370 STUDIES FOR STUDENTS 



which descends several thousand feet in a distance of several hundred 

 miles. 



The streams of a rainy cHmate on the other hand have greater 

 carrying power over their lower and flatter courses, owing to the 

 continually augmented volume with distance from source. The 

 material carried is also more largely fine-grained than in the case 

 of the sub-arid climate, a considerable portion consisting of true clay. 

 There is consequently less tendency to build piedmont slopes and 

 more to deliver the waste to the delta and the sea. In warm humid 

 chmates thoroughly oxidized and leached clays form a maximum 

 percentage in the land waste, giving a river grade of maximum flatness 

 after escaping from the mountains. The Amazon furnishes an excel- 

 lent example, its bed being but 370 feet in elevation at the junction 

 of the Marafion and Ucayah at a distance of about 1,800 miles from 

 the ocean. The grade of the Amazon is doubtless, however, some- 

 what flatter and lower in its middle course than is called for by the 

 present relations of volume and waste, since it is characterized by 

 braided streams, distributaries, and shallow lakes. In cold rainy 

 climates frost becomes a powerful disintegrating agent in mountainous 

 regions, developing with great rapidity vertical chffs skirted by coarse 

 talus slopes and accentuating during the early portion of the cycle 

 the steepness and roughness characteristic of such regions. The 

 waste supphed to the rivers is coarser than that from a warmer region 

 possessing the same broad topographic features, and the river con- 

 sequently must flow on a somewhat steeper grade for the same volu- 

 metric relation of river water to sediment. 



Effects of Varying Climates upon Transportation 



In short streams of steep grade, such as those draining from the 

 coast chains of Cahfornia toward the ocean; or in longer ones of low 

 srade, such as most of those of the Atlantic coast of the United 

 States, the deposits of alluvium under all circumstances are largely 

 marine and the chief effect of a change of cHmate upon the sedimenta- 

 tion is due to the resultant changes in the rate and kind of erosion 

 rather than changes in the transportation. As pointed out under the 

 subject of the relation of topography and climate to erosion this alone 

 should lead to well marked differences in the sediment. In the larger 



