The region of Attica is a mountainous area, alternating with valleys formed between its peaks. Athens itself is surrounded by several mountains, while the southwest part of the city is open to the sea.
The two main rivers of the otherwise ‘dry and thin-earthed’ Athens (Thucydides I.2.5, link) are Kifissos and Ilissos. Both were streams during the dry seasons but in periods of intense rainfall they turned into torrents, overflowing their banks and causing floods reported since antiquity (Plato, Critias 111a, link), especially nearby their estuary at the sea.
Kifissos river springs from the Mount Parnitha and flows into the gulf of Faliro, across the western side of the city. Ilissos river, on the other hand, springs from the north-western slopes of Hymettus; collecting rainwater from smaller streams, it runs on the east and south part of the historical centre of Athens, joined by the third river of Athens, Eridanos, in its route towards Faliro gulf, a few kilometres eastern of Kifissos estuary (In antiquity Ilissos merged with Kifissos outside the city. In mid – 20th century AD it was diverted and now it flows on a different river bed, under P. Tsaldari Avenue at Kallithea, http://goo.gl/Cp4SZb). Nowadays, Kifissos, Ilissos and Eridanos are have been covered almost in their entire length.
Considering the natural environment, an important part of the scarce green patches in modern Athens (e.g. Kaisariane monastery, Ardittos, National Garden, Olympieion park) are watered by Ilissos river, thus preserving a notable ecosystem in the urban environment, including different species of birds, trees, bushes and flowers.
In the area of Mets for example, by the St. Foteini church -one of the few open sites of Ilissos- experts have recorded willows (Vitex agnus-castus), oleanders (Nerium Oleander), plane trees (Platanus orientalis), while stroks building their nests could be spotted in historical copperplates. Also, in the gulf of Faliron -Ilissos’ estuary-, a delta is formed hosting more than one hundred species of aquatic birds, some rare in Attica or threatened, as charadriiformes, little ringed plovers, cormorants, alcyons, grey herons, great crested grebes, broad billed sandpipers and even brown necked gooses. In early 20th century 250 plant species have been recorded in the area, but today only a few survive, as some halophytes.