The most significant feature of the Caspian Sea is its constant level fluctuations.
The main reason of the sea level oscillations is the change of its water balance elements. Majority of the sea water balance (80%) is provided with river inflows, and about 85% of it is due to inflow waters of Volga River. According to observations river inflows expose to changes in great interval in relation to average rate. The cause is hydrometeorological processes occurring in its vast catchment basin (roughly 20% of northern hemisphere).
The sea level change held 15 m during historical period (last 3.5 thousand years) while 3.5 m in the last 200 years. The sea level incurs monthly oscillations with the rate of 30-40 cm interval. The highest rate of the sea level occurs during June-August, while the lowest rate appears during December-February. Furthermore water extrusion and tidal fluctuations of the sea level occur due to long term winds. This process happens especially in the Northern Caspian. Water extrusion and tidal change can reach 4.5 m and 2.5 m respectively in strong south-eastern winds.
Historical sources indicate that the level oscillation of the Caspian Sea held 15 m in the last 2.5 thousand years, while 4.5 m in the last 200 years. The basic instrumental observations on the level were started since 1837 in Baku district (Bayil area) and they are continued in 15 settlements over the Caspian Sea and in 7 settlements in Azerbaijan sector of the sea up to date.
According to the observation data, the Caspian Sea level was located in minus 26.00 m BS absolute height (Baltic system) of ocean level during 1830 – 1929 (within 100 years) and amplitude of the sea level oscillations held 0.5 m. Since 1930 the sea level began to decrease and fell 3 m until 1977.
The Caspian Sea level was in minus 29.00 m BS absolute height in 1977, which was the lowest level in the last 500 years.

Annual oscillations of the Caspian Sea level (1837 – 2007), axis – indicates level prognosis.

The Caspian Sea level increased rapidly 15 cm on average beginning from 1978 and reached the highest level (-26.50 m BS) in 1995. At that time the level rise was 2.50 m in relation to 1977.
The Caspian Sea level decreased 35 m during 1996-2000 and re-increased since 2001. In this regard, the sea level rose nearly 30 cm during 2001 – 2005, and decrease of the level up to 3-5 cm was observed in 2006 in comparison with the previous year.
One year prediction method of the Caspian Sea level was based on estimation of its water balance elements and currently its demonstration holds 90-95%.
Prolonged prognosis of the Caspian Sea level (5-10 years) is somewhat complicated issue, because sea level fluctuation is related with climatic elements and period of reliable predictions of these elements is no more than 6 months in the contemporary scientific development stage.
According to prognoses based on regional climate change, the Caspian Sea level is expected to rise about 70-80 cm by 2020s.
Average annual rate of the Caspian Sea level held 26.78 m BS (Baltic system) in 2007.