The Roman History · Amm. 17.13.4

For the Parthiscus waters this land, proceeding with oblique windings till it falls into the Danube. But while it flows unmixed, it passes through a vast extent of country, which, near its junction with the Danube, it narrows into a very small corner, so that over on the side of the Danube those who live in that district are protected from the attack of the Romans, and on the side of the Parthiscus they are secured from any irruptions of the barbarians. Since along its course the greater part of the ground is frequently under water from the floods, and always swampy and full of osiers, so as to be quite impassable to strangers; and besides the mainland there is an island close to the mouth of the river, which the stream itself seems to have separated into its present state.

Ammianus Marcellinus · translated by C. D. Yonge · 1911 reprint
C. D. Yonge translation (preface dated February 1862); G. Bell and Sons, 1911; Project Gutenberg #28587
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