The Eridanos river was the third in size river of Athens in terms of volume of water flow. In reality it was more like a gushing winter torrent rushing from the south slope of Lecabettus hill to eventually flow into the Ilissos river. Nowadays, its course can only be partly seen, as it mainly runs underground, the size of a mere stream.
During the prehistoric period, the river often flooded as its watershed would suddenly receive large quantities of rainwater from the surrounding hills (Areopagus, Acropolis, Pnyx), while its riverbed was unstable, changing direction from time to time. At this time, the Eridanos left the inhabited area through the Kerameikos valley, which was used by the Athenians as a cemetery.
Later, as the settlement extended to the northwest (Kerameikos) its riverbed was aligned with the construction of the Themistoklean city wall of Athens (478 BC). Actually, until the Persian Wars, only the top of the Acropolis hill was surrounded by walls. However, after the Persian invasion and the destruction of the city in 480 BC, Themistocles had a great wall built around the wider inhabited area, to protect it from the Spartans as well. The work was done in great haste, incorporating in this wall the ruins of the monuments and buildings which had been destroyed. During these works the Eridanos River was lined with stone masonry thus creating a stable direction for the water flow. In the built riverbed, parts of broken statues were also used, some of which have been discovered and are displayed today in the Kerameikos museum.
The riverbed was aligned again several times in later periods, as part of the periodical works on the city’s fortifications. Through the years, certain parts of the river were covered and already by the 2nd century AD it flowed mainly underground as it had been turned into a drain channel for the wastewaters of the densely inhabited city of Athens.