86 



PHYSIOGRAPHY 



do, the stream becomes a network of little streams, sometimes called 

 a braided stream (Fig. 87). The condition shown in the figure exists 

 only at low water. When the river is high, the whole flat through 

 which the minor streams shown in Fig. 87 and PL XXIV (p. 87) 

 flow is covered with water, and becomes the bed of a single river. 



Streams sometimes deposit sand-bars in their channels (Fig. 2, 

 PL XXI, p. 84), especially in low water. Bars interfere with navi- 

 gation, especially when the rivers are low. The bars are often 



I i 



Scale. 



Fig. 87. A braided river, Dawson Co., Neb. (U. S. Geol. Surv.) 



swept away in times of flood, when the streams are swift, but in 

 many cases they form again when the flood is past. Occasionally 

 bars become more or less enduring islands. If they become covered 

 with forests, they are not likely to be washed away, for the roots 

 tend to hold the soil against the force of the current. 



The alluvial plains along rivers are almost flat, though they 

 slope gently down stream. They have a few features, however, 

 which deserve mention, and among them are the natural levees. 

 This term is applied to the low ridges on stream flats along the banks 

 of the channel (Fig 88). They are built in times of flood. At such 



