84 PHYSIOGRAPHY 



2. Another but less common cause of loss of velocity is decrease 

 of volume. Streams generally increase in size as they flow, but 

 to this rule there are some exceptions. (1) If, for example, a 

 stream flows through a very dry region, it may receive little water 

 from tributaries and springs. Evaporation, on the other hand, is 

 great, and some of the water may be absorbed by the thirsty soil 

 and rock through which it flows. This is especially the case if the 

 ground-water surface (p. 30) is below the level of the stream. In a 

 dry region, therefore, a stream may diminish as it flows, and may 

 even disappear altogether (Fig. 42). (2) A stream sometimes breaks 

 up into several streams (Fig. 2, PI. XXI, p. 84). The volume of each 

 is less than that of the original stream, and decrease of volume means 

 decrease of velocity. (3) Still again, many streams, especially in 

 arid regions, have much of their water withdrawn for purposes of 

 irrigation. Many streams in the West are made smaller in this 

 way. 



3. A stream may make deposits because it gets more load than 

 it can carry, even though it does not become slower. Thus tributary 

 streams witli high gradients may bring to the streams into which 

 the}' flow more sediment than the latter can carry away. 



Allurud Deposits: Their Positions and their Forms 

 The sediments left by running water on the land are alluvial 

 dcjiosits. They are found chiefly where the flow of the water is 

 checked. Such situations fall into several classes. 



1 . At the bases of steep slopes. Every shower washes fine sed- 

 iment down the slopes of hills and mountains, and much of it is left 

 at their bases. In such situations, fences are sometimes buried, little 

 by little, by the mud lodged against them. The temporary streams 

 which follow showers sometimes flow down steep slopes, and are 

 suddenly chocked at tlioir bases. Such streams gather much debris 

 on the slopes, but abandon it where their velocity is suddenly 

 checked. Thus at the lower end of every new-made gully on a hill- 

 side, there is a mass of debris which was washed out of the gully 

 itself (Fig. 53 and PI. XXII, p. S. r >). Material in such positions accu- 

 mulates in the form of an allnrial cone. An alluvial fan is the same 

 as an alluvial cone, except that it is spread out more, and has a lesser 



