The Caspian Sea, the largest inland body of water on Earth, presents a unique geographical riddle. Its massive size, salt content, and name suggest a sea, yet its completely landlocked nature aligns with the definition of a lake. Understanding the Caspian Sea requires moving beyond a simple classification and examining the physical processes that govern this exceptional water body.
Defining the Difference
The conventional distinction between a sea and a lake hinges primarily on a connection to the global ocean. Seas are large bodies of saltwater partially enclosed by land but connected to the world’s oceans, allowing for constant water and nutrient exchange. Conversely, a lake is a body of water, typically freshwater, that is entirely surrounded by land with no natural outlet to the ocean.
Chemical composition offers a secondary measure, as seas are consistently highly saline, while most lakes are fresh. However, landlocked bodies like the Great Salt Lake or the Dead Sea accumulate salt due to high evaporation rates. The Caspian Sea’s vast scale, spanning approximately 371,000 square kilometers, further blurs the line, making it larger than some bodies that are uncontroversially called seas.
Geological History and Salinity
The Caspian Sea’s composition is a direct consequence of its deep geological past, linking it to the world’s ancient ocean system. It is a remnant of the vast Tethys Ocean, which fractured and closed millions of years ago as continental plates collided. The Caspian is specifically a vestige of the Paratethys Sea, an inland sea that became isolated from the global ocean during the Oligocene epoch.
This ancient marine origin explains why the Caspian is a brackish water body and why it harbors relict species of fauna that evolved in a fully marine environment. The average salinity is approximately 1.2 percent, about one-third the salinity of the world’s oceans. This concentration is not uniform; the northern portion is nearly fresh due to massive river inflow, while the southern and eastern parts are considerably saltier.
Hydrology: A Closed System
Despite its marine heritage and saltwater content, the Caspian Sea functions, from a hydrological perspective, as the world’s largest lake. It occupies an endorheic basin, meaning it is a closed drainage system with no natural outflow to the ocean. This lack of connection makes its water level entirely independent of the global ocean’s eustatic level.
The water balance is maintained solely by the inflow from rivers, atmospheric precipitation, and evaporation. The Volga River is the dominant source of fresh water, contributing approximately 80 percent of the total annual inflow. This massive influx of river water is the reason the Caspian’s overall salinity remains relatively low compared to a true sea.
Because there is no outlet, the water level is sensitive to climatic and hydrological shifts across its vast catchment area. Fluctuations over the last century have been dramatic, with the sea level falling three meters between 1929 and 1977, followed by a rise of the same amount by 1995. Recently, increased evaporation driven by regional climate warming is causing a rapid decline in the water level. This closed-system dynamic is the definitive scientific reason for its classification as a lake.
Why the Name “Sea” Persists
The persistence of the name “Sea” is rooted less in physical geography and more in geopolitics and international law. The classification has enormous practical consequences for the five bordering nations: Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan. Designating the Caspian as a “sea” or a “lake” determines the legal framework for dividing its oil and natural gas reserves, fishing rights, and navigation.
If recognized as a lake, traditional international law might mandate a straightforward division of the surface and seabed among the five littoral states. Calling it a “sea,” however, allows states to invoke principles from the United Nations Convention on the Law of the Sea (UNCLOS). This enables the establishment of sovereign territorial waters and exclusive fishing zones, which are typically larger under maritime law.
To resolve decades of disputes, the five countries signed the Convention on the Legal Status of the Caspian Sea in 2018. This document avoids a definitive classification, referring to the Caspian simply as a “body of water,” thereby creating a unique legal regime. The convention grants each state a 15-nautical-mile territorial water zone and an additional 10-nautical-mile exclusive fishing zone, leaving sub-seabed resources to be divided by bilateral agreements.