The river Euphrates is the first of the two great streams of the Middle East which gave rise to and determined the historical course of the Land between the Rivers, Mesopotamia. I intend to give only a brief survey of the river, its nature, and its effect on civilization there.
Let's start, following Webster with the name and etymology. The earliest names, the "Sippar River" or simply "the River" in the oldest sources, soon gave way to the names "Burananu" and "Purattum". With a prothetic vowel of uncertain origin, found in Arabic and Syriac, the name was transformed by the Greeks to the form "Euphrates", by which it is known today in most western languages.
The Euphrates proper is formed through a confluence of the rivers Kara Su and Marat Su in south central Anatolia, fed by mountain rivulets. Cutting its way sharply through valleys and gorges it finally reaches the high plain of Syria at Samsat, continuing more than 1,400 kilometers to the borders of the Mesopotamian alluvium at the city of Hit. At this point it is joined by its last great tributary, the Khabur. Here the rate of flow is reduced sharply, with an elevation of about 50 m above sea level. The technical term for the pattern through the flood plain is "anastomosing", that is, the river naturally forms a succession of diversions and channels, separating and rejoining at regular intervals. Just south of the modern town of Nasiriya, the Euphrates flows into the marshy lake Haur al-Hammar, then joins the Tigris to form the Shatt al-Arab and empties into the Persian Gulf near Basra.
But to come to the point: the Euphrates by no means numbers among the great rivers of the world, with a rate of flow 1/3 that of the Rhine or Nile and a length of just over 2,100 kilometers from its origin proper to the Shatt al-Arab. Its importance for the origins of civilization, however, cannot be overestimated. Though I am by no means competent to explain the exact hydrology or geography of the Euphrates, I would like to emphasize two points, based on the above, which determine how the river interacted with the peoples along its banks.
Firstly, the rate of flow strongly determines the limits of its use. Rainfall, though regular, is by no means sufficient to allow rain-fed agriculture, and the farmers depend wholly on the water provided by the streams and channels to irrigate their crops. The river reaches high water in late April or early May, when the rainy season and melting snows of the Turkish highlands swell the sources, and the river at Hit reaches a flow of 2,100 cumecs (cubic meters/second). But the chief growing season is in the winter, from October to April, meaning the river reaches its highest point when water is no longer needed, or, otherwise, has a flow of only 350 cumecs in October during the sowing season. Surveys funded by the United Nations in the 1950's estimate that the amount of water then available allows for the cultivation of only 6,058 square kilometers annually, seriously limiting production. A postponing of the sowing season, taking advantage of greater availability of water, would only expose the crops to devastating diseases, devouring insects, and withering heat in the summer during harvest.
Secondly, the nature of the river (the anastomosing pattern, rate of flow, meandering course, and situation in a semiarid and arid environment) leads to regular and hardly predictable shifts in its course. Sediments are carried naturally by every river, either as moving bed loads or in suspension, and deposited on its banks at a rate depending on varying factors. A greater rate of flow in a river means that the stream cuts a relatively constant bed. But the deposition of sediment maintains an elevated bed with levees formed when the river overtops the banks. For the farmer, this means that coarse sediment is deposited close to the stream, fine silt further away, providing a natural framework for flood irrigation requiring little technology to maintain. When the river tops its banks, however, new diversions may form while the velocity of the river is checked. Sediments accumulate to fill the gap, and, if the new diversion is more efficient, may lead to a permanent lateral shift. This, part of the complex geometry of meandering (of which, admittedly, I understand very little), leads to regular, both gradual and sudden shifts in the watercourses of the Euphrates, thus also of shifts in the settlement patterns along its banks.
Both points depend on the unchanging nature of the river itself, and therefore build two of the many obstacles every inhabitant of Iraq has had to face through the millenia. These are no more than general statements about the Euphrates in particular. I hope that at some point, a good-hearted noder, better informed than I, would develop the general hydrology and geography of rivers in another node. Other statements, on the particular confluence of social and political history and geography of Mespotamia, belongs elsewhere.
Chief Source:
Robert McC. Adams. Heartland of Cities: Survey of Ancient Settlement and Land Use on the Central Floodplain of the Euphrates. (Chicago 1981).