In the broad lower course of the Euphrates, the stream rarely
acquires a velocity of more than three miles an hour, while the
lower Tigris attains double that rate in times of flood. The
water of both great rivers is mainly derived from the northern
and eastern highlands in Armenia and in Kurdistan, and stands at
its lowest level in early autumn and in January. But when the
snows accumulated in the upper basins of the great rivers,
during the winter, melt under the hot sunshine of spring, they
rapidly rise,<1> and at length overflow their banks, covering
the alluvial plain with a vast inland sea, interrupted only by
the higher ridges and hummocks which form islands in a seemingly
boundless expanse of water.
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.
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