In spite of these points of uncertainty, it will be noted that every city of Sumer and Akkad, the site of which has been referred to, was situated on the Euphrates or one of its branches, not upon the Tigris, and the only exception to this rule appears to have been Opis, the most northern city of Akkad. The preference for the Euphrates may be explained by the fact that the Tigris is swift and its banks are high, and it thus offers far less facilities for irrigation. The Euphrates with its lower banks tends during the time of high water to spread itself over the surrounding country, which doubtless suggested to the earliest inhabitants the project of regulating and utilizing the supply of water by means of reservoirs and canals. Another reason for the preference may be traced to the slower fall of the water in the Euphrates during the summer months. With the melting of the snow in the mountain ranges of the Taurus and Niphates during the early spring, the first flood-water is carried down by the swift stream of the Tigris, which generally begins to rise in March, and, after reaching its highest level in the early part of May, falls swiftly and returns to its summer level by the middle of June. The Euphrates, on the other hand, rises about a fortnight later, and continues at a high level for a much longer period. Even in the middle of July there is a considerable body of water in the river, and it is not until September that its lowest level is reached. On both streams irrigation-machines were doubtless employed, as they are at the present day,[14] but in the Euphrates they were only necessary when the water in the river had fallen below the level of the canals.