Why did the Caspian Sea water level fall between 1929 and 1977?

The decline in the water level of the Caspian Sea between 1929 and 1977 was primarily driven by climatic changes that altered the balance between water gains and losses. Specifically, these climatic shifts led to increased evaporation and a reduction in river influx.1

This natural decline was further amplified by human activity, specifically the construction of reservoirs on the Volga River. These projects diverted water to support irrigation and industrial needs, which reduced the volume of water flowing into the sea. By 1977, these combined factors resulted in the sea reaching its lowest level in approximately 400 to 500 years, dropping to 96 feet (29 metres) below sea level.1