The intense fragmentation of the relief and its proximity to the coast as well as the scarce rainfall did not allow the formation in the Hellenic territory of long rivers and vast catchment areas. The watercourses have almost all irregular flows and a typically torrential regime, with winter floods and prolonged summer lean periods. The main rivers are those of Macedonia and Thrace, which interest Greece only with the terminal stretch of their course; they are the Vardar (in Greek Axiós), which descends from the Republic of Macedonia and flows into the Gulf of Thessaloniki, the Struma (in Greek Strymón) and the Mesta (in Greek Néstos), which both originate in Bulgarian territory and pay tribute to the Aegean, the first in the Gulf of Orfánion, the other facing the island of Thasos forming a large delta system, and finally the Marica or Marizza (in Greek Ébros), which was born in Bulgaria and marks the border between Greece and European Turkey. Of the entirely Greek rivers, the main ones are the Aliákmōn, which flows into the Gulf of Thessaloniki, and the Pēneiós, which with its floods gave rise to the plain of Thessaly, both on the eastern side, and also the Aráchthos, which flows between the limestone plateaus of Epirus and the Pindus chain then flowing into the Gulf of Arta, the Aspropotamo (in Greek Achelos), which crosses Aetolia and Acarnania, and the Alphaeus, a tributary of the Gulf of Arcadia, on the Ionian side: the latter, due to the greater quantity of precipitation collected from their catchment basins and the lower permeability of soils, have higher flow rates and more constant regime. The karst manifestations are extensive in the calcareous areas with sub-horizontal stratification, with the development of a vast underground hydrographic network. The main lakes are those of Vólvē and Koroneia, at the base of the Chalkidiki Peninsula, of Vegorrítis, of Prespa and of Doiránes, also in Macedonia, of Giánnina, in Epirus, and of Voiveís, in Thessaly.
According to iamhigher, the Greek population was the protagonist of a conspicuous demographic increase: in fact, while in the middle of the nineteenth century it was around 1,020,000 residents, at the beginning of the twentieth century it doubled, and in 2003 it increased tenfold, exceeding 11,000,000 residents. residents. The birth rate began to decline after 1951 and, in the following decades, showed a tendency to decrease even more rapidly until it stabilized around the levels of the more developed European countries. At the same time, mortality has changed. The current composition of the Greek population is the result of complex historical events. Among these is the one dating back to 1923, at the end of the Greek-Turkish war (1921-23), when, in application of the Treaty of Lausanne, a population exchange took place between Greece and Turkey, under the control of the League of Nations: 607,000 Turks left Greece and 1,222,000 Greeks from Turkey arrived in Greece and were settled for more than half in western Thrace and Macedonia in the areas left by Turkish refugees, bringing significant benefits to the economy of large areas that were previously poorly and under-valued. The ethnic picture of the Hellenic peninsula is consequently very homogeneous, even if slight changes have begun to take place since the 1990s. The population in its absolute majority is Greek (93.7%), there is also a large and growing number of different ethnic groups. The most conspicuous are those of Albanian and Balkan origin. Greece has, in fact, become the destination of a large number of refugees from the crisis areas of the Balkans and immigrants originating from the newly established Asian republics, North Africa and the Near East. Emigration, which in the first decades of the twentieth century deprived the country on average of 30,000 units per year (mainly directed towards America), it has almost completely stopped; conversely, the number of repatriations increased. Internal migratory movements remain towards the economically more dynamic and more urbanized areas, mainly the area of Athens, Thessaloniki, Patras and the island of Crete, all port centers with various industrial complexes. This migratory flow accentuates the already strong imbalance in the distribution of the population, thus increasing the phenomenon of urbanism. The most densely populated area is Attica, especially due to the presence of the Athensconurbation; Central Macedonia follows with the province of Thessaloniki and western Peloponnese, where the port of Patras and the island of Corfu are located. The least populated are the most inaccessible mountainous areas of the interior, especially in Epirus. Athens above all has experienced a huge demographic and building expansion, passing from 100,000 residents from the late nineteenth century to the great conurbation with Piraeus (the so-called Greater Athens), which has a quarter of approx. of the entire Greek population. The other conurbations that exceed 100,000 residents I am Thessaloniki, Patras, Heraklion (or Iràkleio),. Among the cities over 50,000 residents there are Larissa, Giánnina, Kavála, Lamía and Chaniá.