Since 8000 b. c.e. the Near Eastern environment overall has been much as it is today, but there have been many changes in the detail, owing both to natural factors and to human activity, sometimes affecting individual settlements, at other times impacting entire regions.
Changes in the Course of the Rivers
As rising postglacial sea levels progressively flooded the Gulf, the changing location of the coast affected the gradient of the rivers, causing the upper reaches to become more deeply incised and therefore generally stable in their location, but slowing their flow across the plains so that an increased volume of alluvium was deposited there. Just north of Sippar, the plain broadens out and its gradient declines, causing the rivers to become far less stable. The Euphrates in particular has split into a number of major and minor branches, and in years of heavy flooding these can change their course, with devastating effects for their dependent settlements. Before 3000 b. c.e., the Euphrates flowed in three channels, passing through Kutha, Kish, and Jemdet Nasr; by the third millennium, the Kish branch had become the principal one. Around the end of ED
In early antiquity the Tigris and Euphrates Rivers flowed separately into the Gulf, considerably north of its present shoreline. The rivers, and particularly the Euphrates, have changed their course many times. The copious volumes of silt they deposited created a delta within which the two rivers eventually met. This nineteenth-century engraving shows their confluence in recent times. (Ridpath, John Clark, Ridpath's History of the World, 1901)
(Early Dynastic period) I (ca. 2750 b. c.e.), the main flow of the Euphrates farther south moved eastward to run down a previously minor branch, reducing water in its former main channel. Uruk, on the latter, though still a major city, declined from its earlier preeminence, while cities on the eastern branch, including Umma, grew in importance. During the later third millennium, a minor branch developed though Babylon, growing to become the principal branch by the late second millennium b. c.e. Today the principal bed of the Euphrates lies farther west.
The Euphrates is not alone in changing its course. Below Kut, the Tigris is also unstable and is known to have shifted at least three times. In antiquity, it flowed directly into the Gulf, having followed a much more direct course than that of today. The Diyala anciently joined the Tigris considerably south and east of their present confluence near Baghdad. And until around a millennium ago, the Karun and Karkheh in Susiana formed a joint estuary.
Evidence from pollen cores and oxygen isotope profiles from mountain lakes gives some indication of variations in vegetation, rainfall, and temperature, although many uncertainties remain. Temperatures exceeded those of today by 1-2 degrees C around 5000-2000 b. c.e. This, combined with higher rainfall, allowed trees to grow at higher altitudes and in regions of steppe that today have sparse vegetation. A period of reduced rainfall in the northern mountains from 3800 b. c.e. may have caused the marshes of southern Babylonia to become progressively drier. Further evidence suggests that the years around 2900 b. c.e. saw particularly severe inundations, giving rise to the story of the Flood (see chapter 11). During the third millennium b. c.e. river levels may have fallen, particularly around 2350-2000 b. c.e., necessitating increasing investment in irrigation works. Later episodes of fluctuating river levels included peak volumes of water in both the Tigris and the Euphrates around 1350-1250 B. C.E., followed by a reduction that lasted until around 950 b. c.e.
The Consequences of Human Activities
Many changes in the Mesopotamian environment were the result of human activity. The creation of canals and dams could significantly alter drainage patterns, often unwittingly, although at times the interference was malicious. Upstream communities sometimes channeled water to serve their own needs at the expense of communities farther downstream, a catalyst to conflict. Several kings diverted the Euphrates to starve hostile cities of essential water supplies, or to destroy them by flooding, often with devastating long-term effects.
Even more damaging was the impact of millennia of exploitation of upland forests. Trees were cleared to create arable land or to provide timber for building and wood and brush for fuel. Grazing also took its toll, preventing the regeneration of trees and scrub. The lowlands similarly suffered degradation and destruction: Originally the northern plains were covered by savannah with small trees, while the steppe was densely vegetated. The destruction of plant cover caused further changes, promoting desertification of the steppe and producing erosion in the mountains, which increased runoff, swelling the rivers and causing aggravated flooding downstream. However, the effects of these activities were probably not significant before the later first millennium b. c.e.
Prolonged irrigation had another significantly detrimental effect—salinization. Small amounts of salts carried down by the rivers from the sedimentary rocks of the north have over the millennia accumulated in the groundwater of southern Mesopotamia. The deposition of salts is greater in periods, like that between 2350 and 2000 b. c.e., when the volume of water in the rivers is reduced. Intensive irrigation raises the water table, bringing this saline water close to the surface, where it is drawn up by capillary action, causing salts to accumulate in the subsoil and on the surface where water spread for irrigation also contains salts. When moisture is removed by evapotranspiration, the salts are deposited on the land, progressively reducing fertility. Many scholars believe this can be seen during the earlier second millennium b. c.e. when land once suitable for wheat was sown with the more salt-tolerant barley (although scholars opposed to this theory argue that barley was selected for its higher yields rather than its salt tolerance). Eventually the land became too saline for cultivation. Many southern cities were abandoned, and their inhabitants moved north or turned to pastoral nomadism.
World History
