MIGRATIONS OF THE LOP-NOR. 359
high degree to the preservation of the last-mentioned lake. On Prschevalskij's map the connecting link between the two lakes is only 20 versts long, and in that short distance the river had no opportunity to pick up any notable quantity of sedimentary matter. The water upon issuing from the upper lake was perfectly clear, and the erosion of the bed was in no case more than insignificant; indeed it was confined to the sides, though they again are tolerably well protected by vegetation. Consequently upon reaching Kum-tschapghan the water was but very slightly muddied, and at the present day the maximum depth of the Kara-koschun is sounded in its western part, though that quarter is also at the same time much less exposed to be filled up with drift-sand than the eastern part.
The sedimentary masses of the Tarim were thus employed, in the time when the Kara-buran still existed, partly to fill up this lake, partly to build up the pier-like ramparts which now form, and will also continue to form in the future, the framework and containing banks of the river. In consequence of this the sediment with which the river is charged is carried on past these enclosing margins, and the water, upon reaching Kum-tschapghan, is not only incomparably less turbid, but contributes to fill up the basin of the Kara-koschun at a greatly increased ratio. Now that the Kara-buran has for the most part disappeared, the fate of the terminal lake is sealed, and it is only a question of time for it to follow the example of its sister lake.
It is very interesting to note the unambiguous parallelism which has characterised the changes that have taken place in the two lakes during the last decennia. The shrinkage of the Kara-buran has entailed a similar diminution in the Kara-koschun, and the disappearance of the former would now seem to entail as a necessary consequence the disappearance of the latter. One is indeed strongly tempted to believe, that the disappearance of the Kara-buran is the reason why the terminal lake has become restless and has begun to travel; but that would be »the last drop that makes the cup run over». On the other hand the Kara-buran appears to be less intimately connected than the Kara-koschun with the upper marginal lakes, for the consequence of the creation of these upper lakes has been a diminution in the volume of the lower Tarim, entailing as a further consequence a decrease in its power to carry sediment in suspension, to say nothing of the clarifying process the water undergoes in the course of its passage through all these upper lakes. At the same time, as an even further consequence of the same course, the filling up of the Kara-buran has been rather checked.
I possess no sure data which would enable me to calculate, even approximately, how long a time would be required to fill up completely the basin of the Karakoschun with solid material. Before any such calculation could be made, it would be necessary to carry on investigations, extending over at least one year, into the capacity of the river to carry sediment. I have already stated that 2000 million cubic meters is the volume, or rather the capacity, of the Kara-koschun at the beginning of April, and that total is too high rather than too low. I have also assumed, that the whole of that volume is lost every year, especially through evaporation; and if the lake is to be maintained at its normal level, it must be fed by an average inflow of 64 cub. m. per second all through the year. If we take it, that every 20,000
Iledin, ,journey in Central Asia. H. 46
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