482 DIVEESIOTT OF EIVEES. 



arms in their lower course, and enter the sea by different mouths. 

 There are also cases where rivers send ofE lateral branches to con- 

 vey a part of their waters into the channel of other streams.* 

 The most remarkable of, these is the jimction between the Ama- 

 zon and the Orinoco by the natural canal of the Cassiquiare and 

 the Rio !Negro. In India, the Cambodja and the Menam are 

 connected by the Anam ; the Saluen and the Irawaddi by the 

 Panlaun. There are similar examples, though on a much smaller 

 scale, in Europe. The Tornea and the Cahx Rivers in Lapland 

 communicate by the Tarando, and in Westphaha, the Else, an 

 arm of the Haase, falls into the Weser.f 



The change of bed in rivers by gradual erosion of their banks 

 is famihar to aU, but instances of the sudden abandonment of a 

 primitive channel are by no means wanting. At a period of un- 

 known antiquity, the Ardeehe pierced a tunnel 200 feet wide 

 and 100 high, through a rock, and sent its whole current through 

 it, deserting its former bed, which gradually filled up, though its 

 course remained traceable. In the great inundation of 1827, the 

 tunnel proved insufficient for the discharge of the water, and the 



* Some geographical writers apply the term bifurcation exclusively to this 

 intercommunication of rivers ; others, with more etymological propriety, use 

 it to express the division of great rivers into branches at the head of their del- 

 tas. A technical word is wanting to designate the phenomenon mentioned in 

 the text, and there is no valid objection to the employment of the anatomical 

 term anastomosis for this purpose. 



f The division of the currents of rivers, as a means of preventing the over- 

 flow of their banks, is by no means a remedy capable of general application, 

 even when local conditions are favorable to the construction of an emissary. 

 The velocity of a stream, and consequently its delivery in a given time, are 

 frequently diminished in proportion to the diminution of the volume by di- 

 version ; and on the other hand, the increase of volume by the admission of a 

 new tributary increases proportionally the velocity and the quantity of water 

 delivered. Emissaries may, nevertheless, often be useful in carrying off water 

 which has already escaped from the channel, and which would otherwise be- 

 come stagnant and prevent further lateral discharge from the main current, 

 and it is upon this principle that Humphreys and Abbot think a canal of di- 

 version at Lake Providence might be advisable. Emissaries serve an import- 

 ant purpose in the lower course of rivers where the bed is nearly a dead level, 

 and where the water moves from previously acquired momentum and the pres- 

 sure of the current above rather than by the force of gravitation ; and it is, in 

 general, only under such circumstances, as for example in the deltas at the 

 mouths of great rivers, that nature employs them. 



