Why is the Great Salt Lake salty?

The Great Salt Lake in northern Utah is one of the largest hypersaline lakes in the world, and its saltiness is its most defining characteristic. Unlike freshwater lakes, where water enters and exits freely, the Great Salt Lake has no outlet to the ocean, making it a terminal lake. Over time, this unique hydrological setup—combined with natural geology, climate, and human impact—has led to extremely high concentrations of salt.

A Terminal Lake with No Outlet

The most fundamental reason the Great Salt Lake is salty is that water flows in but never flows out. Instead of draining into a river or ocean, the lake’s only method of water loss is evaporation. Each year, the sun and dry desert climate cause millions of acre-feet of water to evaporate from the lake’s surface.

However, salts and minerals dissolved in the inflowing water are left behind. These come primarily from:

• The Bear River
• The Weber River
• The Jordan River

These rivers carry not just fresh water, but dissolved minerals like sodium chloride (common table salt), magnesium, sulfate, and potassium. As evaporation continues and the water cycle repeats, the salts accumulate year after year, making the lake saltier over time.

Geologic History: The Legacy of Lake Bonneville

The Great Salt Lake is a remnant of ancient Lake Bonneville, a massive freshwater lake that covered much of present-day Utah during the last Ice Age. About 14,000 years ago, climate change caused Lake Bonneville to shrink dramatically, leaving behind smaller bodies of water—most notably the Great Salt Lake.

As Lake Bonneville evaporated and diminished, the water became more concentrated with minerals. Over thousands of years, this concentration process continued in what became the Great Salt Lake, resulting in the hypersaline conditions we see today.

Evaporation Outpaces Inflow

Utah’s arid climate—with hot summers, low precipitation, and high evaporation rates—further contributes to the lake’s saltiness. On average, 2.5 to 3 million acre-feet of water evaporate from the lake each year, far more than what is added by precipitation or river inflow. The disproportionate rate of evaporation ensures that salts are not flushed out but rather left behind, steadily increasing the lake’s salinity.

The Role of Human Activity

Human water consumption has also played a role in making the lake saltier, particularly in the south arm of the lake. Over the decades, significant volumes of water have been diverted from the inflowing rivers for:

• Agriculture
• Urban use
• Industry

These diversions reduce the overall volume of freshwater entering the lake. With less water and the same amount of salt, the concentration increases. Additionally, the construction of the Southern Pacific Railroad causeway in 1959 has physically divided the lake, restricting the natural flow of water and causing even higher salinity in the north arm, where little freshwater enters.

Different Parts of the Lake, Different Salinities

Because of varying inflows and limited water exchange between sections of the lake, salinity levels can differ dramatically:

• North Arm (Gunnison Bay): Can reach over 27% salinity
• South Arm (Gilbert Bay): Typically ranges from 5% to 15%
• Fringe Bays and Wetlands: Can be much less salty or even brackish

These variations impact which organisms can live where and how salt continues to accumulate in different regions.

Types of Salts in the Lake

While sodium chloride is the most common salt in the Great Salt Lake, the lake also contains significant amounts of:

• Magnesium chloride
• Potassium sulfate
• Sodium sulfate
• Calcium carbonate

These minerals are not only responsible for the lake’s salinity, but also provide raw materials for local industries. The lake supports a robust mineral extraction industry, harvesting tons of salt and other compounds each year.

Ecological and Economic Impacts

Despite its extreme salinity, the Great Salt Lake supports a unique and rich ecosystem. Brine shrimp, brine flies, and halophilic microbes thrive in the salty waters, forming the base of a food web that attracts millions of migratory birds, including eared grebes and Wilson’s phalaropes.

Salt levels also impact commercial activities. Brine shrimp harvesting and mineral extraction are multimillion-dollar industries. However, excessive salinity in the south arm (due to prolonged drought or water diversion) could jeopardize brine shrimp populations, affecting both ecology and economy.

Conclusion

The Great Salt Lake is salty because it’s a closed basin with no natural outlet, receiving inflow rich in minerals but losing water only through evaporation. This geological and climatic setup has turned it into one of the saltiest lakes in the world, with salinity levels that rival and often exceed those of oceans. Over time, both natural processes and human activities have influenced this unique inland sea, making it not only a geographic curiosity but a critical ecological and economic resource.