Nar Sharri or "King's Canal",
the Nahar Malkha of the Greeks and the Nahr el-Malik of the
Arabs, protected the right bank of the Tigris by its own
high artificial banks, which can still be traced for
hundreds of miles; but it took its supply from the Euphrates
at Sippar, where the ground is some 25 ft. higher than on
the Tigris. The Tigris usually flooded its left bank; it was
the right bank which was protected, and a breach here meant
disaster. Cf. Willcocks, op. cit., and _The Near East_,
Sept. 29, 1916 (Vol. XI, No. 282), p. 522.

It was only by constant and unremitting attention that disaster from
flood could be averted; and the difficulties of the problem were and are
increased by the fact that the flood-water of the Mesopotamian rivers
contains five times as much sediment as the Nile. In fact, one of
the most pressing of the problems the Sumerian and early Babylonian
engineers had to solve was the keeping of the canals free from silt.(1)
What the floods, if left unchecked, may do in Mesopotamia, is well
illustrated by the decay of the ancient canal-system, which has been
the immediate cause of the country's present state of sordid desolation.
That the decay was gradual was not the fault of the rivers, but was
due to the sound principles on which the old system of control had been
evolved through many centuries of labour. At the time of the Moslem
conquest the system had already begun to fail. In the fifth century
there had been bad floods; but worse came in A.D. 629, when both rivers
burst their banks and played havoc with the dikes and embankments. It
is related that the Sassanian king Parwiz, the contemporary of Mohammed,
crucified in one day forty canal-workers at a certain breach, and yet
was unable to master the flood.(2) All repairs were suspended during the
anarchy of the Moslem invasion. As a consequence the Tigris left its
old bed for the Shatt el-Hai at Kut, and pouring its own and its
tributaries' waters into the Euphrates formed the Great Euphrates Swamp,
two hundred miles long and fifty broad. But even then what was left of
the old system was sufficient to support the splendour of the Eastern
Caliphate.

(1) Cf. _Letters of Hammurabi_, Vol. III, pp. xxxvi ff.; it
was the duty of every village or town upon the banks of the
main canals in Babylonia to keep its own section clear of
silt, and of course it was also responsible for its own