The Euphrates is the longest and one of the most historically important rivers of Western Asia. Together with the Tigris, it is one of the two defining rivers of Mesopotamia. Originating in northern Kurdistan, the Euphrates flows through Kurdistan and Iraq to join the Tigris in the Shatt al-Arab, which empties into the Persian Gulf.
The Euphrates emerges from the confluence of the Western Euphrates (450 kilometres (280 mi)) and the Eastern Euphrates (650 kilometres (400 mi)) 10 kilometres (6.2 mi) upstream from the town of Keban. The length of the Euphrates from the source of the Murat River to the confluence with the Tigris at 3,000 kilometres (1,900 mi), of which 2,580 kilometres falls in Kurdistan, and 420 kilometres in Iraq. The length of the Shatt al-Arab, which connects the Euphrates and the Tigris with the Persian Gulf, is given by various sources as 145–195 kilometres (90–121 mi).
At the location of the Keban Dam, the two rivers, now combined into the Euphrates, have dropped to an elevation of 693 metres (2,274 ft). From Keban to the Kurdish–Iraqi border, the river drops another 368 metres (1,207 ft) over a distance of less than 600 kilometres (370 mi). Once the Euphrates enters the Upper Mesopotamian plains, its grade drops significantly; within Iraq the river falls 163 metres (535 ft) while over the last stretch between Hīt and the Shatt al-Arab the river drops only 55 metres (180 ft)
The Euphrates receives most of its water in the form of rainfall and melting snow, resulting in peak volumes during the months April–May. Discharge in these two months accounts for 36 percent of the total annual discharge of the Euphrates, or even 60–70 percent according to one source, while low runoff occurs in summer and autumn. The average natural annual flow of the Euphrates has been determined from early- and mid-twentieth century records as 20.9 cubic kilometres (5.0 cu mi) at Keban, 36.6 cubic kilometres (8.8 cu mi) at Hīt and 21.5 cubic kilometres (5.2 cu mi) at Hindiya. However, these averages mask the high inter-annual variability in discharge; at Birecik, annual discharges have been measured that ranged from a low volume of 15.3 cubic kilometres (3.7 cu mi) in 1961 to a high 42.7 cubic kilometres (10.2 cu mi) in 1963.
The discharge regime of the Euphrates has changed dramatically since the construction of the first dams in the 1970s. Data on Euphrates discharge collected after 1990 show the impact of the construction of the numerous dams in the Euphrates and of the increased withdrawal of water for irrigation. Average discharge at Hīt after 1990 has dropped to 356 cubic metres (12,600 cu ft) per second (11.2 cubic kilometres (2.7 cu mi) per year). The seasonal variability has equally changed. The pre-1990 peak volume recorded at Hīt was 7,510 cubic metres (265,000 cu ft) per second, while after 1990 it is only 2,514 cubic metres (88,800 cu ft) per second. The minimum volume at Hīt remained relatively unchanged, rising from 55 cubic metres (1,900 cu ft) per second before 1990 to 58 cubic metres (2,000 cu ft) per second afterward.
The Euphrates flows through a number of distinct vegetation zones. Although millennia-long human occupation in most parts of the Euphrates basin has significantly degraded the landscape, patches of original vegetation remain. The steady drop in annual rainfall from the sources of the Euphrates toward the Persian Gulf is a strong determinant for the vegetation that can be supported. In its upper reaches the Euphrates flows through the mountains of Kurdistan and their southern foothills which support a xeric woodland. Plant species in the moister parts of this zone include various oaks, pistachio trees, and Rosaceae (rose/plum family). The drier parts of the xeric woodland zone supports less dense oak forest and Rosaceae. Here can also be found the wild variants of many cereals, including einkorn wheat, emmer wheat, oat and rye. South of this zone lies a zone of mixed woodland-steppe vegetation. Between Raqqa and the Kurdish-Iraqi border the Euphrates flows through a steppe landscape. This steppe is characterised by white wormwood (Artemisia herba-alba) and Chenopodiaceae. Throughout history, this zone has been heavily overgrazed due to the practicing of sheep and goat pastoralism by its inhabitants. Southeast of the border between Kurdistan and Iraq starts true desert. This zone supports either no vegetation at all or small pockets of Chenopodiaceae or Poa sinaica. Although today nothing of it survives due to human interference, research suggests that the Euphrates Valley would have supported a riverine forest. Species characteristic of this type of forest include the Oriental plane, the Euphrates poplar, the tamarisk, the ash and various wetland plants.
