230 



NA TURE 



''[Jan. 2 1, 1875 



volumes of the summer and winter discharges than there would 

 be if the river wtre in a state of nature, and if a large portion of 

 its flood-water were not diverted by man for the purposes of 

 irrigation. This equalisation between the summer and winter 

 discharges of the Amu, below the irrigated tract, has great sig- 

 nificance, and has suggested to me a cause which may probably 

 account for the cliange in the course of the river from the Cas- 

 pian to the Aral. It is a matter of notoriety that the waters of 

 the Amu carry in suspension an enormous quantity of mud and 

 sand, and it is to the deposition, in its old bed, of this suspended 

 matter, that I am inclined to think the change in the course of 

 the river may with much probability be ascribed. 



In speaking of the aqueous causes of the changes taking place 

 on the surface of the earth, .Sir Charles Lyell, in his "Principles 

 of Geology," has made the following remarks on the transporting 



power of water. As they cannot be put In plainer and better 

 language, and as they bear intimately on the theory I have 

 hazarded, I will quote them verbatim and i)i extetiso : — "The 

 force," he says, "of mountain torrents is easily understood ; but 

 a question naturally arises, how the more tranquil rivers of the 

 valleys and plains, flowing on comparatively level ground, can 

 remove the prodigious burden which is discharged into them by 

 their numerous tributaries, and by what means they are enabled 

 to convey the whole mass to the sea. If they had not this 

 removing power, their channels would be annually choked up, 

 and the valleys of the lower country and plains at the base of 

 mountain chains would be continually strewed over with frag- 

 ments of rock and sterile sand. But this evil is prevented by a 

 general law regulating the conduct of running water : thus, two 

 equal streams do not, when united, occupy a bed of double sur- 





face. Nay, the width of the principal river, after the junction 

 of a tributary, sometimes remains the same as before, or is even 

 lessened. The cause of this apparent paradox was long ago 

 explained by the Italian writers, who had studied the confluence 

 of the Po and its feeders in the plains of Lombardy. The addi- 

 tion of a smaller river augments the velocity of the main stream 

 often in the same proportion as it does the quantity of water. 

 The cause of the greater velocity is, firstly, that after the union 

 of two rivers, the water, in place of the friction of four shores, 

 has only that of two to surmount ; secondly, because the main 

 body of the stream, being further distant from the banks, flows 

 on with less interruption ; and lastly, because a greater quantity 

 of water, moving more swiftly, digs deeper into the river's bed. 

 By this beautiful adjustment the water which drains the interior 

 country is made continually to occupy less room as it approaches 



the sea, and thus the most valuable part of our continents, the 

 rich deltas and great alluvial plains, are prevented from being 

 constantly under water." 



Now, if we apply these principles to the Amu Darya, it is 

 manifest that, when it fell into the Caspian, the conditions 

 of its flow were such that the volume and velocity of its summer 

 or flood-stream were sufficiently great to clear its bed annually 

 of the deposition of silt due to the smaller volume and velocity 

 of its winter stream. The figures given in the table show that 

 the volume of water passing Nukus on the loth of September was 

 a little more than one-third only of the total discharge of the 

 river on the 3rd of August, on which date, it is probable, the 

 Amu Darya reached its maximum height for 1S74. I am in- 

 clined to think, from a consideration of the winter discharges 

 which are recorded by Wood in his work on the Upper Oxus, 



