120 



PHYSICAL GEOLOGY 



banks. The downstream slope of a flood plain varies with the vol- 

 ume of the water in the stream and its load. The slope of the flood 



plain of the lower 

 Mississippi, for ex- 

 ample, is only two to 

 three inches a mile, 

 while streams carry- 

 ing coarse material 

 may build up flood 

 plains with slopes of 



Fig. 107. — The flood plain of a river. The natural 

 levees on each side are shown, as is also the structure of 

 flood-plain deposits. 



50 to 75 feet to the 

 mile. Flood plains are highest near the river and slope gradually 

 away from it (Fig. 107). This is due to the fact that, at flood, 

 the coarser and more abundant material is deposited where the 

 silt-laden main current is checked by contact with the slow-moving 

 waters of the sides. 



Meanders, Owens valley, California. (U. S. Geol. Surv.) 



