658 PROCEEDINGS OF THE CORDILLEBAN SECTION 



Diomeutarily, so as to make short leaps; they constitute the bottom load. The 

 smaller are lifted far from the bottom, are sustained for long periods, and are 

 distributed through the whole body of the current; they constitute the sus- 

 pended load. The classification of load in each particular cause depends 

 chiefly on velocity of current ; a relatively swift stream will include in its 

 suspended load relatively coarse detritus. 



Many determinations have been made of the suspended load of streams, the 

 method including (1) the measurement of the total quautity of water, and (2) 

 the measurement of the ratio of load to water in samples taken from the 

 stream. The determination of the bottom load is comparatively difficult, and 

 there is much- need of information as to its relative and absolute amount. 

 The writer has taken advantage of an unusually favorable opportunity to 

 measure the bottom load of the Yuba river, one of the tributaries of the Sacra- 

 mento river in California. 



The river issues from a mountain gorge at the Smartsville narrows, and 

 thence to its mouth, a distance of 20 miles, traverses a valley. In consequence 

 of a special condition brought about by hydraulic mining, it is now aggrading 

 its channel in this part of its course. The growing deposit is of coarse gravel 

 near the narrows, and of sand near the mouth, with gradual transition. Dur- 

 ing high floods the suspended load near the mouth of the river includes silt 

 only ; near the narrows it probably includes sand also, and possibly some small 

 gravel. 



In the sunnner of 1004 a dam 6 feet high was built at a point 5 miles below 

 the narrows, where the channel is l.ijOO feet wide ; and the basin created by 

 this dam was filled with detritus by the floods of the following winter and 

 spring. In the summer of 1005 an addition was made to the dam. increasing 

 its height 8 feet, and during the construction of this addition a contour map 

 of the channel above the dam was made b.y the U. S. Geological Survey. The 

 river remained low and the water clear until the following January, when a 

 large flood occurred, lasting about one week. After the flood had passed the 

 channel was resurveyed, and the two maps were afterward used to determine 

 the quantity of detritus arrested. The character of the material deposited 

 was investigated by means of pits and borings in the summer of 1906. During 

 the flood the river was gaged to determine its discharge, and at the time of 

 flood maximum the water was thoroughly sampled near the river mouth to 

 determine the percentage of suspended load. 



The discussion of the data thus obtained is not yet complete, but certain 

 results of geologic bearing may be given at this time. All of the figures are 

 approximate and subject to correction. The detritus which passed the mouth 

 of the canyon during the flood of January, 1006, would cover a square mile to 

 a depth of about 17 inches. The ratios of its chief components were : Silt, 24 ; 

 sand, 24; gravel, 52. In this statement whatever sand was deposited in the 

 interstices of the gravel is accounted with the gravel ; by transferring it to 

 the sand category the ratios are changed to : Silt, 24 ; sand, 34 ; gravel, 42. 

 Near the river mouth, where the suspended load consisted of silt and the bot- 

 tom load of sand, the ratio of bottom load to suspended load was about 1 : 1. 

 Near the narrows, if the silt only be accounted as suspended load, its ratio to 

 the bottom load was as 1:3; but if. as is pvobable, the sand also should be 

 classed with the suspended load, the ratio is about 1 : 1. 



