The Euphrates (English pronunciation: /juːˈfreɪtiːz/, Arabic: الفرات: al-Furāt, Turkish: Fırat) is the longest and one of the most historically important rivers of Southwest Asia. Together with the Tigris, it is one of the two defining rivers of Mesopotamia. Originating in eastern Turkey, the Euphrates flows through Syria and Iraq to join the Tigris in the Shatt al-Arab, which empties into the Persian Gulf.
The earliest references to the Euphrates come from cuneiform texts found in Shuruppak and pre-Sargonic Nippur and date to the mid-third millennium BCE. In these texts, written in Sumerian, the Euphrates appears as Buranuna (logographic: UD.KIB.NUN). The name could also be written KIB.NUN.(NA) or dKIB.NUN, with the prefix "d" indicating that the river was deified. In Sumerian, the name of the city of Sippar in modern-day Iraq was also written UD.KIB.NUN, indicating a historically strong relationship between the city and the river. In Akkadian, the Euphrates was called Purattu. The modern spelling of the Euphrates derives from the Old Persian Ufrātu via Middle Persian Frat into Turkish Fırat. The Persian Ufrātu (meaning the good) is also the source of the Greek spelling Εὐφράτες (Euphrates).
The Euphrates emerges from the confluence of the Kara Su or Western Euphrates () and the Murat Su or Eastern Euphrates () upstream from the town of Keban in Turkey. Daoudy and Frenken put the length of the Euphrates from the source of the Murat River to the confluence with the Tigris at , of which falls in Turkey, in Syria and in Iraq. The same figures are given by Isaev and Mikhailova. The length of the Shatt al-Arab, which connects the Euphrates and the Tigris with the Persian Gulf, is given by various sources as .
Both the Kara Su and the Murat Su rise northwest from Lake Van at elevations of and amsl, respectively. At the location of the Keban Dam, the two rivers, now combined into the Euphrates, have dropped to an elevation of amsl. From Keban to the Syro–Turkish border, the river drops another over a distance of less than . Once the Euphrates enters the Upper Mesopotamian plains, its grade drops significantly; within Syria the river falls while over the last stretch between Hīt and the Shatt al-Arab the river drops only .
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 at Keban, at Hīt and at Hindiya. However, these averages mask the high inter-annual variability in discharge; at Birecik, just above the Syro–Turkish border, annual discharges have been measured that ranged from a low volume of in 1961 to a high 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 per second ( per year). The seasonal variability has equally changed. The pre-1990 peak volume recorded at Hīt was per second, while after 1990 it is only per second. The minimum volume at Hīt remained relatively unchanged, rising from per second before 1990 to per second afterward.