Message #26118

[Bianca]

Another important factor contributing to the formation of the marshlands is that the lower Mesopotamian plain becomes very narrow towards the Gulf. This is created by the large alluvial fan of Wadi Batin and the Al Dibdibah plain drawing in from the Nejid in the west, and the Karkheh and Karun river systems descending from the Zagros Mountains in the east. The Karkheh disperses into the marshes on the eastern bank of the Tigris, whose waters eventually overflow into the Shatt-al-Arab via Al Suwaib River. For its part, the Karun joins the Tigris-Euphrates system below their confluence in the lower section of the Shatt-al-Arab, at the port city of Khorramshahr 72 km from the Gulf. Both rivers, but particularly the latter, carry a large sediment load. By fanning out at the head of the Gulf, the Wadi Batin/Al Dibdibah, the Karkheh and Karun constrict the lower Mesopotamian valley to a width of less than 45 km and prevent the twin rivers from flowing directly into the sea (Rzóska, 1980). In so doing, the natural drainage of the Tigris and Euphrates is impeded and they are forced to deposit their sediment loads inland. This results in the creation of a double delta composed of a continental marshland complex and a marine estuary. As mentioned earlier, a notable feature of both the Tigris and Euphrates is the large fluctuation in their water discharge volumes. Spring floods, occurring form February to May, are caused by snowmelt in the headwater region in Turkey and the Zagros Mountains in Iran and northern Iraq. These short-lived but intense seasonal floods, which formerly have been on the order of 1.5 to 3 meters (with a record of 9 meters on the Tigris in 1954) cause large-scale inundations (Scott, 1995). As a result of the flat topography, the flood pulses are able to maintain an extensive complex of interconnected shallow lakes, backswamps and marshlands in the lower Mesopotamian plain. The marshlands, which are of great though changing extent, may dry up completely in shallower areas under the influence of high summer temperatures, leaving salt flats and reverting back to desert conditions. This highly dynamic ecosystem is therefore dependent on spring floods for its replenishment and very existence.