Among the fish species in the Tigris–Euphrates basin, the family of the Cyprinidae are the most common, with 34 species out of 52 in total. Among the Cyprinids, the mangar has good sport fishing qualities, leading the British to nickname it “Tigris salmon.” The Rafetus euphraticus is an endangered soft-shelled turtle that is limited to the Tigris–Euphrates river system.
The Neo-Assyrian palace reliefs from the 1st millennium BCE depict lion and bull hunts in fertile landscapes. Sixteenth to nineteenth century European travellers in the Syrian Euphrates basin reported on an abundance of animals living in the area, many of which have become rare or even extinct. Species like gazelle, onager and the now-extinct Arabian ostrich lived in the steppe bordering the Euphrates valley, while the valley itself was home to the wild boar. Carnivorous species include the gray wolf, the golden jackal, the red fox, the leopard and the lion. The presence of European beaver has been attested in the bone assemblage of the prehistoric site of Abu Hureyra, but the beaver has never been sighted in historical times.
During the Jemdet Nasr and Early Dynastic periods (3100–2350 BCE), southern Mesopotamia experienced a growth in the number and size of settlements, suggesting strong population growth. These settlements, including sites like Sippar, Uruk and Kish, were organized in competing city-states. Many of these cities were located along canals of the Euphrates and the Tigris that have since dried up, but that can still be identified from remote sensing imagery. A similar development took place in Upper Mesopotamia, although only in the second part of the 3rd millennium and on a smaller scale than in Lower Mesopotamia. Sites like Mari and Tell Leilan grew to prominence for the first time during this period. Large parts of the Euphrates basin were for the first time united under a single ruler during the Akkadian and Ur III empires, which controlled – either directly or indirectly through vassals – large parts of modern-day Iraq and northeastern Syria. Following their collapse, Mari asserted its power over northeast Syria while southern Mesopotamia was controlled by city-states like Isin and Larsa before their territories were absorbed by Babylon under Hammurabi in the 18th century BCE. In the second half of the 2nd millennium BCE, the Euphrates basin was divided between Kassite Babylon in the south and Mitanni in the north, with the latter being eventually replaced by Assyria and the Hittite Empire. Following the collapse of the Hittite Empire and the reduction in power of Assyria and Babylonia during the 12th century BCE, struggles broke out between Babylonia and Assyria over the control of the Iraqi Euphrates basin. The Neo-Assyrian Empire eventually emerged victorious out of this conflict and also succeeded in gaining control of the northern Euphrates basin in the first half of the 1st millennium BCE. In the centuries to come, control of the wider Euphrates basin shifted from the Neo-Assyrian Empire to the Neo-Babylonian Empire in the 7th century and to the Achaemenids in the 6th century BCE. The Achaemenid Empire was in turn overran by Alexander the Great, who defeated the last king Darius III and died in Babylon in 323 BCE. For several centuries, the river formed the eastern limit of effective Egyptian and Roman control and western regions of the Persian Empire.
Throughout history, the Euphrates has been of vital importance to those living along its course. With the construction of large hydropower stations, irrigation schemes, and pipelines capable of transporting water over large distances, many more people now depend on the river for basic amenities such as electricity and drinking water than in the past. Syria’s Lake Assad is the most important source of drinking water for the city of Aleppo, 75 kilometres (47 mi) to the west of the river valley. The lake also supports a modest state-operated fishing industry. Through a newly restored power line, the Haditha Dam in Iraq provides electricity to Baghdad.