Download Talk plan - Geological Society of America

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.