Development of a Plan to Restore a Healthy Lake Ecosystem

Great Salt Lake brine shrimp (Artemia franciscana) and cysts (eggs), (a male shrimp is in upper right, a female shrimp at lower right. The cysts (eggs) are the small, bright circles scattered throughout the photo.
Photo courtesy of the U.S. Geological Survey.

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.

Schematic diagram showing bi-directional flow through a causeway culvert, and through the causeway fill. Low-density, south-arm brine flows northward at the surface while high-density, north-arm brine flows southward at depth into the bottom of the south arm forming a layer of intermediate-density brine. The low-density brine (top) is separated from the intermediate-density brine (bottom) by the south-arm interface.

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-to south, 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.

Summary

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.

Acknowledgments

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.