The Tigris-Euphrates Basin watershed's topography is such that the headwater catchment generating Tigris and Euphrates flows is wholly located in the north and eastern parts of the basin in the highlands of Turkey, Iraq and Iran. The waters of the Tigris and Euphrates are essential to the life of the country, but they may also threaten it. The rivers are at their lowest level in September and October and at flood in March, April, and May when they may carry forty times as much water as at low mark. Moreover, one season's flood may be ten or more times as great as that in another year. In 1954, for example, Baghdad was seriously threatened, and dikes protecting it were nearly topped by the flooding Tigris.
Both the Tigris and the Euphrates break into a number of channels in the marshland area, and the flow of the rivers is substantially reduced by the time they come together at Al Qurnah. Moreover, the swamps act as silt traps, and the Shatt al Arab is relatively silt free as it flows south. Below Basra, however, the Karun River enters the Shatt al Arab from Iran, carrying large quantities of silt that present a continuous dredging problem in maintaining a channel for ocean-going vessels to reach the port at Basra. This problem had been superseded by a greater obstacle to river traffic, however, namely the presence of several sunken hulks that had been rusting in the Shatt al Arab since early in the war.
The Euphrates [Greek Euphrates, meaning "sweet water", Assyrian Purat, meaning "great stream", Arabic Al Furat, Turkish Furat] originates in Turkey, is augmented by the Nahr (river) al Khabur in Syria, and enters Iraq in the northwest. Here it is fed only by the wadis of the western desert during the winter rains. It then winds through a gorge, which varies from two to sixteen kilometers in width, until it flows out on the plain at Ar Ramadi. Beyond there the Euphrates continues to the Hindiyah Barrage, which was constructed in 1914 to divert the river into the Hindiyah Channel; the present day Shatt al Hillah had been the main channel of the Euphrates before 1914. Below Al Kifl, the river follows two channels to As Samawah, where it reappears as a single channel to join the Tigris at Al Qurnah. Arising near Mount Ararat at heights of around 4,500 m near lake Van, the Euphrates traverses an expanse of about 3,000 kilometers. This is a considerably longer distance than that travelled by the Tigris, which rises in the small mountain lake of Hazar and whose total length is around 1,900 kilometres. Although the Euphrates drains a larger surface area (579,314 km2) than the Tigris (371,562 km2), an overwhelming 88-98% of Euphrates runoff is produced in the highlands of southeastern Turkey while the remainder of its catchment is an arid region that contributes little inflow.
The Tigris [Greek Tigris = "tiger", Pahlavi tigr means "arrow"] also rises in Turkey but is significantly augmented by several rivers in Iraq, the most important of which are the Khabur, the Great Zab, the Little Zab, and the Uzaym, all of which join the Tigris above Baghdad, and the Diyala, which joins it about thirty-six kilometers below the city. At the Kut Barrage much of the water is diverted into the Shatt al Gharraf, which was once the main channel of the Tigris. Water from the Tigris thus enters the Euphrates through the Shatt al Gharraf well above the confluence of the two main channels at Al Qurnah. In contrast to the Euphrates, the Tigris is less dependent on the headwater region in Turkey, which contributes an estimated 32-50% of its discharge. The balance of Tigris flows is produced by a series of major left bank tributaries descending from the Zagros Mountains of Iran and Iraq. Prior to dam construction, estimates of the mean annual runoff of the Tigris at Baghdad ranged from 49.2 - 52.6 BCM (billion cubic meters), which is considerably greater than that of the Euphrates at Hit ranging between 28.4 - 32.4 BCM.
Superimposed on the natural drainage basin of the Tigris and Euphrates is a complex geopolitical reality, with several countries controlling the upper, middle and lower courses of the twin rivers. Geographically, the principal riparian nations are aligned above each other, creating a classical "dominant upstream vs. vulnerable downstream relationship". In this hierarchical order, the Euphrates rises in Turkey and flows through Syria and Iraq, while the Tigris catchment whose source is also in Turkey is shared with Iraq and Iran. Syria also has access to a 20 km stretch on the right bank of the Tigris, which forms its extreme northeast border with Turkey. The aforementioned difference in runoff generation patterns between the Tigris and Euphrates has important implications for water management projects. With the Euphrates' main tributaries all located in its upper catchment, Turkey, by damming the river, is able to exert almost full command over the river's hydrological regime. For the Tigris, dams in Turkey would also have an important impact, but only partial control of the river's waters would be possible as it receives major water flows from tributaries along its middle course. A more complex series of dams on Tigris tributaries in Iran and Iraq are therefore necessary to yield a similar level of river control as on the Euphrates.
