Caspian Sea
The Caspian Sea, like the Black Sea, Namak Lake, and Lake Urmia, is a remnant of the ancient Paratethys Sea. It became landlocked about 5.5 million years ago due to tectonic uplift and a fall in sea level. During warm and dry climatic periods, the landlocked sea almost dried up, depositing evaporitic sediments like halite that were covered by wind-blown deposits and were sealed off as an evaporite sink when cool, wet climates refilled the basin. (Comparable evaporite beds underlie the Mediterranean.) Due to the current inflow of fresh water, the Caspian Sea is a freshwater lake in its northern portions, and is most saline on the Iranian shore, where the catchment basin contributes little flow. Currently, the mean salinity of the Caspian is one third that of Earth's oceans. The Garabogazköl embayment, which dried up when water flow from the main body of the Caspian was blocked in the 1980s but has since been restored, routinely exceeds oceanic salinity by a factor of 10.
Caspian Sea
Over 130 rivers provide inflow to the Caspian, with the Volga River being the largest. A second affluent, the Ural River, flows in from the north, and the Kura River flows into the sea from the west. In the past, the Amu Darya (Oxus) of Central Asia in the east often changed course to empty into the Caspian through a now-desiccated riverbed called the Uzboy River, as did the Syr Darya farther north. The Caspian also has several small islands; they are primarily located in the north and have a collective land area of roughly . Adjacent to the North Caspian is the Caspian Depression, a low-lying region below sea level. The Central Asian steppes stretch across the northeast coast, while the Caucasus mountains hug the western shore. The biomes to both the north and east are characterized by cold, continental deserts. Conversely, the climate to the southwest and south are generally warm with uneven elevation due to a mix of highlands and mountain ranges; the drastic changes in climate alongside the Caspian have led to a great deal of biodiversity in the region.