Alexander now ordered Hephaestion to lead the main body of the infantry as far as the Persian Sea, while he himself, his fleet having sailed up into the land of Susiana, embarked with the shield-bearing guards and the body-guard of infantry; and having also put on board a few of the cavalry Companions, he sailed down the river Eulaeus to the sea.[855] When he was near the place where the river discharges itself into the deep, he left there most of his ships, including those which were in need of repair, and with those especially adapted for fast sailing he coasted along out of the river Eulaeus through the sea to the mouth of the Tigres. The rest of the ships were conveyed down the Eulaeus as far as the canal which has been cut from the Tigres into the Eulaeus, and by this means they were brought into the Tigres. Of the rivers Euphrates and Tigres which enclose Syria between them, whence also its name is called by the natives Mesopotamia,[856] the Tigres flows in a much lower channel than the Euphrates, from which it receives many canals; and after taking up many tributaries and its waters being swelled by them, it falls into the Persian Sea.[857] It is a large river and can be crossed on foot nowhere as far as its mouth,[858] inasmuch as none of its water is used up by irrigation of the country. For the land through which it flows is more elevated than its water, and it is not drawn off into canals or into another river, but rather receives them into itself. It is nowhere possible to irrigate the land from it. But the Euphrates flows in an elevated channel, and is everywhere on a level with the land through which it passes. Many canals have been made from it, some of which are always kept flowing, and from which the inhabitants on both banks supply themselves with water; others the people make only when requisite to irrigate the land, when they are in need of water from drought.[859] For this country is usually free from rain. The consequence is, that the Euphrates at last has only a small volume of water, which disappears into a marsh. Alexander sailed over the sea round the shore of the Persian Gulf lying between the rivers Eulaeus and Tigres; and thence he sailed up the latter river as far as the camp where Hephaestion had settled with all his forces. Thence he sailed again to Opis, a city situated on that river.[860] In his voyage up he destroyed the weirs which existed in the river, and thus made the stream quite level. These weirs had been constructed by the Persians, to prevent any enemy having a superior naval force from sailing up from the sea into their country. The Persians had had recourse to these contrivances because they were not a nautical people; and thus by making an unbroken succession of weirs they had rendered the voyage up the Tigres a matter of impossibility. But Alexander said that such devices were unbecoming to men who are victorious in battle; and therefore he considered this means of safety