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FIEST TEAR SCIENCE 



branches of the lower Mississippi when the coastal plain 

 bordering the Gulf of Mexico was elevated. These for- 

 merly entered the extended Gulf by separate mouths, but 

 when the land rose and forced the water of the sea back, 

 their extended courses joined them to the great central 

 river, thus vastly increasing its drainage area and the 

 volume of water it poured through its mouth into the 

 sea. Many of the great river systems of the. world have 



been built in this way. 

 These may be called en- 

 grafted rivers. 



In Figure 116 the 

 rivers all enter the sea 

 at the old coast line GrH 

 by separate mouths. 

 When this region is 

 elevated so that the 

 coast is at IK, the rivers 

 E, D, B find that their 

 easiest course to the sea 

 is by engrafting them- 

 selves upon the river (7, 

 and thus they all four 

 The rivers F and A 



Fig. 116. 



find their outlet at one point L. 



still maintain their independent courses. 



It may be that the elevation takes place over only a 

 part of the river's course. Then the river is dammed back 

 and laJced on the landward side of the elevation and 

 obliged to seek a new course for itself, thus becoming a 

 reversed river, or else it is strong enough to cut its bed 

 down as fast as the land rises, and thus maintain its course. 

 Such a river is called an antecedent river, as its course 

 antecedes the uplift which naturally would have deter- 

 mined its course. The Columbia River has maintained 



