THE WORK OF STREAMS 



123 



Natural Levees. — A study of a topographic map of the lower 

 Mississippi River shows that it flows between banks which rise ten or 

 more feet above the surrounding swamps, and occasionally constitute 

 the only dry land for long distances. Such embankments are called 

 natural levees (Fig. 107). They are gradually built up in time of flood 

 when the water is swift and contains much sediment. The current 

 in the channel is sufficient to carry the sediment onward, but its 



Fig. 112. — Map showing the changes in the course of the Hoang Ho on its delta 

 (shaded). The river. is useless for navigation because it is so changeable, and its 

 waters are restrained only by an elaborate system of dikes and canals. (Richtofen.) 



velocity is checked when it comes in contact with the slow-moving 

 flood water on the sides, sediment is deposited, and an embank- 

 ment is thus erected above the swamp. Natural levees are often 

 strengthened and heightened artificially to prevent floods, but it is 

 readily seen that during a flood a river may break through its 

 levees, spread over its swamps, and perhaps change its course. The 

 Mississippi River broke through its levees in 191 2, causing great 

 destruction of life and property. The levees of the Hoang Ho in 



