They therefore begged Jovian to allow them to swim the stream.

It was in vain that he and his officers opposed the project; mutinous cries arose; and, to avoid worse evils, he was compelled to consent that five hundred Gauls and Sarmatians, known to be expert swimmers, should make the attempt.

It succeeded beyond his hopes.
The corps crossed at night, surprised the Persians who held the opposite bank, and established themselves in a safe position before the dawn of day.

By this bold exploit the passage of the other troops, many of whom could not swim, was rendered feasible, and Jovian proceeded to collect timber, brushwood, and skins for the formation of large rafts on which he might transport the rest of his army.
These movements were seen with no small disquietude by the Persian king.
The army which he had regarded as almost a certain prey seemed about to escape him.

He knew that his troops could not pass the Tigris by swimming; he had, it is probable, brought with him no boats, and the country about Dura could not supply many; to follow the Romans, if they crossed the stream, he must construct a bridge, and the construction of a bridge was, to such unskilful engineers as the Persians, a work of time.