RIVER DEPOSITS AND FLOODS. 161 



fluenciiig the whole system of dependent brooks and rivers. The na- 

 ture and shape of the detritus — whether fine sand or earth, smaller or 

 larger rock masses, stones, roundish, square, or flat — cause much difter- 

 ence; and this in turn depends upon many conditions, geological and 

 climatical. 



According to the nature of the rock from which it is deriA'ed, the 

 detritus appears in different shapes, and again changes in form dur- 

 ing its further transportation by the waters in diU'erent ways, and 

 therefore exerts n varying influence npon the run. Thus the detritus, 

 which appears in large plates or shales, is carried more easily than the 

 square or round rocks; the former, even when deposited, hinders the 

 flow of water between the plates but little, and therefore gives less 

 cause for stow water than the heavy square rocks, which resist the 

 transportation and obstruct the flow more efl'ectuall^'. 



Sand and gravel detritus is easily carried, easily accumulated, and 

 again with a new flood easily removed; it otters, therefore, little resist- 

 ance to the flow of water, but becomes objectionable in filling the lower 

 channels of rivers, etc. 



Clay detritus, although easily carried, is apt to compact and cement 

 the rock detritus together, and thus becomes one of the worst impedi- 

 ments of water flow and is the cause of the worst dangers from flood 

 waters. 



Fr(mi tlu'so exani])les it is apparent that two rivers, although under 

 similar conditions of rain fall, i)hysical conditions of soil and topog- 

 rai)hy, may yet show different behavior, according to varying char- 

 acter of the detritus. 



As we have stated in the beginning of this chapter, the underground 

 drainage gives to rivers their permanent regime, while the surface 

 riiii off gives to them their rapid variations of stage, 



According to whether a river is mainly supplied by surface waters 

 that riin iu trackless courses and open runs over the slopes into its 

 bed, or wliether it is supplied by underground waters, its water condi- 

 tions must follow a different course, fitful in the first, even in the second 

 case. Few rivers depend on one of these sources of sui>ply alone; 

 Oiost of them are supplied in the different parts of their course by both, 

 so that a streanj nuiy begin as a torrent and later in its coarse find 

 additional supplies from springs or ground wat<n's — or the reverse may 

 take place, being originated by a spring, it may have no other addi- 

 tions except from surface waters, 



The greater the proportion of the supply by surface run -off the more 

 lialde to disturbances must be the flow. Finally in a larger river, like 

 the Ohio or Mississippi the question of floods becomes still farther 

 com]>licated. For here not cmly the regime of the main river, but also 

 that of all its affluents and the topographic, stratigraphic, climatic, and 

 surface conditions of their catchment basins become elements of dis- 



12411— No. 7 11 



