section_path: Geography
section_name: Geography
paragraph_text: Geography
  
  Mesopotamia encompasses the land between the Euphrates and Tigris rivers, both of which have their headwaters in the neighboring Armenian highlands. Both rivers are fed by numerous tributaries, and the entire river system drains a vast mountainous region. Overland routes in Mesopotamia usually follow the Euphrates because the banks of the Tigris are frequently steep and difficult. The climate of the region is semi-arid with a vast desert expanse in the north which gives way to a 15,000-square-kilometre (5,800 sq mi) region of marshes, lagoons, mudflats, and reed banks in the south. In the extreme south, the Euphrates and the Tigris unite and empty into the Persian Gulf.
  
  The arid environment ranges from the northern areas of rain-fed agriculture to the south where irrigation of agriculture is essential. This irrigation is aided by a high water table and by melting snows from the high peaks of the northern Zagros Mountains and from the Armenian Highlands, the source of the Tigris and Euphrates Rivers that give the region its name. The usefulness of irrigation depends upon the ability to mobilize sufficient labor for the construction and maintenance of canals, and this, from the earliest period, has assisted the development of urban settlements and centralized systems of political authority.
  
  Agriculture throughout the region has been supplemented by nomadic pastoralism, where tent-dwelling nomads herded sheep and goats (and later camels) from the river pastures in the dry summer months, out into seasonal grazing lands on the desert fringe in the wet winter season. The area is generally lacking in building stone, precious metals, and timber, and so historically has relied upon long-distance trade of agricultural products to secure these items from outlying areas. In the marshlands to the south of the area, a complex water-borne fishing culture has existed since prehistoric times and has added to the cultural mix.
  
  Periodic breakdowns in the cultural system have occurred for a number of reasons. The demands for labor has from time to time led to population increases that push the limits of the ecological carrying capacity, and should a period of climatic instability ensue, collapsing central government and declining populations can occur. Alternatively, military vulnerability to invasion from marginal hill tribes or nomadic pastoralists has led to periods of trade collapse and neglect of irrigation systems. Equally, centripetal tendencies amongst city-states have meant that central authority over the whole region, when imposed, has tended to be ephemeral, and localism has fragmented power into tribal or smaller regional units. These trends have continued to the present day in Iraq.