A Lake Divided – A History of the Southern Pacific Railroad Causeway and Its Effect on Great Salt Lake, Utah
By J. Wallace Gwynn
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-mile rock-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.
Fluctuations in Lake Level Introduce New Challenges
From the time of the causeway’s construction in 1959 until 1987, Great Salt Lake experienced its greatest recorded changes in surface elevation, from its low of 4,191.35 feet in 1963 to its high of 4,211.85 feet in 1987. Within this range of over 20 feet, the lake rose above and fell below its “normal” surface elevation of about 4,200 feet. Beginning in 1982-83, the lake began to rise from its “normal” elevation of 4,200 feet. The south arm rose five feet in 1983, over four feet more in 1984, and nearly three feet more by 1987 to its historical high of 4,211.85 feet….. With this rise came extensive flooding, especially around the southern arm of the lake. Roads, farms, wildlife management areas, and other facilities were inundated. State officials reviewed a number of options, and decided that breaching the causeway would bring the most immediate relief from the flooding. The breach would be constructed as a bridged opening 300 feet long, with a design bottom elevation of about 4,195 feet. Unfortunately, during construction the bottom elevation of the breach was not built at the design elevation of 4,195 feet, but was completed somewhat higher at about 4,200 feet.
The breach was quickly completed, and on August 1, 1984, south-arm water flooded into the north arm. Within two months, the head differential between the south and north arms had decreased to less than one foot. As the whole lake continued to rise, however, the hydrostatic conditions within the breach opening became favorable for bi-directional flow to occur. During the period from 1984-88, large volumes of south-arm water flowed through the upper portion of the breach opening into the north arm. By 1987, the salinity of the north arm had dropped from its 1981 level of about 27 percent salt to about 18 percent. At the same time, large volumes of north arm brine were flowing into the south arm as return flow, adding to the south arm’s intermediate density brine layer. Between mid-1984 and mid-1986, the elevation of the south-arm interface had risen about 12 feet due to the large influx of dense northarm brine.
Even as the State opened the breach in 1984, the lake continued to rise, and the State decided to pump water from the lake westward into the Great Salt Lake desert (info mally known as the West Desert) to provide additional evaporation area. This project became known as the West Desert Pumping Project. Three large pumps were installed near Hogup, about 13 miles west of Lakeside, that lifted brine from the north arm of the lake into a 4.1- mile canal, where it flowed westward into a shallow depression called the West Pond, located west of the Newfoundland Mountains. Pumping started on April 1, 1987, and continued through June 30, 1989. During this time, about 2.2 million acre-feet (153 billion gallons) of brine was pumped from the north arm of the lake into the West Pond. Concentrated brines were returned to the lake through the East Pond. Pumping contributed about 26 inches to the total lake-level decline of 5 feet during that period of time.
From 1989 through the mid-1990s, the lake level continued to drop. From 1993-94 through 1997-98 there was some south-to-north flow through the breach opening, but no north-to south return flow. Then, as the lake started to rise again in 1998, large volumes of south-to-north flow moved through the breach opening, but still no northto- south flow occurred. As a result, the salinity of the south arm of the lake experienced a steady decline from 1994 through 1999. During this time, the south-arm salinity dropped from about 14 weight-percent salt in 1994 to only about 7 weight-percent in 1999. The north-arm salinity, on the other hand, remained near 25 weight percent salt.
Development of a Plan to Restore a Healthy Lake Ecosystem
The steady decline of the south-arm salinity raised concerns about the future of the brine shrimp industry, the south-arm mineral extraction industry, and the overall ecological health of Great Salt Lake. One of the main effects of declining salinity on the lake’s ecological health was a decline in the brine shrimp (Artemia franciscana) population, determined in part by the quantity and quality of the cysts collected during the annual brine-shrimp-cyst harvest. From the 1995-96 through 1999-2000 seasons, there was a steady decline in the total pounds of harvested raw biomass (shrimp cysts and debris), and a decline in the hatch rate of the eggs. Ongoing studies by the Utah Division of Wildlife Resources also suggested that the declining salinities of the lake would not only affect the brine shrimp population adversely, but also the birds that eat the brine shrimp. Additionally, lower salinity could result in an increase in algae in the lake (because fewer brine shrimp would be present to eat the algae). Eventually the overall ecological balance of the lake could be affected. The declining brine concentrations also meant less concentrated feed brines for salt companies, resulting in reduced yearly salt or brine harvests, and lower profits.
Because of these concerns, the State intensified its study of the lake system to determine (1) the cause(s) for the decline in south-arm salinity, (2) the effects of declining salinity on the overall ecology of the lake, and (3) what could be done to minimize or reverse the decline.
To help determine the cause(s) of the decline in south-arm salinity, the U.S. Geological Survey (USGS) updated its “water and salt balance” model of the lake. The model was designed to (1) help predict the long-term changes in the lake if nothing were done, (2) determine the effects that modifying the causeway would have on north- and southarm salinities, (3) measure the hydraulic conductivity of the causeway fill (a critical component of brine flow through the causeway), (4) determine the effect of keeping the two culverts clean versus letting them remain unattended and plugged most of the time, and (5) help in developing future West Desert pumping scenarios should the lake rise again.
USGS computer modeling determined that the hydraulic conductivity of the causeway (a measure of how easily the brine flows through the causeway fill) was significantly lower after the 1980s flooding than it was before. A possible cause for this decline was the addition of fill material by the Southern Pacific Railroad during the 1980s, as it raised the level of the causeway to keep the tracks above water. Over 500,000 tons of crushed ballast and over 3 million cubic yards of quarry-run rock were used.
In the State’s search for a solution to the declining salinity of the south arm, further computer modeling was done by the USGS and the Utah Division of Water Resources. This modeling suggested that the amount of north-tosouth, high-density brine moving through the breach opening could be increased by deepening the existing breach opening, and keeping the two culverts free of debris. Through this action the overall salinity of the south-arm brines would be increased over time.
Based on this information, the State decided to deepen the breach. By December 2000, crews had deepened the breach opening to a completed bottom elevation of 4,193 feet. Flow measurements made by the USGS show that the new average-flow rate through the deepened breach is 330 percent greater than for the years 1998 through 1999. Density profiles show that the increased flow of brine is both increasing the salinity and the thickness (volume) of the deep, south-arm brine.
The SPRR causeway has played, and continues to play, a significant role in the history and health of Great Salt Lake. The causeway’s history is an interesting story about how human attempts to re-engineer nature produce unexpected impacts. While the causeway has served as an important transportation corridor for rail traffic, it has divided the lake into two separate bodies of water, restricted the mixing of brine throughout the lake, and caused the two arms to develop their own chemical and hydrological characteristics over time. The high salinities in the north arm have been more favorable for mineral extraction than the low salinities in the south arm. The variable south-arm salinities have also presented a challenge to the mineral and brine shrimp industries, and a threat to the overall ecology of the lake. To reverse the salinity decline in the south arm of the lake, the causeway breach was deepened in 2000 to increase the return flow of high-salinity brine from the north arm into the south arm, which hopefully will restore some balance to the complex Great Salt Lake ecosystem.
Data used in this article came from the Utah Geological Survey; Utah Division of Water Resources; Eckhoff, Watson, and Preator Engineering; Southern Pacific Railroad; and U.S. Geological Survey.
Note: The Great Salt Lake Management plan is available at the Natural Resources Bookstore.
Survey Notes, v. 34 no. 1, January 2002