vnt RIVERS AND LAKES 329 



suppose that rivers always tend to deepen 

 their valleys. This is only the case when the 

 slope exceeds a certain angle. When the fall 

 is but slight they tend on the contrary to 

 raise their beds by depositing sand and mud 

 brought down from higher levels. Hence in 

 the lower part of their course many of the 

 most celebrated rivers the Nile, the Po, the 

 Mississippi, the Thames, etc. run upon em- 

 bankments, partly of their own creation. 



A n 



Fig. 48. Diagrammatic section of a valley (exaggerated) 



R R, rocky basis of valley; A A, sedimentary strata; B, ordinary level 

 of river; C, flood level. 



The Reno, the most dangerous of all the 

 Apennine rivers, is in some places as much as 

 30 feet above the adjoining country. Rivers 

 under such conditions, when not interfered 

 with by Man, sooner or later break through 

 their banks, and leaving their former bed, 

 take a new course along the lowest part of 



