The geography of Iraq is diverse and falls into five main regions: the desert (west of the Euphrates), Upper Mesopotamia (between the upper Tigris and Euphrates rivers), the northern highlands of Iraq, Lower Mesopotamia, and the alluvial plain extending from around Tikrit to the Persian Gulf.
The mountains in the northeast are an extension of the alpine system that runs eastward from the Balkans through southern Turkey, northern Iraq, Iran, and Afghanistan, eventually reaching the Himalayas in Pakistan. The desert lies in the southwest provinces along the borders with Saudi Arabia and Jordan and geographically belongs in the Arabian Peninsula.
Major geographical features Most geographers, including those of the Iraqi government, discuss the country's geography in terms of four main zones or regions: the desert in the west and southwest; the rolling upland between the upper Tigris and Euphrates rivers (in Arabic the Dijla and Furat, respectively); the highlands in the north and northeast; and the alluvial plain through which the Tigris and Euphrates flow. Iraq's official statistical reports give the total land area as , whereas a United States Department of State publication gives the area as .
Upper Mesopotamia
The uplands region, between the Tigris north of Hamrin Mountains and the Euphrates north of Hit, is known as Al Jazira (the island) and is part of a larger area that extends westward into Syria between the two rivers and into Turkey. Water in the area flows in deeply cut valleys, and irrigation is much more difficult than it is in the lower plain. The southwest areas of this zone are classified as desert or semi-desert. The northern parts, which include such places like the Nineveh Plains, Duhok, Zakho and Amedi, mainly consist of Mediterranean vegetation. The vegetation cyclically dries out and appear brown in the virtually arid summer and flourish in the wet winter.
Lower Mesopotamia
An alluvial plain begins north of Baghdad and extends to the Persian Gulf. Here the Tigris and Euphrates rivers lie above the level of the plain in many places, and the whole area is a river delta interlaced by the channels of the two rivers and by irrigation canals. Intermittent lakes, fed by the rivers in flood, also characterize southeastern Iraq. A fairly large area () just above the confluence of the two rivers at Al Qurnah and extending east of the Tigris beyond the Iranian border is marshland, known as Hawr al Hammar, the result of centuries of flooding and inadequate drainage. Much of it is permanent marsh, but some parts dry out in early winter, and other parts become marshland only in years of great flood.
Because the waters of the Tigris and Euphrates above their confluence are heavily silt- laden, irrigation and fairly frequent flooding deposit large quantities of silty loam in much of the delta area. Windborne silt contributes to the total deposit of sediments. It has been estimated that the delta plains are built up at the rate of nearly twenty centimeters in a century. In some areas, major floods lead to the deposit in temporary lakes of as much as thirty centimeters of mud.
The Tigris and Euphrates also carry large quantities of salts. These, too, are spread on the land by sometimes excessive irrigation and flooding. A high water table and poor surface and subsurface drainage tend to concentrate the salts near the surface of the soil. In general, the salinity of the soil increases from Baghdad south to the Persian Gulf and severely limits productivity in the region south of Al Amarah. The salinity is reflected in the large lake in central Iraq, southwest of Baghdad, known as Bahr al Milh (Sea of Salt). There are two other major lakes in the country to the north of Bahr al Milh: Buhayrat ath Tharthar and Buhayrat al Habbaniyah.
Baghdad area
Between Upper and Lower Mesopotamia is the urban area surrounding Baghdad. These "Baghdad Belts" can be described as the provinces adjacent to the Iraqi capital and can be divided into four quadrants: northeast, southeast, southwest, and northwest. Beginning in the north, the belts include the province of Saladin, clockwise to Baghdad province, Diyala in the northeast, Babil and Wasit in the southeast and around to Al Anbar in the west.
Highlands
The northeastern highlands begin just south of a line drawn from Mosul to Kirkuk and extend to the borders with Turkey and Iran. High ground, separated by broad, undulating steppes, gives way to mountains ranging from near the Iranian and Turkish borders. Except for a few valleys, the mountain area proper is suitable only for grazing in the foothills and steppes; adequate soil and rainfall, however, make cultivation possible. Here, too, are the great oil fields near Mosul and Kirkuk. The northeast is the homeland of most Iraqi Kurds.
Desert
The desert zone, an area lying west and southwest of the Euphrates River, is a part of the Syrian Desert and Arabian Desert, which covers sections of Syria, Jordan, and Saudi Arabia and most of the Arabian Peninsula. The region, sparsely inhabited by pastoral Bedouins, consists of a wide stony plain interspersed with rare sandy stretches. A widely ramified pattern of wadis–watercourses that are dry most of the year–runs from the border to the Euphrates. Some wadis are over long and carry brief but torrential floods during the winter rains.
Western and southern Iraq is a vast desert region covering some , almost two-fifths of the country. The western desert, an extension of the Syrian Desert, rises to elevations above . The southern desert is known as Al-Hajarah in the western part and as Al-Dibdibah in the east. Both deserts are part of the Arabian Desert. Al Hajarah has a complex topography of rocky desert, wadis, ridges, and depressions. Al-Dibdibah is a more sandy region with a covering of scrub vegetation. Elevation in the southern desert averages between . A height of is reached at Mount 'Unayzah at the intersection of the borders of Jordan, Iraq and Saudi Arabia. The deep Wadi Al-Batin runs in a northeast–southwest direction through Al-Dibdibah. It has been recognized since 1913 as the boundary between western Kuwait and Iraq.
Tigris–Euphrates river system
The Euphrates originates in Turkey, is augmented by the Balikh and Khabur rivers 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 16 kilometers in width, until it flows out on the plain at Ar Ramadi. Beyond there the Euphrates continues to the Hindiya 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. The Tigris 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 Adhaim, 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-Hayy, which was once the main channel of the Tigris. Water from the Tigris thus enters the Euphrates through the Shatt al-Hayy well above the confluence of the two main channels at Al Qurnah.
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 has been superseded by a greater obstacle to river traffic, however, namely the presence of several sunken hulls that have been rusting in the Shatt al Arab since early in the Iran-Iraq war.
The waters of the Tigris and Euphrates are essential to the life of the country, but they sometimes 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. 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.
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.
Iraqi coastal waters boast a living coral reef, covering an area of 28 km2 in the Persian Gulf, at the mouth of the Shatt al-Arab river (). The coral reef was discovered by joint Iraqi–German expeditions of scientific scuba divers carried out in September 2012 and in May 2013. Prior to its discovery, it was believed that Iraq lacks coral reefs as the local turbid waters prevented the detection of the potential presence of local coral reefs. Iraqi corals were found to be adapted to one of the most extreme coral-bearing environments in the world, as the seawater temperature in this area ranges between 14 and 34 °C. The reef harbors several living stone corals, octocorals, ophiuroids and bivalves. There are also silica-containing demo-sponges.
Settlement patterns In the rural areas of the alluvial plain and in the lower Diyala region, settlement almost invariably clusters near the rivers, streams, and irrigation canals. The bases of the relationship between watercourse and settlement have been summarized by Robert McCormick Adams, director of the Oriental Institute of the University of Chicago. He notes that the levees laid down by streams and canals provide advantages for both settlement and agriculture. Surface water drains more easily on the levees' back-slope, and the coarse soils of the levees are easier to cultivate and permit better subsurface drainage. The height of the levees gives some protection against floods and the frost that often affect low-lying areas and may kill and/or damage winter crops. Above all, those living or cultivating on the crest of a levee have easy access to water for irrigation and household use in a dry, hot country.