314 THE BASIN OF EASTERN PERSIA AND SISTAN.
A decrease in the size of the lake ensued because of decreased rainfall or increased warmth.
Next, the lake stood at the level of the lower beach, with an area greatly diminished from that of the time of the upper beach. As this is the level at which the lake overflows permanently to the God-i-Zirrah, the water may have stood here twice. The last time was probably very recent, because the bluffs left by it are so fresh that they can scarcely have existed more than a few hundred years.
From this time of relatively high water the lake appears to have shrunk gradually to its present condition, as is shown by the transition from the lower of the old beaches to the present shoreline. Colonel McMahon believes that the condition of the reed-beds proves this decrease in size to be still in progress.
The agreement between the, traditional and the physiographic history of Sistan is so close as to amount almost to identity. If we assume that they are identical, and put them together, we find that they match a third set of facts, the historical, which have already been mentioned, and a reasonable sequence of events presents itself. In this we begin with what was probably the last fluvial epoch, either when the lakes of Sistan and Zirrah were united and stood at the 25-foot beach, or more probably when the lake of Sistan stood for the first time at the 15-foot level and overflowed to Zirrah. As the fluvial epoch began to wane, springs dried up, the rivers decreased in volume, and the level of the lake fell. As the water retired the abandoned shores were occupied by human inhabitants, who we may suppose began to practice irrigation at an early date. At first the largest tract of irrigable land lay along the relatively elevated neck through which runs the Shila. Accordingly the largest canals were dug in this direction. Thus it happened that the Godi-Zirrah, which was now separated from the Hainun-i-Sistan, received the greater share of water. Indeed, it is probable that practically the whole stream of the Helmund flowed to Zirrah, for Smith (p. 285) says that in prehistoric ages the Helmund is reported to have flowed from the dam of Kamal, where it now turns north, in a southwest direction to the lake of Zirrah, and tradition has it that Khai Khusru sailed down it in a vessel. Sykes (p. 365) quotes Istakhri to the effect that in his day, the tenth century, the river flowed in the same way. Earlier classical writers speak of only one lake in this part of the world. It is probable that at this time Sistan was entirely dry, and the towns of Sabari, watered presumably by the Harud or Farah River, and of Kaha-Kaha, watered from the Shila, were built in what is now the lake. It is not to be supposed that the absence of water in the lake of Sistan indicates extreme aridity, for at this time not only was the lake of Zirrah a hundred miles long, according to Istakhri, but Sistan was in its glory. At the height of its prosperity a region which, as Curzon (I, p. 227) puts it, " contains more ruined cities and habitations than are perhaps to be found within a similar space of ground anywhere in the world," must have consumed an immense amount of water in the irrigation of its fields. To furnish this and at the same time fill the great lake of Zirrah, the rivers must have been larger than now. The limits of this period of prosperity and of abundant water-supply can not be stated, but they seem to have included Alexander, 30o B. C., and Istakhri, 900 A. D.