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Slide 1. Mesopotamia, i.e. 'the land between two rivers', Tigris and Euphrates, was the birthplace of the earth's earliest civilization that emerged about 3500 BC and declined about 1000 AD. It is believed to be home to one of the seven wonders of the world, hanging gardens of Babylon, and legendary Tower of Babel both shown on my title slide. Among major achievements of this early civilization are the inventions of wheel, codes of law, irrigational agriculture, flood control systems, and the first writing system, i.e. cuneiform. Slide 2. But, most importantly, lower Mesopotamia was a birthplace of the first cities on earth. It was the most urbanized society of antiquity where, at some point, about 90% of population lived in cities. The largest cities during third millennium BC contained as many as 80,000 people. Ancient city of Uruk achieved approximately half of the size of ancient Rome, the center of vast empire, but 3000 years later. Many different opinions exist regarding origins of early cities. During my presentation, I am going to discuss the role that channel networks resulting from avulsions, could have played in the evolution of settlement patterns and emergence of urban settlements in lower Mesopotamia. Slide 3. Mesopotamia is subdivided into two regions with distinctly different natural conditions, upper or northern, and lower or southern. Northern Mesopotamia receives more precipitation whereas in lower Mesopotamia located in desert climate with annual precipitation of only 100-150 mm only irrigational agriculture is possible. Lower Mesopotamian depression is a part of foreland basin located between Zagros Mountains and Arabian Platform. Tigris and Euphrates flow from mountains of southern Turkey and join near Qurna forming Shatt al Arab estuary of Persian Gulf. Lower Mesopotamia is subdivided into three physiographic units: alluvial plain of Tigris and Euphrates, the zone of marshes and lakes ("Ahwar') and estuary. Ancient settlements are located within alluvial plains. It is believed that ancient Mesopotamia was mainly irrigated by Euphrates rather than Tigris waters. Tigris though carrying much larger discharge is characterized by violent unpredictable floods, much higher the Euphrates sediment load and flows in an incised valley throughout most of its length making it difficult to manage. Due to differences in geological sources, soils on Euphrates alluvium are more fertile. Tigris water began to be extensively used for irrigation only since early medieval times. After decades of archaeological research, there is still an unresolved problem why first urban settlements appeared in deserts of lower Mesopotamia instead of upper Mesopotamia with much more favorable natural conditions. Slide 4. Archaeological surveys (e.g. Adams, 1981) demonstrate certain dynamics of settlement patterns in lower Mesopotamia. Very little data exist about Ubaid period of Mesopotamian history (ended 5500 BP). Though these data are controversial, it is believed that lower Mesopotamia during Ubaid period was populated by small farming communities. At the end of Ubaid and during Uruk (5500-5100 BP) and, possibly, Jamdet Nasr (5100-4900 BP) periods many urban settlements emerged (settlement with size of more than 40 ha). Beginning from Early Dynastic period (4900 BP) and through Old Babylonian period (ended 3600 BP), 'urban explosion' took place in lower Mesopotamia when cities reached maximum extent with some of them exceeding the size of 200 ha. Most cities were first located in the southern part of lower Mesopotamia known as Sumer. Then, after decline of many settlements, major shift in population took place toward more northern part of lower Mesopotamia, i.e. Akkad or Babylonia. From the end of Old Babylonian period (3600 BP) and through Cassite period (3600-3150 BP), major population decline occurred in lower Mesopotamia when many cities and rural settlements became abandoned. Prior to onset of settlement: very little data Period I: small settlements and few larger centers along multiple channel networks. Joint Tigris-Euphrates? Abandonment of many sites at the end. Period II: sudden emergence of many large cities and small settlements, ‘urban explosion’ along several Euphrates channels. Abandonment of many sites and population shift to the north at the end. Period III: shift in Euphrates flow westward toward two-channel system, extensive ‘statecontrolled’ canal construction, some large centers and many small settlements, progressive abandonment of many large centers toward the end, using Tigris water for irrigation, establishment of large centers on Tigris River. General decline of Lower Mesopotamian civilization