The Sumerian World (Routledge Worlds)

Tony J. Wilkinson

irrigation. They include floodbasins where water tables are higher and salinisation

can be a chronic problem.

• The delta plain o f the Tigris and Euphrates where the rivers are prone to branch into
  a network o f channels. Here drainage is slow, water tables are high, many soils have
  a tendency to be waterlogged, and flood basins are often covered by water or marshy.


• The marsh region where the rivers are virtually lost in reed-fringed extensive shallow
  marshes and where groundwater is always high.


• The estuarine zone - here the tidal effect o f the G u lf is felt so that rises and falls o f
  water levels contribute to both irrigation and drainage.

The nature o f the river systems

The most obvious, but profoundly deceptive, feature o f the lower Mesopotamian
plains is that they appear almost flat. W hat this means is that even minor differences
o f elevation can make a significant difference to the flow o f water. Therefore the
gradient, which can be as low as 5—10 cm per km (see Table 2.1), is sufficient for rivers
to flow and to discharge that flow into the Gulf. Because the channels in the lower
plain have a tendency towards straightness rather than being meandering, it is difficult
to distinguish between natural and artificial channels (Adams 1981:19; Wilkinson 2003:
82-85) — an ambiguity reflected in the cuneiform texts that do not distinguish between
natural rivers and artificial, dug canals. Two features o f the Tigris and Euphrates are
particularly significant for the development o f the hydraulic landscape: first, the rivers
tend to branch thereby forming what are termed anastomosing rivers; and second, they
flow on slightly raised levees up to a few metres high that result from the preferential
deposition o f sands and silts closer to the trunk channels, and finer clays further away
(Figure 2.1). Such low sinuosity anastomosing rivers are ideal for navigation by boats
and therefore for the transportation o f bulk products (Algaze 2008: 50-63). Moreover,
the location o f channels on levee crests enabled early irrigators to avoid one o f the
fundamental constraints o f the plain, namely, its low gradient. In other words, because
the gradient down the levee slope away from the river is much greater than the
longitudinal gradient along the river, it is easier to lead canals down the levee rather
than dig much longer canals that follow the overall gradient o f the plain (Adams 1981:
8; see below).

Levee

Crest

Figure 2.1 Section through levees showing the deposits o f canals and ancient channels

(re-drawn from Diyala report)

2.0m

1.0

0

• 1.0

2.0 m

1.0

0

1.0

Fine sand + silt of

relict channel Silt

Silt + clay of

flood basins

Flood basin with

Gilqai