346 THE LOP-NOR PROBLEM.
the river-bed presents the appearance of an artificially made canal. The considerable volume of water which the river contains moves at an extremely slow rate, and so far from exercising any erosive energy and absorbing solid material from the sides of its bed, it even allows any material that does chance to find its way into it, blown thither for the most part by the wind, to settle at the bottom. Upon entering the Avullu-köl therefore the water of the Ilek is pure, or at all events the sediment it carries is so insignificant that it would require a very long time to make itself perceptible as a deposit at the bottom of the lake. As one might with good reason expect, the Avullu-köl is somewhat shallower than the lakes to the south of it. It may be regarded as a sheer physical necessity that, unless other changes take place
Fig. 169. PART OF SADAK-KÖL IN 1896, SHOWING A REGULAR BAJIR-DEPRESSION FILLED WITH WATER. THE DEAD FOREST SHOWS, THAT THERE WAS AN EARLY WET PERIOD FOLLOWED BY A DRY PERIOD, WHICH KILLED THE TOGHRAKS. AT THE PRESENT TIME THE DEPRESSION IS ONCE MORE FILLED.
in the meantime, the chain of lakes — Avullu-köl, Tajek-köl, and Arka-köl — will inevitably be filled, for vast quantities of drift-sand and drift-dust settle in their basins, and to them large quantities of materials are added by the decaying vegetation. Since then the cubic capacity of these lakes is being in this way lessened, it follows that simultaneously, and in a corresponding degree, the stream which issues from the lowermost of the series, the Arka-köl, increases, the ultimate result being, that it will form a direct continuation of the upper Ilek. If now we project our minds backwards in time, we must picture this stream as growing smaller and smaller, in the same proportion as the lakes were correspondingly bigger than they are now. Indeed the time cannot be very distant when the Ilek emptied itself into the Avulluköl without at the same time any water issuing from the Arka-köl, because the lakes were then so great that an even balance was maintained between evaporation and influx. Nor is the cogency of this reasoning at all impaired by the fact that
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