ACCIDENTS IN RWER DEVELOPMENT 



347 



its course through uplifts which have reached thousands 

 of feet. 



Not only may a river be elevated, but it may be de- 

 pressed. In this case its rate of erosion is diminished, 

 and the river becomes marshy where the grade is low. 

 Where the river valleys approach the sea they will be sub- 

 merged or drowned. 



ALLUVIAL CONES. 

 Formed at the foot of each mountain gully. 



These drowned river valleys form some of the finest 

 harbors on the coast. San Francisco Bay, Narragansett 

 Bay and New York harbor are examples of protected 

 harbors due to the submergence of rivers. The mouth 

 of the Hudson was formerly some seventy miles to the 

 east of Long Island, that of the St. Lawrence to the east 

 of Nova Scotia. In fact the Atlantic coast north of the 



