Southern Tibet : vol.7 / 710 ページ（カラー画像）

0710	Southern Tibet : vol.7
南チベット : vol.7

3. The Tso-ngombo has an effluent to the Panggong-tso the whole year round,
but the Panggong-tso was so long ago cut off from a connection with the Drugub-
Shayok, that it has become salt. It must therefore be regarded as no longer belonging
to the Indus system and oceanic drainage, but essentially to the Tibetan plateau-land.

We now have to consider the most important differences in the three pairs
of lakes, viz., their form and their beach-lines. The Satlej and Hwang-ho lakes are
of the same kind; they are rounded or elliptic, and have only badly developed,
quite recent beach-lines at 2 or 3 m. above the present water level. Their hydro-
graphic fluctuations make the formation of beach-lines nearly impossible. If the
Saloma and Jagiyn-gol suddenly should carry ten times as much water as now, the
only result would be that the Hwang-ho at the exit from Orin-nor would increase
in volume, without giving the lake an opportunity to rise more than one or two feet.
But if the two rivers should decrease so much that the outflow from the Jarin-nor
channel ceased for several years, then, of course, the Orin-nor would be cut off from
the Hwang-ho, and its surface would sink gradually. Under such circumstances the
actual outlines of the lake would be clearly visible in the form of a beach-line.
For the erosion of the Hwang-ho is so slow that the surface of the Orin-nor may
be regarded as stationary. The action of the waves and of the ice, especially where
it breaks up during hard storms in spring, is much stronger and quicker than the
action of the erosion in a river bed.

In August 1907 the highest point in the channel Ngangga was at 2,263 m. above
the surface of the Manasarovar. This much the lake had to rise before delivering
any water to the Rakas-tal. The affluents had to bring down to the Sacred
Lake 1,267 millions cub. m. of water to bring about a rise of 2,263 m. As the
2,475,360 cub. m. in 24 hours which I measured in all the affluents only were
spent for compensating the loss by way of evaporation, the 1,267 millions mean a
very considerable increase in all the affluents. At a level of 2,263 m. above the
lake a beach-line could be seen at several points around the lake, but above that
height no other marks of the action of waves and ice could be noticed. Therefore,
as the water above a certain threshold in the bed of an effluent always escapes,
it is quite natural that in the case of such lakes as those of the Satlej and
Hwang-ho no beach-lines at a considerable altitude may have been formed. On the
other hand the possibility always remains that the erosion of the outflowing river
may have been strong enough to cut down its bed to a considerable depth. The
surface of the lake then sinks at the same rate so slowly and gradually that beach-
lines at certain heights scarcely can be formed. The result of the gradual action
will rather be an abrasion slope.

Turning now our attention to the Panggong-tso and Tso-ngombo, we find
that their form is of a quite different type from that of the two other pairs of lakes.

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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
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