What Really Happens When a Salt Lake Dries Up?

Have you ever wondered about the strange, beautiful landscapes of salt flats or the vibrant colors of some salt lakes? These unique environments are the result of a powerful natural process: evaporation. This article provides an in-depth look at the science behind what happens when the water in a salt lake disappears, leaving its hidden contents behind.

The Unique Nature of Salt Lakes

Before diving into evaporation, it’s important to understand what makes a salt lake different from a typical freshwater lake. Most lakes have an outlet, like a river, that allows water to flow out. This constant flushing keeps mineral concentrations low.

Salt lakes, on the other hand, are often found in endorheic basins. This is a geological term for a closed drainage basin that retains water and allows no outflow to other external bodies of water, such as rivers or oceans. Water flows in from rivers or streams, but it can only leave through evaporation.

Over thousands or even millions of years, the inflowing water brings tiny amounts of dissolved minerals and salts from the surrounding rocks and soil. Because only the pure water evaporates, these minerals are left behind. Year after year, the concentration of these dissolved solids builds up, creating the highly saline water that defines a salt lake. Famous examples include the Great Salt Lake in Utah, the Dead Sea between Israel and Jordan, and Salar de Uyuni in Bolivia.

The Evaporation Engine: Water Leaves, Salt Stays

The core of the phenomenon is a simple principle of physics. The sun’s energy heats the surface of the lake, giving the water molecules enough energy to change from a liquid to a gas (water vapor) and escape into the atmosphere. This is evaporation.

The crucial point is that this process is a natural form of distillation. Only the H₂O molecules escape. The heavier, dissolved mineral salts, like sodium chloride (table salt), magnesium, calcium, and potassium, are left behind in the remaining water. As more and more water evaporates, the volume of the lake shrinks, but the total amount of salt remains the same. This causes the concentration of salt, or salinity, to rise dramatically.

The Hypersaline Transformation and Its Effects

As evaporation continues, the lake’s water becomes hypersaline, meaning it is much saltier than seawater. Seawater has an average salinity of about 3.5%, but a hypersaline lake can easily exceed this. For example, the Great Salt Lake’s salinity ranges from 5% to 27%, depending on water levels.

This extreme environment has profound effects on the lake’s ecosystem:

• Life Finds a Way: Most fish and aquatic plants cannot survive in such conditions. However, a specialized group of organisms called extremophiles thrives here. These include brine shrimp, brine flies, and specific types of algae and bacteria.
• A Splash of Color: The iconic pink or red colors seen in many salt lakes, like Lake Hillier in Australia, are not caused by the salt itself. They are produced by microorganisms that flourish in high-salt environments. An alga called Dunaliella salina and a type of archaea called halobacteria produce reddish pigments that protect them from the intense sunlight, giving the water its striking hue.

Reaching Saturation: The Birth of Crystals

There is a limit to how much salt can be dissolved in water. This limit is called the saturation point. Once the water becomes so concentrated that it can no longer hold all the dissolved minerals, the minerals begin to precipitate, or “fall out” of the solution, and form solid crystals.

This process does not happen all at once. Different minerals crystallize at different levels of concentration. This is known as fractional crystallization.

1. Carbonates and Gypsum: The least soluble minerals, like calcite (a calcium carbonate) and gypsum (a calcium sulfate), are the first to crystallize as the water becomes more concentrated.
2. Halite (Rock Salt): As more water evaporates, the concentration of sodium chloride increases until it reaches its saturation point. This is when halite, the mineral form of table salt, begins to crystallize in massive quantities.
3. Potash and Magnesium Salts: Finally, the most soluble salts, often called “bitterns” like potassium and magnesium salts, are the last to precipitate from the remaining brine.

This predictable sequence is not just a scientific curiosity; it is the basis for the commercial extraction of different minerals from salt lake brines.

The Final Stage: The Salt Flat

If the evaporation process continues to completion, or if the lake is seasonal, it can dry up completely, leaving behind a vast, barren landscape known as a salt flat, or playa. The surface is a thick crust of salt and other minerals.

One of the most recognizable features of a salt flat is the hexagonal pattern that often forms on the surface. These patterns are created as the salt crust repeatedly expands and contracts with changes in temperature and moisture, causing it to crack in a honeycomb-like design.

The Bonneville Salt Flats in Utah, a remnant of the ancient Lake Bonneville, is a perfect example. Its surface is so flat and hard that it is used for land speed racing. Salar de Uyuni in Bolivia is the world’s largest salt flat and becomes a stunning natural mirror when covered by a thin layer of rainwater.

Frequently Asked Questions

Why are some salt lakes pink or red? The color is not from the salt but from microorganisms. A type of algae called Dunaliella salina and a class of archaea known as halobacteria produce protective red pigments called carotenoids in response to high salinity and intense sunlight, which tints the water pink or red.

Is the Great Salt Lake actually disappearing? Yes, the Great Salt Lake has been shrinking to record-low levels. This is caused by a combination of factors, including long-term drought, climate change, and the diversion of water from the rivers that feed the lake for agriculture and urban use. The exposed lakebed creates dust that can be a significant air quality hazard for nearby residents.

Can anything live in the water of a salt lake? While most fish cannot survive, salt lakes support a unique ecosystem of extremophiles. The most common inhabitants are brine shrimp, brine flies, and various species of algae and bacteria that have adapted to the extreme salinity. These organisms form the base of a food web that supports millions of migratory birds.