The Caspian Sea is not connected to the global ocean system, making it a massive inland body of water. While commonly called a sea due to its immense size and saline composition, it is technically the world’s largest lake by both surface area and volume. Its surface spans approximately 371,000 square kilometers, and it holds about 78,200 cubic kilometers of water, which is nearly five times the volume of all five North American Great Lakes combined. This enormous basin is entirely landlocked, lying between Europe and Asia, and its water level is not influenced by the tides or sea level of the world’s oceans.
Defining an Endorheic Basin
The physical geography of the Caspian Sea confirms its isolation, classifying it as an endorheic basin. This term describes a drainage system that retains water internally and has no natural outflow connection to any external body of water. Water flowing into the basin remains there until it is lost solely through evaporation, which leads to the accumulation of dissolved minerals over long periods.
The water balance of the Caspian Sea is maintained primarily by massive river inflow, particularly from the Volga River, which contributes roughly 80% of the total water input. Other major rivers, including the Ural and Terek, also feed the sea, carrying fresh water and suspended sediments into the basin. This reliance on river input and subsequent evaporation means the sea’s water level is constantly fluctuating, independent of global sea levels.
These fluctuations can be quite dramatic over decades, demonstrating the sensitivity of the closed system to climate and human activity. For example, the water level dropped significantly between 1929 and 1977, followed by a substantial rise in the subsequent two decades.
How Salty is the Caspian Sea
The enduring name “sea” creates a common misconception about the water’s salt content, suggesting a salinity similar to the open ocean. However, the Caspian Sea is brackish, with an average salinity of approximately 1.2 percent (12 parts per thousand, or ppt). This is significantly lower than the average salinity of the world’s oceans, which is about 3.5 percent (35 ppt).
The salt concentration is not uniform across the entire basin but varies dramatically depending on the location. The northern section, where the Volga and Ural rivers discharge their massive freshwater load, is the least saline, approaching near-fresh conditions. Conversely, salinity increases toward the south and is highest in the eastern Garabogazköl lagoon, a shallow bay where intense evaporation concentrates the minerals.
The existing salt content results from the continuous inflow of minerals and the lack of an outlet. Rivers naturally dissolve and carry salts from the soil and rocks across the catchment area into the sea. Since water is only lost through evaporation, the dissolved salts are left behind, gradually concentrating the water over millennia.
Geological History of Isolation
The Caspian Sea’s current state as a landlocked, brackish lake is a result of immense geological forces acting over millions of years. This body of water is a remnant of the ancient Paratethys Sea, a vast, shallow ocean covering much of Central Eurasia and descending from the larger Tethys Ocean.
Around 5.5 million years ago, during the Late Miocene epoch, tectonic activity began to reshape the region. The collision of continental plates caused significant uplift of mountain ranges, including the Caucasus and Kopet Dag mountains, which effectively sealed off the Paratethys from the global ocean system. This continental uplift, combined with a general fall in sea level, created the closed basin that would become the Caspian Sea.
The isolation led the ancestral sea to transform into a series of large, ever-changing brackish lakes, with the modern Caspian Sea being the largest surviving piece. This history explains why the southern part of the basin still preserves an oceanic-type crust, a relic from its former connection to the open sea. The ancient separation allowed the marine life trapped within the basin to evolve in isolation, leading to many endemic species found nowhere else.