Over these it spreads itself, forming the well-known "Chaldaean marshes," which absorb the chief proportion of the water that flows into them, and in which the "great river" seems at various times to have wholly, or almost wholly, lost itself.

No such misfortune can befall the Tigris, which runs in a deep bed, and seldom varies its channel, offering a strong contrast to the sister stream.
Frequent allusion has been made, in the course of this description of the Tigris and Euphrates, to the fact of their having each a flood season.

Herodotus is scarcely correct when he says that in Babylonia "the river does not, as in Egypt, overflow the corn-lands of its own accord, but is spread over them by the help of engines." Both the Tigris and Euphrates rise many feet each spring, and overflow their banks in various places.

The rise is caused by the melting of the snows in the mountain regions from which the two rivers and their affluents spring.

As the Tigris drains the southern, and the Euphrates the northern side of the same mountain range, the flood of the former stream is earlier and briefer than that of the latter.