The Caspian Sea is a unique body of water that defies simple classification. It is neither purely freshwater nor entirely saltwater, but rather a vast expanse of brackish water. This inland basin, situated between Europe and Asia, covers approximately 371,000 square kilometers. Its distinctive hydrological characteristics result in a salinity profile that sets it apart from both typical freshwater lakes and Earth’s oceans.
Defining its Water: Not Quite Either
Brackish water describes a mixture of fresh and saline water, with a salt content higher than most freshwater bodies but lower than average ocean water. The Caspian Sea’s average salinity is around 12 parts per thousand (ppt), approximately one-third the salinity of the world’s oceans. For comparison, typical freshwater lakes have salinity levels below 0.5 ppt, while oceans average about 35 ppt. This intermediate salinity places the Caspian Sea firmly in the brackish category.
The Caspian Sea’s unique water composition stems from its geological history, as it is a remnant of the ancient Paratethys Sea, which became landlocked millions of years ago. The dissolved salts from its oceanic past remained, contributing to its current salinity. Unlike typical freshwater lakes, its substantial salt concentration prevents it from being classified as such, despite its enclosed nature.
Geographical and Hydrological Influences
The Caspian Sea’s landlocked nature is a primary reason for its brackish character, as it has no natural outflow to the global ocean. Water entering the basin can only exit through evaporation, leading to the concentration of dissolved salts over time. This closed basin system means that any minerals carried in by rivers accumulate within the sea.
Over 130 rivers flow into the Caspian Sea, with the Volga River being the most significant contributor. The Volga, Europe’s longest river, accounts for approximately 80% of the total freshwater inflow into the Caspian. Other notable rivers, such as the Ural and Kura, also contribute substantial freshwater volumes. The balance between this freshwater input and high evaporation rates dictates the sea’s overall salinity.
Salinity Variations Across the Basin
Despite its general brackish classification, the Caspian Sea exhibits notable regional differences in salinity. The northern part of the sea, which is shallower and receives the overwhelming majority of freshwater inflow from the Volga and Ural rivers, is considerably less saline. Salinity in this northern region can be as low as 1 part per thousand near the river outlets, making it almost fresh.
Conversely, the southern and eastern parts of the Caspian Sea tend to be saltier. The middle and southern basins typically have a salinity range of 10 to 14 parts per thousand. The eastern sections, particularly the highly saline Garabogazköl lagoon, can reach extreme salinity levels, sometimes exceeding 200 parts per thousand due to intense evaporation and limited water exchange with the main basin. These variations are influenced by factors such as riverine input, localized evaporation rates, and the sea’s internal currents and depths.