RIVERS AND THEIR WORK 



239 



These bars may become permanently joined to the shore or 

 may remain as islands. 



The Mississippi River, flowing very slowly in its lower 

 course, has been filling up its channel. As the bed of the 

 stream has thus been raised, the water would in many places 

 overflow the banks every year if artificial banks (called 



FIG. 127. EROSION OF ELEVATED SURFACES, MONUMENT PARK, 



COLORADO 



This is a stratum of coarse sandstone which had a layer of harder 

 rock over it. In its elevation, vertical cracks were made, and erosion re- 

 sulted in the making of columns. How was this assisted by freezing water? 



levees) had not been built. The levees must be high enough 

 to restrain the water at flood times. The higher the river, 

 the greater pressure there is against the side of the levee. 

 Constant watch is maintained when the river is full, in order 

 to prevent disastrous breaks, and millions of dollars are spent 

 annually to prevent and repair damage. 



The Ohio, the Arkansas, the Amazon, and the Yellow 

 rivers have great floods. In thickly populated China, the 

 loss of life is often great and the destruction of rice fields 

 results in famine. On the other hand, the flood of the 



