Marathon Reservoir was created by the construction of a concrete dam, 54 meters high at the junction of the Charadros and Varnava Torrents. The construction began in 1925 and the project began its operation in 1931. The reservoir has a surface area of 2.4 square kilometres, a watershed of 132 square kilometres, a maximum capacity of 41 million m³ of water and an operational volume of 34 million m³. The reservoir operates as a backup source for the water supply system of the greater Attica region and as a primary regulating reservoir.