50 



PHYSIOGRAPHY 



The force of running water is evident even when streams are not 

 in flood. Many small mountain brooks are so swift that it is 

 difficult to stand or wade in them, although they are no more than 

 a foot or two deep. The force of running water is also seen in the 

 rapids and falls of streams. It is most impressive in the case of 

 such a fall as Niagara, the force of which is estimated at four million 

 horse-power. Even sluggish streams may have great force, for 



Fig. 38. Scene in the freight yards of Kansas City after the flood of 1903. 

 (Drawn from photo. U. S. Weather Bureau.) 



their currents are made to work the machinery of thousands of mills 

 throughout the land, and the force of the machinery in the mills is 

 often impressive, even when the moving water which turns it makes 

 little show of strength. 



Sources of stream water. Most streams derive the larger part 

 of their water from the immediate run-off and from ground-water; 

 but many of them receive contributions from ponds and lakes as 

 well, and a few, like the St. Lawrence, receive most of their water 

 from such sources. Others get water from the snow and ice of 

 mountains. The Mississippi, as well as most other great rivers, 

 receives water in all these ways. The immediate run-off, the ground- 



