'Lake Marathon or the Marathon Reservoir (Greek: Limni Marathonos)a is a water supply reservoir formed from the construction of Marathon Dam at the junction of Charadros and Varnavas Torrents near the town of Marathon, in Greece. Since 1931 when the reservoir became operational was the primary the water supply facility for Athens until 1959 when water supply from Yliki Lake and Mornos Reservoir (operational since 1981) used, today is a backup source for water supply of Athens.
The area of the lake at the height of the spilway of the dam is 2.45 square kilometres, the maximum deth is 54 m , the lake concetrates water from a watershed of 118 square kilometres with an average runoff of 14,400,000.0 m³ per year in an average rainfall of 580mm per year (standard deviation 135mm), the average inbound volume is 12,000,000.0 m³ per year and the maximum reservoir capacity is 41,000,000.0 m³ (effective volume 34,000,000.0 m³).
The dam has a maximum height (from foundation to crest) 54m, maximum width 28m at base and 4.5 m at crest, length 285m , crest altimeter 227 m above sea level, toe altimeter 173 m above sea level (at lower point), the spilway is at 223 m above sea level and has a disgarge of 520 m³/s. The dam is costructed from concrete and is a gravity dam and is considered unique worldwide because of the external cladding of white pentelikon marble (the same marble used in constuction of the Parthenon and all the other buildings in the Acropolis. The dam constucted from the American firm ULEN (which had, in a BOT type contract, the ownership of water supply company of Athens until 1974) due to increased water supply needs from the rapid increasing of population of Athens from the huge influx of refugees from Asia Minor in after the end of Greek - Turkish war in 1922. The construction lasted from 1926 to 1929.