Most of the water flowing into the sea comes from coastal rivers – currently supplying 300 to 310 cubic km a year. The Volga alone accounts for 80% of inflow. But it has dropped substantially during the 20th century, declining from about 400 cubic km in the 1920-30s to between 260 cubic km and 270 cubic km at present, due to various climatic factors and human activities such as dams built for hydroelectric energy production.