202 WJMCGEE OUTLINES OF HYDROLOGY 



Gilbert is once more engaged in a notable investigation of the capacity 

 of streams for load of given size of particle, and is soon to take up loads 

 of variable particles ; yet there is thus far no formula for the capacity of 

 streams for loads of the widely variable particles found in nature. In 

 the lower Colorado the load may exceed 5 per cent of the volume or 10 

 per cent of the weight of the water; in some of its tributaries, when in 

 freshet, the load may temporarily reach 20 per cent of the volume, while 

 in the sheetfloods of Arizona and Sonora the load occasionally increases 

 until the mass becomes a mere mud-bed, gradually coming to rest as the 

 water is partly evaporated and partly absorbed into the earth below. 

 Manifestly the capacity of a stream for solid matter of variable dimen- 

 sions and weights varies with the velocity much more widely than does 

 the competence. In a general way it may be safe to average the effective 

 competence (that is, the mean difference between the rate of movement of 

 the saltatory particles and that of the stream) at the fourth power, and 

 the capacity for load of variable sizes at something over the competence 

 or sixth power of the velocity; or, since the velocity increases geomet- 

 rically with the declivity, at something like the seventh or eighth power of 

 the declivity. 



Still less is there any formula, or even consistent opinion, expressing 

 that sum of the positive mechanical work of running water (washing, 

 eorrasion, suspension, and saltation) which may be denoted efficiency, or 

 of the variability in that work attending increase of load; though, mani- 

 festly, washing and eorrasion must within undefined limits increase with 

 the load. Powell, who raised LyelFs doctrine of uniformism to a highei 

 order and in framing the laws of hydraulic degradation outlined hydro- 

 dynamic geology, held that the effectiveness of stream-work must vary 

 approximately as the weight of water and load combined; while Gilbert, 

 whose experiments promise again to revolutionize constructive geology, 

 inclines to regard the carrjdng of load as commensurate with the expendi- 

 ture of a corresponding part of the total energy of the stream, thus re- 

 ducing the measure of transportation to an equation of volume and 

 velocity of the water alone (though it seems probable that this equation 

 should be modified at least to the extent of adding the gravity component 

 and momentum of the saltatory particles, and also a factor for lubrica- 

 tion — an extreme case arising in landslides in which the water acts 

 almost wholly as a lubricant). In a general way the efficiency must vary 

 geometrically with the exercised capacity, that is, the load — say as the 

 second power of the capacitjr, or, if reduced to actual stream conditions, 

 the eighth or ninth power of the declivity. 