occurred during mid-Islamic time. Slide 5. Several archaeologists suggested that the changing channel networks in lower Mesopotamia could have been have been important factor in the evolution of settlement patterns. Channel networks are created by the process called avulsion. Avulsion is a major river diversion to the lower elevation on a floodplain by which older channel belt may become abandoned and new channel belts initiated. Avulsions in meandering streams are especially typical for low-gradient alluvial and deltaic plains where, due to high aggradation rates, river, confined by levees, flows above the surrounding floodplain. Avulsions normally occur during floods when river overtops its banks and find a new course that provides gradient advantage compared to the previous course. Slide 6. Evidence of avulsions in lower Mesopotamia includes: - abandoned river channels identified on aerial/satellite photos and during archaeological surveys. Close association between abandoned river courses and ancient settlements is widely recognized; - presence of necessary conditions for avulsions: low floodplain gradient, high aggradation rates, channels flowing above surrounding floodbasins; - presence of triggering mechanisms of avulsions: episodic high floods, channel blockage by sediments and vegetation; - Holocene sea-level changes, climate fluctuations, tectonic movements and changes in aggradation rates that may be important underlying causes of avulsions. Slide 7. The role of avulsion in ancient Mesopotamia has been widely recognized. However, it was regarded in rather simplistic and purely negative terms: when old channel becomes abandoned, settlements along it decline and new settlements emerge along the new course. I am going to suggest that the role of avulsion was much more complex and more constructive rather than destructive. Studies of avulsion processes in many different settings suggest that several factors determine overall effects of avulsions: avulsion rate, avulsion frequency, avulsion style and channel pattern following avulsion. Slide 8. I will consider three avulsion scenarios that result in different channel and sedimentation patterns: - reoccupational avulsion toward new single channel with crevasse splays. Single channel results from abrupt avulsion and two-channel system- from gradual avulsion; - gradual progradational avulsion followed by development of relatively short-lived (natural life-time of ~100 yrs) anastomosed channel networks and avulsion belts. Avulsion belt is relatively large (few 100 km2) area of floodplain inundated by avulsion with multiple channel networks that consist of predominantly lacustrine deltas and splay complexes). Avulsion belts provide increase in naturally-irrigated area of the floodplain composed of relatively well-drained soils; - gradual and/or frequent avulsions followed by long-lasting (102-103 yrs) coexistence of many active channels, some with avulsion belts; These scenarios have different implications for settlement distribution because of different requirements to the sizes of irrigated floodplain needed for rural and urban settlements. Slide 9. City: refer to slide; Slide 10. Mesopotamian farmers used natural floodplain morphology to their advantage. Gravity or flow irrigation was a major technique supplemented only during modern times by use of pumps and other mechanical devices. Gravity irrigation utilizes elevated position of channels above the floodbasin. Levee breaks at crevasse channels or made artificially may be used as intake points for irrigation water. Irrigational enclaves may develop within distal levee and crevasse splays where soils are coarser, better drained and undergo less salinization. Relatively small naturally-irrigated area is needed for rural settlements. For example, assuming density of population of about 100 persons/ha (Adams, 1981) and sustainable area of 0.5-1.5 ha/person, irrigated area of ~5-15 km2 is needed for a large rural settlement with 1000 inhabitants. Slide 11. Keeping this in mind, I now can analyze the consequences of three avulsion scenarions suggested above for settlement distribution. Refer to slide. Scenario A the irrigated area of floodplain is limited. Scenario B may produce local relatively short-term increase in irrigated area of floodplain. Third scenario produced conditions for dramatic increase in population, possible concentration in large centers due to increase in irrigated area of the floodplain and generation of food surpluses. Slides 12-15. Now I am going to try to combine geological and archaeological data from lower Mesopotamia and analyze changes in avulsion types and resulting channel networks. Refer to slides. Slide 16. Conclusions.