by J. Wallace Gwynn

Introduction

The State of Utah is faced with the task of providing a balance between using Great Salt Lake’s natural resources and maintaining a healthy lake ecosystem. Often at odds are the production of mineral salts and brines worth over $200 million annually, hydrocarbon exploration, and brineshrimp- cyst harvesting worth over $100 million annually; implementing flood-control measures; maintaining proper brine salinities; and protecting thousands of acres of wetlands and the islands which are home to millions of birds including the American white pelican and California gull. The lake’s ecosystem, its wetlands in particular, have very high environmental value.

This balancing task is complicated by the unpredictable rise and fall of the lake, and the effects of the Southern Pacific Railroad (SPRR) causeway on the lake’s salinity and chemistry. The causeway serves as a major transportation corridor and divides the lake into two parts. This article focuses on the history of the causeway, its effects on the lake (in particular brine flow and concentration), and the measures the State has taken to maintain a healthy ecosystem.

Effects of the SPRR Causeway on Brine Flow

On May 10, 1869, the completion of America’s first transcontinental railroad was celebrated by the driving of the Golden Spike at Promontory Summit, near the north end of Great Salt Lake. Thirty-five years later, the Southern Pacific Railroad (SPRR) completed the Lucin Cutoff. The Cutoff traversed a route east from Lucin, located about 8 miles east of the Nevada/Utah state line, then across the lake, and on to Ogden.

The original Central Pacific Railroad traversed a more difficult route, about 42 miles longer, from Lucin, around the north end of the lake to Brigham City, and then southward to Ogden. The portion of the Cutoff crossing the main body of Great Salt Lake consisted of two earth- and rock-fill embankments, one extending eastward into the lake from Lakeside and the other extending westward from Promontory Point, with a 12-mile open, wooden trestle in between. A shorter section of rock-fill embankment extended eastward from Promontory Point to the mainland. The open trestle offered little resistance to the movement and circulation of brine throughout the lake.

By the mid-1950s, Southern Pacific personnel deemed the trestle to be in need of major repairs or replacement. Engineering studies led to a decision to construct a 13-milerock-fill causeway parallel to and 1,500 feet north of the trestle. Construction began in 1956 and was completed in 1959 at a cost of $53 million.

With the causeway’s completion, the main body of the lake was partitioned into two bodies of water, the north arm and the south arm. The causeway immediately reduced the mixing of brine between the north and south arms, and three notable changes were observed.

First, the south-arm brine became less saline than the north-arm brine because all three of the major tributaries (the Bear, Weber, and Jordan Rivers) flow into the south arm. The north arm mainly received salty water that moved through the causeway from the south arm. The north arm also received less annual precipitation, and experienced slightly higher evaporation rates than the south arm, accentuating the salinity imbalance.

Second, a surface-elevation differential developed across the causeway between the two arms of the lake, with the surface elevation of the south arm three and one-half feet higher than that of the north.

Third, the brine in the south arm became density stratified shortly after the causeway was completed, a condition in which a brine of greater density lies on the bottom of the lake, and is overlain by an upper layer of less dense brine. The two brines are separated by a transitional zone called the interface. The greater density south-arm brine comes from, and is maintained over time by, the north to- south flow of northarm brine moving through the lower part of the causeway fill, and through the deeper portions of the two culverts in the causeway.

Lake brines can flow simultaneously both to the south and to the north through the causeway and its openings (referred to as bi-directional flow). Under the right hydrostatic conditions, including a surface-elevation differential (the south arm being the highest), and a density difference (the north-arm brines being the densest), there is a critical depth below the water surface at which the hydrostatic pressure of the column of brine on both sides of the causeway is the same. Above this depth, brines flow from south to north through culvert openings or through the causeway fill material; below this depth, brines flow from north to south.