THE WORK OF RUNNING WATER. 181 



of a river are so nicely balanced that slight disturbance at one point 

 causes disturbance at all points below. 



The deposits. 



Types. — Turning from the principles which underlie river deposition 

 to the deposits themselves, they are found to occur in various situa- 

 tions. Running water usually descends from steeper slopes above to 

 gentler slopes below, and ends at the sea, or in a lake or inland basin. 

 Wherever there is a sudden decrease in its gradient, as at the base of 

 a hill, ridge, or mountain, running water is likely to leave a large part of 

 its load, building an alluvial fan or co7ie (Figs. 67, 68, and PL VI). Even 

 where there is no sudden decrease in the gradient of a stream, there 

 is likely to be a gradual one, and in spite of the fact that the increased 

 volume of a stream in its lower course tends to overcome the effect of 

 diminished gradient on velocity, deposition is hkely to take place 

 as the gradient is reduced. Deposits occasioned by the gradual re- 

 duction of a stream's velocity often have great extent in the direction 

 of a stream's flow. They cover the flood plains of streams, making them 

 alluvial plains (Fig. 73). When a stream reaches the sea or a lake its 

 current is destroyed and its load dropped, unless taken in charge by 

 the waves and currents of the standing water. Sediment accumulated 

 in quantity at the debouchures of streams gives rise to deltas (Figs. 

 169, 187). Alluvial cones and fans, alluvial plains, and deltas, are the 

 principal tj^pes of river deposits. Apart from these well-defined types 

 there are hars in the channels of depositing streams, and much ill- 

 defined allmium wliich does not allow of ready classification. 



Alluvial fans and cones.— The only distinction between the al- 

 luvial fan and the alluvial cone is one of slope, the cones (thej^ are 

 but half-cones at best) being steeper than the fans. Allu^dal fans 

 and cones have their most striking development where temporary 

 torrents, occasioned by showers or the rapid melting of snow, issue 

 from mountain ra^dnes. Such streams usually carry hea\^ loads of 

 detritus, the coarser part of which is hkely to be deposited at the base 

 of the mountain slope. Cones and fans built by such streams have 

 a periodic rather than a steady groT\i;h. 



At the beginning of its development the material of the alluvial 

 cone is deposited much as in a talus cone (compare Fig. 170 with Figs. 

 67 and 68). Its deposition chokes the channel of the stream, and 



