64 FROM CENTRAL TIBET TO LADAK.
Launching our skiff again on the branch that I have mentioned, we paddled north, the river winding but little. In its lower part it is joined from the left by a tributary, which likewise issues from the actually existing principal stream, although accidentally cut off from it. At length our deltaic arm widened out and entered the extreme western part of the Naktsong-tso; which part again forms an independent basin, though of no great extent. Along the line by which we crossed it it was very shallow. At first the depth was barely half a meter, and we were surrounded on all sides by long mud-banks and peninsulas and promontories lifting themselves only one or two decimeters above the surface of the water, and all pointing towards the north-east. Beyond them followed a number of islands and islets of a similar character. The circumstance that they were all grass-grown, as was the actual channel, rendered it probable that the eastern river-arm had not been used at all recently; for had it been, the vegetation would not have been able to establish itself, but would repeatedly have been covered with fresh deposits of mud. This however is not true of the first deltaic arm into which we penetrated from basin No. I. It was destitute of plant-life, no vegetation having succeeded in establishing itself there; hence it would appear to have been used at a later date than the deltaic arm that discharges towards the north. Outside the last-named, between the peninsulas and the mud-banks, we were able to trace distinctly a deeper channel, having at first the mean depth stated above, namely 1/2 m., though this subsequently increased to 0.65, 0.91, and I.00 m. Close to the sides of this deeper channel the stream was so shallow that our light skiff was quite unable to make her way. This channel, which we were able to follow a pretty long way out into the lake, proves however, not only that the now abandoned eastern branch formerly possessed a not inconsiderable erosive power, but that it cannot be very long since it ceased to function, otherwise the lake-bottom outside its mouth would have been levelled up by the beat of the waves, the movement of the current, and the deposition of sediment. Two facts are undoubtedly true, namely that the river is heavily charged with sediment and that the delta is growing. It is not so very long since the peninsula, with the many east-west ranges of mountains on it, really was an island, entirely surrounded on every side by water. That part of the lake which lies immediately west of it was no doubt at first relatively deep, for towards the south we still find a depth of 11.68 m., while farther north it goes down to 19 m. But the intervening section of this part of the lake has on the contrary become increasingly filled up with sediment, which has been carried down and deposited by the river that comes from the west. Originally the river emptied into the lake through its western side; its latitudinal valley still opens out there between two parallel ranges. Gradually a delta grew up, stretching its mud-flats north-east, east, and south-east. This delta then spread out more and more towards the east, gradually contracting the open waterway (the sound), until at last the most easterly, most advanced part of the delta came into contact with the extreme western peninsula of the large island, thus connecting it with the land on the opposite side, and the island consequently ceased to be a true island. Still going on increasing in size, the delta at length, as we have seen, filled up no inconsiderable part of the sound itself. It is however everywhere so flat that the river-arms are extremely sensitive to the slightest changes in the sedimentary deposits.
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