Ilissos

Ilisos
River
The only uncanalised part of the Ilisos river bed
Country	Greece
Region	Attica
City	Athens
Source
- location	Mount Hymettus
Mouth
- location	Phaleron Bay
- elevation	0 m (0 ft)

The Ilisos or Ilisus (Greek: Ιλισός) is a river in Athens, Greece. Originally a tributary of the Kifissos River, it is now largely channeled underground.

Its name is in all probability Pre-Greek: it features the -sós/-ssós/-ttós ending, which it shares with many other toponyms in Attica and other rivers in Greece, all of which are considered linguistic substratum survivals.

During antiquity, it ran outside the defensive walls of Athens: Plato wrote in Critias that the river was one of the borders of the ancient walls. Its banks—in the busy intersection that today features the Hilton Hotel and the National Gallery—were grassy and shaded by plane trees, and were considered idyllic in antiquity; they were the favored haunts of Socrates for his walks and teaching. The temple of Pankrátēs, a local hero, was located there, giving its name to the modern suburb of Pagkrati. Ilisos was also a demi-god, the son of Poseidon and Demetra, and was worshipped in a sanctuary on the Ardittos Hill, next to the current Panathinaiko Stadium. This area was called Cynosarges in antiquity, and the spring of Kallirrhóē was located there.

The stream drains the western slopes of Mount Hymettus, and originates from multiple converging seasonal creeks. As urban Athens expanded in the 19th and early 20th centuries, the river became a source of pollution and was gradually converted into a rainwater runoff conduit, covered with streets that track its original, twisting route along the lay of the land. Its bed proper flows under Mesogeion Avenue at the Old Gendarmerie Academy, runs under Michalakopoulou (the modern-day Ilisia suburb) and Vasileos Konstantinou Avenues, and passes in front of the Panathinaiko Stadium, where it was bridged in the 19th century. It then flows to the southeastern flank of the ruined Columns of Olympian Zeus, where it is still visible amidst reed beds, next to the Byzantine chapel of Saint Photeini "of the Ilisos". In older times the river at this point would expand into shallow marshland, called "Vatrachonisi"" ("Frog Island") in the vicinity of the ancient spring of Kallirrhóē, now submerged under Kallirois Avenue. It then flows under Theseos Avenue, in the suburb of Kallithea, its original course turning sharply northwest to join the Kifissos River, of which it was once a tributary. The Ilisos is now routed straight to sea, coming to surface and running into the Saronic Gulf in the middle of Phaleron Bay.