The country is intersected by many rivers, the largest of which are the Euphrates and the Tigris; next to the Indian rivers, the rivers in the southern parts of Asia are said to hold the second place. The Tigris is navigable upward from its mouth to Opis and to the present Seleucia. Opis is a village and a mart for the surrounding places. The Euphrates also is navigable up to Babylon, a distance of more than three thousand stadia. The Persians, through fear of incursions from without and for the purpose of preventing vessels from ascending these rivers, constructed artificial cataracts. Alexander, on arriving there, destroyed as many of them as he could, those particularly (on the Tigris from the sea) to Opis. But he bestowed great care upon the canals, for the Euphrates, at the commencement of summer, overflows. It begins to fill in the spring, when the snow in Armenia melts; the ploughed land, therefore, would be covered with water and be submerged, unless the overflow of the superabundant water of the Nile is diverted. Hence the origin of canals. Great labour is requisite for their maintenance, for the soil is deep, soft, and yielding, so that it would easily be swept away by the stream; the fields would be laid bare, the canals filled, and the accumulation of mud would soon obstruct their mouths. Then again, the excess of water discharging itself into the plains near the sea forms lakes and marshes and reed grounds, supplying the reeds with which all kinds of platted vessels are woven; some of these vessels are capable of holding water when covered over with asphaltus; others are used with the material in its natural state. Sails are also made of reeds; these resemble mats or hurdles.
It is not, perhaps, possible to prevent inundations of this kind altogether, but it is the duty of good princes to afford all possible assistance. The assistance required is to prevent excessive overflow by the construction of dams, and to obviate the filling of rivers produced by the accumulation of mud, by cleansing the canals and removing stoppages at their mouths. The cleansing of the canals is easily performed, but the construction of dams requires the labour of numerous workmen. For the earth being soft and yielding does not support the superincumbent mass, which sinks, and is itself carried away, and thus a difficulty arises in making dams at the mouth. Expedition is necessary in closing the canals to prevent all the water flowing out. When the canals dry up in the summer-time they cause the river to dry up also; and if the river is low (before the canals are closed) it cannot supply the canals in time with water, of which the country, burnt up and scorched, requires a very large quantity, for there is no difference, whether the crops are flooded by an excess or perish by drought and a failure of water. The navigation up the rivers (a source of many advantages) is continually obstructed by both the above-mentioned causes, and it is not possible to remedy this unless the mouths of the canals were quickly opened and quickly closed, and the canals were made to contain and preserve a mean between excess and deficiency of water.
Aristobulus relates that Alexander himself, when he was sailing up the river and directing the course of the boat, inspected the canals, and ordered them to be cleared by his multitude of followers; he likewise stopped up some of the mouths, and opened others. He observed that one of these canals, which took a direction more immediately to the marshes and to the lakes in front of Arabia, had a mouth very difficult to be dealt with, and which could not be easily closed on account of the soft and yielding nature of the soil; he (therefore) opened a new mouth at the distance of thirty stadia, selecting a place with a rocky bottom, and to this the current was diverted. But in doing this he was taking precautions that Arabia should not become entirely inaccessible in consequence of the lakes and marshes, as it was already almost an island from the quantity of water (which surrounded it). For he contemplated making himself master of this country, and he had already provided a fleet and places of rendezvous, and had built vessels in Phœnicia and at Cyprus, some of which were in separate pieces, others were in parts, fastened together by bolts. These, after being conveyed to Thapsacus in seven distances of a day’s march, were then to be transported down the river to Babylon. He constructed other boats in Babylonia, from cypress trees in the groves and parks, for there is a scarcity of timber in Babylonia. Among the Cossæi [Kossæans] and some other tribes the supply of timber is not great.
The pretext for the war, says Aristobulus, was that the Arabians were the only people who did not send their ambassadors to Alexander; but the true reason was his ambition to be lord of all.
When he was informed that they worshipped two deities only, Jupiter and Bacchus, who supply what is most requisite for the subsistence of mankind, he supposed that, after his conquests, they would worship him as a third, if he permitted them to enjoy their former national independence. Thus was Alexander employed in clearing the canals, and in examining minutely the sepulchres of the kings, most of which are situated among the lakes.
Eratosthenes, when he is speaking of the lakes near Arabia, says, that the water, when it cannot find an outlet, opens passages underground, and is conveyed through these as far as the Cœle-Syrians, it is also compressed and forced into the parts near Rhinocolura and Mount Casius, and there forms lakes and deep pits. But I know not whether this is probable. For the overflowings of the water of the Euphrates, which form the lakes and marshes near Arabia, are near the Persian Sea. But the isthmus which separates them is neither large nor rocky, so that it was more probable that the water forced its way in this direction into the sea, either under the ground, or across the surface, than that it traversed so dry and parched a soil for more than six thousand stadia: particularly, when we observe, situated midway in this course, Libanus, Antilibanus, and Mount Casius.
Such, then, are the accounts of Eratosthenes and Aristobulus.
But Polycleitus says, that the Euphrates does not overflow its banks, because its course is through large plains; that of the mountains (from which it is supplied) some are distant two thousand, and the Kossæan Mountains scarcely one thousand stadia, that they are not very high, nor covered with snow to a great depth, and therefore do not occasion the snow to melt in great masses, for the most elevated mountains are in the northern parts above Ecbatana; towards the south they are divided, spread out, and are much lower; the Tigris also receives the greater part of the water (which comes down from them) and thus overflows its banks.
The last assertion is evidently absurd, because the Tigris descends into the same plains (as the Euphrates); and the above-mentioned mountains are not of the same height, the northern being more elevated, the southern extending in breadth, but are of a lower altitude. The quantity of snow is not, however, to be estimated by altitude only, but by aspect. The same mountain has more snow on the northern than on the southern side, and the snow continues longer on the former than on the latter. As the Tigris therefore receives from the most southern parts of Armenia, which are near Babylon, the water of the melted snow, of which there is no great quantity, since it comes from the southern side, it should overflow in a less degree than the Euphrates, which receives the water from both parts (northern and southern), and not from a single mountain only, but from many, as I have mentioned in the description of Armenia. To this we must add the length of the river, the large tract of country which it traverses in the Greater and in the Lesser Armenia, the large space it takes in its course in passing out of the Lesser Armenia and Cappadocia, after issuing out of the Taurus in its way to Thapsacus (forming the boundary between Syria below and Mesopotamia), and the large remaining portion of country as far as Babylon and to its mouth, a course in all of thirty-six thousand stadia.