Historically and prior to dam construction, two characteristic features distinguished the hydrological regime of the Tigris-Euphrates river system. The first is sudden and violent flooding. Annual precipitation in the Anatolian and Zagros highlands, where most of the water of the twin rivers is generated, typically exceed 1,000 mm. As most of this precipitation occurs in winter, it falls as snow and may remain fixed in a solid state for half the year. With snow melt in spring, vast quantities of water are rapidly released, creating periodic flooding downstream. These flood waves are a major driving force in the ecology of the Tigris-Euphrates river system, particularly of the alluvial-deltaic plains of lower Mesopotamia as it constantly shifts between desert and marsh.
The other distinguishing mark is the highly irregular regime of the twin rivers, both between and within years. Hydrological records (1937-1964) at the Turkish-Syrian border for the Euphrates range from a minimum flow of 16,871 MCM/y (1961) to a maximum discharge of 43,456 MCM/y (1963), while that of the Tigris at Cizre in the upper Tigris dropped to 7,891 MCM/y (1961) rising by a factor of four to 34,340 MCM/y (1969). Similarly, there are steep differences between maximum and minimum monthly flows, which for the Tigris is nearly 80-fold and for the Euphrates 28 times as much.
The concentration of discharge over a short time period causes extensive spring flooding, inundating wide areas and creating a mosaic of permanent and temporary water bodies in the flat alluvial plain between the two rivers. The seasonal flood regime is thus of critical importance to the ecological dynamics of the marshlands, governing not only the physical extent of the marshes but also the range and distribution of flora and fauna it supports.
The diversion of floodwaters into seasonal lakes and natural depressions along the middle course of the Euphrates and Tigris signalled the first major change in river management activity. The Ramadi barrage was built in 1951 to divert water from the Euphrates River to Habbaniyah depression and in the event of extreme floods, excess water was channelled by a canal into the larger Abu Dibbis depression creating the Razzaza Lake. On the Tigris, the Samarra barrage was completed in 1954 to divert flood peaks into the Tharthar depression. While the earlier barrages aimed to regulate flood flows and divert water for irrigation purposes ( e.g. Al Hindiyah, 1918, Al Kut 1939), those of Ramadi and Samarra created water reservoirs and effectively functioned as dams.
Until the mid-twentieth century, most efforts to control the waters were primarily concerned with irrigation. Some attention was given to problems of flood control and drainage before the revolution of July 14, 1958, but development plans in the 1960s and 1970s were increasingly devoted to these matters, as well as to irrigation projects on the upper reaches of the Tigris and Euphrates and the tributaries of the Tigris in the northeast. During the war, government officials stressed to foreign visitors that, with the conclusion of a peace settlement, problems of irrigation and flooding would receive top priority from the government. Since Syria built a dam on the Euphrates, the flow of water has been considerably diminished and flooding was no longer a problem in the mid-1980s. In 1988 Turkey was also constructing a dam on the Euphrates that would further restrict the water flow.
A decisive restructuring in the basin's development occurred in 1977, when Turkey merged all its water management schemes on the Tigris and Euphrates into the Southeast Anatolia Development project (commonly referred to by its Turkish acronym GAP) and which it re-launched in 1989 as an integrated regional development program. Composed of 13 individual but related schemes, the GAP aims to bring into operation 22 dams and 19 hydropower plants in the upper Tigris and Euphrates, as well as provide an irrigation network serving 1.7 million hectares of land. Set to double the country's energy production, this massive engineering programme is costing $32 billion. In the mid-1990s, Iran embarked on a multibillion water management scheme on the Karun river. Originally modelled on the famous river engineering programme of the Tennessee Valley Authority, the Karun River Development Project comprises a series of dams and related irrigation and hydroelectric power schemes in the southwestern province of Khuzestan. An estimated 150,000 hectares of formerly marginal land is to be placed under irrigated agriculture.
All in all, the gross storage capacity of all existing hydraulic works on the Euphrates is 143.19 BCM or five times the river average annual flow. Although on the Tigris water retention capacity is presently less than on the Euphrates, it is nonetheless considerable. Iraq currently exercises the greatest control on Tigris waters. The massive Tharthar diversion reservoir accounts for 69% of the country's 105.95 BCM gross storage capacity, which is double the average annual flow of the Tigris.
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