76 



PHYSIOGRAPHY 



Rapids and falls. The bed of a stream is often steeper at some 

 point than at others, and there the stream flows more rapidly. In 

 such a case as that shown in Fig. 1, PL XVI, p. 61, the quickened 

 flow constitutes a rapids. If the water in a stream's bed drops over 

 a cliff, it makes a waterfall (Fig. 2, PL XVI, p. 61), and between 

 waterfalls and rapids there are all gradations. Steep rapids are 

 often called falls, and both are sometimes called cascades. 



The falls and the rapids of many rivers add greatly to their 

 beauty, and sometimes enhance their value to mankind by affording 



-c 



Fig. 72. Diagram representing an early stage in the history of a waterfall. 

 The diagram represents a vertical section through the rock. H is a hard 

 layer, and b a represents the slope over which the water began to run. In 

 time, the gully develops the profile 6 c a, and the water flows more swiftly 

 just below the hard layer, making a rapids. A little later, the profile of 

 the valley becomes c c', Fig. 73, and the rapids become more rapid. 



Fig. 73. This figure shows a further development of the process illustrated 

 in Fig. 72. The profile of the stream becomes d d', when there is a pro- 

 nounced fall; e e', when the fall becomes lower; //'. when the fall has 

 again become rapids; and g g', when the rapids have disappeared. 



great water-power. Niagara Falls affords about 4,000,000 horse- 

 power. The Falls of St. Anthony did much to make Minneapolis 

 the greatest flour manufacturing city of the world. Some of the 

 great manufacturing cities of New England also grew up about low 

 falls and rapids. But rapids and falls are enemies of navigation. 

 In the early days, the rapids of the Ohio at Louisville prevented the 

 passage of steamers, and so determined the location and early growth 

 of that city. A canal around the rapids was completed in 1830. 



Waterfalls came into existence in various ways. If the rock 

 in the bed of a stream is of unequal hardness, the less resistant part 

 will be worn more rapidly than the more resistant part farther up- 



