THE ACTION OF PLOWING STREAMS. 735 



Turning now to the application of these principles, let us take the 

 case of a stream flowing in a restricted rocky channel and debouching 

 on to a gently sloping plain whose gradient is too gentle to admit of 

 the stream carrying on its burden of debris. This is consequently 

 deposited as a " fan " whose surface has a steeper slope than that of 

 the plain and on which the stream is broken up into several broad 

 and shallow channels, in which it has such a velocity as just suffices 

 to transport its burden. 



In course of time, as the fan increases in size, some of these sepa- 

 rate portions of the stream will reunite, and, by the mere fact of 

 uniting, their velocity will be increased ; since the same body of 

 water flowing over the same gradient has a more rapid current when 

 flowing in a single channel than when flowing in two separate 

 channels. But this increase of velocity will cause the stream to 

 erode the surface of the fan over which it flows, and by forming a 

 defined channel, to increase its hydraulic depth. The velocity and 

 consequently erosion will thus be still further increased. 



But this channel of erosion must at its lower end come out to the 

 surface either of the fan or the plain, while its upper end is depressed 

 below the surface of the fan ; its gradient is therefore less than that 

 of the fan and consequently, in spite of the greater hydraulic mean 

 depth, a limit will be reached where the velocity of current will 

 again be only just sufficient to enable it to transport its burden of 

 debris. 



Where this eroded channel passes out to the general surface of the 

 ground, and its waters are no longer confined within a defined 

 channel, the debris will be deposited in a second " fan." I am aware 

 that the word fan has never been used in this sense ; but the form of 

 deposit is essentially so similar to that of the fan, as ordinarily 

 known, that I am loth to invent a new word, and prefer to define the 

 lower region of deposition as a " secondary fan." 



At the base of the secondary fan an eroded channel will be formed, 

 and at its lower end another fan, till we have a series of stretches 

 in which the stream has alternately a higher gradient with shallow 

 ill-defined channel and a lesser gradient with deeper well-defined 

 channel, the velocity in each case being such as will just enable the 

 stream to transport its burden of debris. I shall define the former 

 of these conditions as a " fan," the latter as a " reach." 



Where a fan passes into a reach the gradient will be steeper than 

 that of the fan, for the upper end of the reach has been excavated 

 out of the latter ; over this steeper gradient the stream will fiow 

 under the same conditions as over the surface of the fan, the velocity 

 will be higher than on the fan, and erosion will take place. We 

 may consequently expect that the reach will be constantly cutting 

 back into the fan, and must consider whether there will not be an 

 equal encroachment by the fan on the lower end of the reach. 



I have already explained that the fan below the reach wiU 

 commence where the lesser gradient of the latter brings it out to 

 the surface, and the waters are scattered abroad instead of being 

 confined to a defined channel. But this spreading of the waters will 



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