Unfortunately I can’t see the previous answers for some reason, but the first thing to be taken into account is the enormous Greek coastline: the land may be small, but the geography of the peninsula is a combination of high altitude observation points (hills and mountains) near the sea, while the coast is simultaneously human-friendly and versatile, with many ports, small or large. The first civilizations of the Hellenic territory were marine: you may remember the Cycladic civilization or the Minoans, both peoples of the sea and of the trade, based on islands- the Cyclades and Crete-Thera respectively. Even the Myceneans, who were more aggressive and fierce, had good knowledge of marine trade and several coastal cities under their control-the sunken city of Paulopetri, as well as the recent discovery of ancient Iolkos in contemporary Volos are poignant examples.

The seafaring peoples of the city states had it relatively easy; even though the Mediterranean is a temperamental sea, the islands are close enough to have regular connections almost all year round, and they worked also as bridges between what is today mainland Greece and the coast of Asia Minor.

That said, there may not have been a major river such as the Nile, or the Mesopotamian rivers, but smaller rivers in various areas, especially the more mountainous ones, created fertile valleys. Even today, Thessaly is a flat and fertile land, with resources to be not only self sufficient, but to trade the spare and feed other city states too. Also, one must consider what had already been there by nature, like honey, that doesn’t need a river per se to thrive, or the goldmine of olive groves.

Greek Nobel laureate Odysseas Elytis describes it quite well:

“If you deconstruct Greece, you will in the end see an olive tree, a grape vine and a boat remain. That is, with as much, you reconstruct her.”