vill 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. 
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 
