112 
EXPLORATIONS IN TURKESTAN. 
occupies about two-fifths of the total area of its catchment basin, the upper Chu 
basin not included. The surrounding mountains, from which most of the water 
sujiply is received, proba1)ly occupies as nnich surface as the lake, leavinj^ one-fifth 
of the catchment basin as valleys and piedmont slopes. The drainage basin of the 
upper Chu system, southwest from the lake, is in area about a quarter of the present 
Issik Kill basin, or roughly a half of the mountain area that drains into the lake. 
Inasmuch as the chief upper branches of the Chu, the Kach-kar and the Juvan- 
arik, come from among high ranges, it is evident that their volume nuist be a 
considerable fraction of the total water supply that tlie lake now receives ; not, 
however, so large a fraction as their drainage area would suggest, for the mountains 
southeast of Issik Kul are lofty, and we were told that many large streams flow 
from them. If liberal allowance be made for this, the upper Chu might be taken as 
yielding perhaps a twentieth of the present supply of Issik Kul ; hence if the upper 
Chu were turned into the lake, its surface ought to expand by a twentieth part, if 
it did Hot overflow to the lower Chu before such an expansion was accomplislied. 
On the other hand, when the lake stood at the 25-foot shoreline, its area could 
hardly have been increased by a twentieth of its present surface. Hence it is 
probable that if the Chu should now be turned into the lake, its surface would rise 
to the 25-foot shoreline, if a barrier of that height existed at the outlet; and there 
might be some water to spare for overflow. It has 
already been shown that the delta-like plains between 
the Chu and the western end of Issik Kul indicate that 
the river has entered the lake. Evidence will now be 
presented to show that an overflowing outlet has prob- 
ablv generally existed during the subrecent histor\- of 
the lake. This evidence is found, first, in the eastward 
increase in tlie depth of valley erosion, and, second, in 
the eastward increase in the depth of valley drowning, already stated. To appre- 
ciate this curious case, a preliminan,- statement will be serviceable. 
The problem before us is to discover the conditions under which a lake, A B, 
(figs. 77, 78) may preserve its shoreline at A nearly constant, while it sinks from 
B to D (E) and rises from D to G (H). Let us first inquire how such changes can 
be produced in a lake without an outlet. 
Let the area of the lake, A B, be such as to strike a balance between the rainfall 
on its entire drainage basin and the evaporation from its surface. If the climate 
remain constant and regional tilting occur, with uplift 
on the right, as in fig. 77, the lake will sink from B to D 
and rise from A to C, thus preser\'ing its area unchanged. 
In order that the shoreline .shall remain at A during 
uplift, the climate must become just enough more arid 
to reduce the lake area so that its surface shall be E A, 
instead of D C. On the other hand, if depression occur 
on the right, as in fig. 78, the lake will change from 
A B to F G. In this case the shore can be held at A only by climatic change 
towards more humidity of just such amoxmt as to expand the surface to H A. It is 
Fig. 77. — Effect of tilting on an in- 
closed lake ; uplift on the right. 
Fig. 78. — Effect of tilting on an in- 
closed lake; uplift on the left. 
