In the occurrence of these annual inundations lies one of
several resemblances between the valley of the Euphrates and
that of the Nile. But there are important differences. The time
of the annual flood is reversed, the Nile being highest in
autumn and winter, and lowest in spring and early summer. The
periodical overflows of the Nile, regulated by the great lake
basins in the south, are usually punctual in arrival, gradual in
growth, and beneficial in operation. No lakes are interposed
between the mountain torrents of the upper basis of the Tigris
and the Euphrates and their lower courses. Hence, heavy rain, or
an unusually rapid thaw in the uplands, gives rise to the sudden
irruption of a vast volume of water which not even the rapid
Tigris, still less its more sluggish companion, can carry off in
time to prevent violent and dangerous overflows. Without an
elaborate system of canalisation, providing an escape for such
sudden excesses of the supply of water, the annual floods of the
Euphrates, and especially of the Tigris, must always be attended
with risk, and often prove harmful.
There are other peculiarities of the Euphrates valley which may
occasionally tend to exacerbate the evils attendant on the
inundations. It is very subject to seismic disturbances; and the
ordinary consequences of a sharp earthquake shock might be
seriously complicated by its effect on a broad sheet of water.
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