it's greenish-blue. The colors are attributed to different species of algae that thrive in different concentrations of salt.

The unusual color scheme is a direct result of the causeway. The original trestle allowed water to flow freely under the railroad tracks, but the causeway that replaced it essentially acts as a 20-mile (32km) dam. The salt imbalance developed over many years because the south arm of the lake gets nearly all the freshwater.

There are no significant creeks or rivers flowing into the north arm, while the south arm gets flushed continually by waters of the Jordan, Weber and Bear Rivers.