PRESENT CHANGES IN THE EARTH. 



93 



through it, occasionally overflowing its banks. In the 

 overflows mud will be deposited over the valley-flat, and 

 7 j after a while the result 



a ' indicated in Fig. 34 



"Will OG l*GclllZGCl v USu 



this process has been 

 gone through with along the Mississippi, so that there 

 is a natural levee, as it is termed, on each side of the 

 river. The inhabitants are obliged to raise this in cer- 

 tain places to prevent inundations from freshets. Some- 

 times a break (crevasse) occurs in the levee, and much 

 damage is done in the country adjoining. The same 

 process goes on, and the same results occur at length in 

 those cases where the land along a river is inundated at 

 all seasons. 



175. Improvement of Rivers. A very pretty illustra- 

 tion of the manner in which the solid material in rivers 

 is deposited is furnished us by a mode of improving river 

 navigation sometimes practiced. We will take a particu- 

 lar case. The river issues from a gorge, and then spreads 

 out over a large flat surface, there being, of course, a chan- 

 nel through the flat for the main body of the water to 

 flow in. The object of the plan of improvement is to 

 prevent accumulation of sediment in the channel, and 

 even, if possible, to carry out sediment already deposit- 

 ed. This is done by building piers of stone across the 

 flats out toward the channel, as indicated in Fig. 35. 



Fig. 35. 



Let a be the gorge, c c the flats across which the piers 

 stretch, as represented, and b the channel. The piers, J, 

 J, d, it is obvious, will somewhat detain the water on the 

 flats, and so allow the sediment to fall from it in larger 



