Action of Water on the Land 



245 



* Including Lake Victoria and its longest tributary, 

 t From Missouri source. 



320. Torrential Track. On account of the forms of 

 the land-slopes (see sections of continents, Figs. 56-62) 

 the course of a typical river falls into three natural divisions : 

 the Torrential Track, with a slope usually exceeding 50 

 feet in a mile ; the Valley Track, with a slope rarely greater 

 than 10 feet, and often less than 2 feet ; and the Plain 

 Track, in which the change of level is only a few inches in 

 a mile. Some rivers have only one or two of these charac- 

 teristic divisions. Torrents dash down the mountain-sides 

 with tremendous speed, often exceeding 20 miles an hour, 

 leaping in cataracts from rock to rock and foaming through 

 ravines. Little soil forms on the steep slopes, hence as a 

 rule torrents swell quickly during rain and dwindle away to 

 a mere thread of water at other times. The work of a river 

 in its torrential track is purely destructive. When wholly 

 immersed in water, rocks are practically reduced in weight 

 from one-half to one-third, and are therefore moved with 

 much less expenditure of energy than would be required in 

 air. Huge boulders are thus hurled along by the flooded 

 stream, and hammer out the hollows in which the water 

 flows. The chips struck off at every concussion get broken 

 into smaller pieces, forming pebbles, gravel, sand, and 

 mud, or, to use a general term, detritus, which is swept 

 away to lower levels. As the ravines are deepened, tribu- 

 tary torrents leaping down the rugged slopes carve out 

 tributary ravines and increase the volume of water and of 

 detritus in the river. 



321. Valley Track. The valley track of a river lies 



