The Caspian Sea, the world’s largest enclosed body of water, presents a geographic paradox due to its name. Located between Europe and Asia, this massive body of water covers a surface area of approximately 371,000 square kilometers, which is comparable to the size of Japan. This sheer scale, combined with its high salinity, leads many to assume it is a sea. Despite its common name, however, the Caspian is technically classified by geographers and hydrologists as the world’s largest lake.
Defining Lakes and Seas: The Classification Criteria
The fundamental distinction between a lake and a sea hinges on connectivity and outflow. A true sea is a large body of saline water that forms part of the global ocean system, maintaining a connection to the world’s oceans either directly or through other seas. This connection ensures its water level is tied to the global sea level.
A lake, conversely, is defined as a body of water completely surrounded by land, meaning it has no natural outflow to an ocean. While lakes are typically freshwater, salinity is a secondary characteristic for classification. The Caspian Sea’s water level fluctuates independently of the world’s oceans, which is the primary criterion for its categorization as a lake.
Salinity causes the most confusion, as oceans have an average salinity of about 35 parts per thousand (ppt). The Caspian Sea is brackish, possessing a salinity of roughly 12 ppt, or approximately one-third that of the ocean. This salt content places it outside the definition of a freshwater lake, yet its lack of an ocean outlet confirms its status as an inland body.
The Geological Past and Present Salinity
The Caspian Sea’s substantial salt content and massive size are rooted in its ancient geological history. It is a remnant of the Paratethys Sea, a vast, shallow body of water that was cut off from the Tethys Ocean millions of years ago following tectonic uplift and a drop in sea level. The Tethys Ocean itself was a prehistoric ocean that existed during the Mesozoic Era.
The Caspian’s isolation allowed the water within the basin to evaporate over geological timescales, concentrating the dissolved mineral salts. This process is typical of landlocked water bodies and resulted in the current brackish conditions. The seabed of the southern Caspian basin even contains basaltic rock, a characteristic of oceanic crust, which further points to its ancient marine origin.
Salinity levels are not uniform across the entire water body. The northern Caspian is the least saline because it receives the vast majority of freshwater input from the Volga River. In contrast, the southern and eastern portions, which receive less river inflow and experience higher evaporation, are noticeably more brackish.
Endorheic Basin: The Defining Hydrological Feature
The defining factor for the Caspian Sea’s classification as a lake is its position as an endorheic basin. An endorheic basin is a geographical depression that retains water and allows no outflow to external bodies of water, such as an ocean. The water that enters the basin can only leave through evaporation or seepage.
The water balance of the Caspian is dominated by river inflow and evaporation. The Volga River is the most significant source, contributing approximately 80% of the total water inflow. Other major rivers, such as the Ural and Kura, also feed the basin. The only natural mechanism for water loss is the massive rate of surface evaporation.
This closed hydrological system dictates the lake’s independent and fluctuating water level. Historically, the Caspian Sea level has shown significant variation over decades, driven by climate-related changes in the Volga River’s discharge and regional evaporation rates.
Legal and Political Ramifications
The debate over the Caspian Sea’s classification extends beyond scientific nomenclature, carrying significant legal and political weight. The five nations bordering the water body—Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan—have long disputed how its vast resources should be managed. The central issue is whether the water should be treated legally as a sea or a lake.
If classified as a sea, the United Nations Convention on the Law of the Sea (UNCLOS) would apply, allowing for the creation of exclusive economic zones and continental shelves for each nation. Classifying it as a lake, however, would necessitate a division of resources based on a consensus agreement among the five states. The stakes involve lucrative oil, gas, and fishing rights, particularly concerning sturgeon and caviar stocks.
Negotiations spanning over two decades culminated in the 2018 Convention on the Legal Status of the Caspian Sea, which established a “special legal status.” This hybrid agreement grants each state 15 nautical miles of territorial waters with sovereign rights and an additional 10 nautical miles for exclusive fishing rights. The remaining central waters are deemed a common maritime space, blending elements of international sea law with shared resource management typical of a large border lake.