10 HAYDEN: GEOLOGY OF THE PROVINCES OF TSANG AND 0. 
flows through a broad and fertile valley to Shigatse where it joins 
the Tsangpo. 
To the east of the Karo La most of the streams fall into a basin 
of internal drainage, in which lies the famous 
Rong Chu. " 
Yamdrok lake. The only exception is the Rong 
Chu, which, rising at a short distance from the north-western shore 
of the lake, flows through a deep and narrow valley to join the 
Tsangpo at some distance below Shigatse. The Rong would appear 
to have been at one time a considerably larger river than it is 
at present and probably flowed out of Yamdrok Tso ; it may still 
derive some of its water from the lake by percolation through the old 
moraine material which now dams the outlet (see below, p. 12). 
Second only in importance to the Tsangpo, the Kyi Chu rises 
in the mountains to the east of Lhasa and is 
Kyi Cha. ... „ . 
jomed by various streams flowing from the north 
and north-west. At Lhasa it flows through a broad, swampy plain 
spreading out into several channels separated by gravel banks (PI. 6). 
Its fall from Lhasa to near Chushii, where it joins the Tsangpo, is a 
little less than 300 feet : its gradient is thus only about i in 600. 
3. Lakes. 
Like most known Tibetan lakes those of Central Tibet were 
once much larger than they are now ; and if, as is usually supposed, 
they are still undergoing desiccation, it is probable that, owing 
to higher rainfall, the rate is not so rapid as in the arid plains of 
western and northern Tibet. 
In the area visited by the mission, the most important lakes 
are Tso Modretung, Hram Tso, Kala Tso and Yamdrok Tso. 
The first of these lies in the north-western corner of the Yaru 
plain and is fed by rivers from the high rangfe to 
Tso Modretang. , , , • , . .. 
the north. Its area is about forty square mues 
and it appears at present to have no outflow, although there is a 
well-marked channel connecting it with the Yaru river, into which, 
however, it probably drains by under ground percolation. 
( ) 
