Southern Tibet : vol.7 / 733 ページ（カラー画像）

0733	Southern Tibet : vol.7
南チベット : vol.7

0733

Southern Tibet : vol.7

南チベット : vol.7

while another was 6555 m. high. On the Lunpo-gangri, WOOD determined
several peaks of about 6700 m. From Tradum, Ryder saw a peak belonging to the
same system and having an altitude of 7058 m. These belong to the same group
of peaks I saw from the north, and of which I have several illustrations in Vol. III.¹
That means that on the Transhimalaya there are peaks rising nearly 2400 m. above
the depression along the northern base of the system.

Now the question arises: what was the hydrographic arrangement in the de-
pression north of Transhimalaya during the wet period preceding the period of
beginning aridity, desiccation and formation of self-contained plateau-basins and
steppes? Considering, to begin with, only the western half of the great depression
we find a series of small rivers each of them having its source on the northern
side of the great water-parting, and each of them being the affluent and feeder of
a saltwater lake. The Soma-tsangpo goes to Teri-nam-tso, the Buptsang-tsangpo to
Tarok-tso, the Pedang-tsangpo to Shovo-tso, and the Sumdang-tsangpo to Ngang-
laring-tso. Although these lakes, as all other lakes in the interior of Tibet, are
subject to the general law of desiccation, the inflow of fresh water to them may be
said to be abundant enough to compensate for the evaporation from the lake. Bet-
ween the river and its lake there exists a state of equilibrium. During the summer
when the snows and ices melt and occasional rains fall, the rivers rise and carry
down so much water that they sometimes may be crossed only with difficulty and
at many places not at all. Then the lakes rise to their maximum level. In the
late autumn the melting and the rains cease, the rivers dwindle and the surfaces of
the lakes sink. In the winter mere brooks come down under the ice, but then the
lakes themselves are frozen and the evaporation diminished. Thus there is an
annual period of rise and fall in the lakes. There is also a period of a higher
degree, viz. the one that is registered by the outflow of the Manasarovar. And
finally we have the great period of general desiccation.

What would happen if the desiccation ceased and the climate again became
more and more moist? The Soma-tsangpo would increase and many other, now
temporary, brooks would by and by grow to permanent rivers. The Teri-nam-tso
would expand in all directions. To the north and south where the mountains are
close upon the present lake the shore-line would be less changed. In the east the
lake would meet an obstacle in the hills separating its basin from that of Dangra-
yum-tso. To the west a rise of 9 m. only would bring the western shore 21 km.
westwards to Mendong-gompa, to which place the mouth of the Soma-tsangpo would
be removed. By increasing moisture the whole Nevem valley would get filled
and the overflowing water would continue W. N. W. always following the lowest

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