238 FIRST YEAR COURSE IN GENERAL SCIENCE 



in its hundreds of miles of water-worn canyons, thousands 

 of feet deep. Its velocity is so great that it carries coarse 

 sand and even large pebbles. The St. Lawrence River is 

 in direct contrast to both the Mississippi and the Colorado. 

 It neither deposits sediment in its bed nor erodes by means 

 of what it carries. Its waters have flowed through lakes 

 whose quiet depths have received the transported material, 

 leaving clear water to flow on to the sea. 



FIG. 126. A FLOOD SCENE 



A railroad bridge and its abutments have been carried away. The 

 position of the tracks is evidence of the force of the current. What 

 shows the direction of the current? 



258. Deposit. By depositing the material that it car- 

 ries, a river may build up its bed and its banks; at the 

 mouth it may form a delta and sand bars. At flood times, 

 when torrential rains or melting snows in the spring have 

 caused the river to overflow its banks, the receding water 

 leaves a thin layer of mud on the land. Wherever the 

 velocity of the current is decreased, some material is de- 

 posited in the river bed, and thus sand bars are formed. 



