198 W J MCGEB — OUTLINES OF HYDROLOGY 



between (1) that strictly dj'nainic agency of the inorganic in which 

 actual energy is transmitted or converted and (2) that partly kinetic 

 activity of the organic in which potential energy is actualized and 

 redirected. 



Concordantly, the smallest but most energetic division of the hydro- 

 sphere (the running water) may be regarded in some measure as kinetic 

 rather than a merely dj^namic agency; the kinetic property arising in its 

 interrelations with other substances. Eunning water does not move 

 uniformly ; a film of water on an inclined surface, however smooth, will 

 not remain a film, but soon gathers (a) into transverse waves and (5) 

 into streams; neither will its movement remain uniform, but accelerates 

 (a) in the transverse waves and (h) in the medial portions of the streams, 

 and slackens between waves and streams. Similarlj^, a filament of water 

 descending freely in space breaks up into drops. The variability of 

 flowing water doubtless arises largely in (1) interadjustment among the 

 water particles and (2) friction between the body of water and its bed, 

 while in falling water surface tension and perhaps other factors enter; 

 yet the undulatory tendency is pronounced in small and large volumes in 

 every natural stream and artificial canal — indeed, in all moving water — • 

 and the tendency may be intensified when the water carries solid matter. 

 And not only is the stream variable in itself ; in a state of nature it intro- 

 duces variability in its bed — or varigradation — in such manner that all 

 streams of whatever size normally alternate from pools to rapids, and 

 vice versa, save measurably in that case (characteristic of arid regions) 

 in which solid matter so abounds as to overload and spread the water in 

 sheetfloods and debar it from gathering into streams. 



Again, streams sxabject to fluctuation in volume develop inner chan- 

 nels adapted to the lower stages when the water runs clear, and larger 

 channels adapted to the freshets of muddy water with adjacent plains 

 built up and shaped by combined erosion and sedimentation; so that 

 under natural conditions every streamway normally comprises a low-water 

 channel and a high-water channel with a bordering floodplain — a difl'er- 

 ence between arid and humid conditions l3'ing chiefly in this, that in arid 

 regions the water tends to spread into sheetfloods rather than to gather 

 into streams, rendering the general surface a floodplain and the channel 

 an aberrant feature. 



The interrelations of moving water are simple, most of the processes 

 well known; the special significance lies in the fact that the stream no 

 less than the animal makes its own bed, the river no less than the man 

 shapes its own valley, by a complex combination of natural interactions. 

 Indeed, the running waters of the hydrosphere as a whole have shaped 



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