国立情報学研究所 - ディジタル・シルクロード・プロジェクト
『東洋文庫所蔵』貴重書デジタルアーカイブ

DESICCATION OF TIBETAN LAKES.   173

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As the real plateau-lakes are all situated at nearly the same height above the sea, they are in the same way exposed to changes of climate. And still we find lakes in all possible states of development. Regarding the fresh-water lakes only it would be in many cases difficult to tell whether they are stationary or sinking, for there may be no terraces at all beside them. Around others, more or less salt, there are low beach-lines or strand-terraces; and yet others, as the Lakor-tso, which are extremely salt, have beach-lines up to i 33m. above the present level. The fresh-water lakes are always deeper than the salt lakes, which are sometimes extremely shallow, for instance the Ngangtse-tso. In some lakes we find only little pools of water surviving amid an expanse of nothing but salt and gypsum; while others are temporary, and others again completely dried up.

The absolute height does not seem to have anything to do with the extent to which the desiccation has proceeded. But the geographical position seems to play an important part. For the desiccation advances more rapidly in the south than in the north, and more rapidly in the west than in the east. It is difficult to account for this, unless it depends upon the fact that the monsoon loses its influence as it proceeds towards the interior of the highland plateau. But to this it may be objected that innumerable lakes in the interior of Asia are diminishing in spite of their being far away from the influence of the monsoon. I

Hardly any European traveller has crossed or visited a portion of the Tibetan plateau without noticing the desiccation of the salt-lakes, for no observation of physical geography is easier to make. The concentric, often very well preserved terraces and beach-lines prove that the lake has been much larger in former times. In times past the desiccation has obviously proceeded gradually towards the complete extinction of the lakes, as this goal has been reached by many of them, as can be easily seen in some depressions where nothing but the old terraces are left. As to the rate and speed of this desiccation we know nothing. And still less are we able to know whether this desiccation is going to continue in the future. The several beach-lines round Lakor-tso only tell us of a depression of the desiccation curve, and there is nothing at all against the plausibility of a future rise of this curve, when the old beach-lines may be reached by the level of the lake one after another.

The only thing we may be pretty sure of is, that the desiccation has not proceeded with constant regularity. For within the great period of desiccation there have been shorter periods, and within them still shorter, with a length of only a few years. Here the history of the Manasarovar comes to our assistance. -What is going on there under our direct control may certainly be said to be going on in every lake on the Tibetan plateau-land. And from this point of view the oscillations in the Manasarovar may serve us as a key for solving the problem, at least partially.

I Compare : »Hypsometry and relief of the Tibetan Plateau», and »I,acustrine problems. Desiccation etc.» Scientific Results of a journey in Central-Asia 1399-1902. Vol. IV, p. 582 and 595.