116 IRRIGATION INVESTIGATIONS IN CALIFORNIA. 



The Sierra foothills as they merge into the valley have an elevation of about 100 



feet above tide. The watershed rises gently in rounded and broken mountains to the 



crest of the Sierras, which at the headwaters of the Yuba has a mean elevation of 



about 8,200 feet, with peaks rising to 9,100 feet. From Mount Lincoln, a point 



common to the watersheds of Yuba, American, and Truckee rivers, to some 2^ miles 



northeast of Mount Webber, the summit of the Sierras divides the watershed of 



Yuba River from that of Truckee River, which discharges into Humboldt Basin. 



Farther on beyond Mount Webber there is a secondary crest which divides the 



watersheds of Yuba and Feather rivers, the watershed of the latter stream reaching 



farther east to a less elevated divide, in which the passes are lower than those in the 



easterly crest. 



PHYSICAIi FEATURES AND GEOLOGY. 



The western and lower portions of the drainage area are slates and kindred rock, 

 very much eroded and merging into gravel and alluvial deposits of the great valley 

 of California. The upper portions are principally lava and granites. All are deeply 

 eroded, particularly the slates and lavas. Some idea of the magnitude of these 

 erosions is gained when it is considered that the great valley of California has 

 required at least 4,000, and possibly 6,000, cubic miles of denuded materials to fill it 

 to its present level, most of which has come from the Sierras. 



A stratum of serpentine traverses the watershed of Yuba River in a direction 

 generally parallel with the crest of the Sierras. It is intersected by the North Fork 

 at Goodyears Bar, by the Middle Fork near Moores Flat, and by the South Fork just 

 east of Washington, and leaves the drainage basin of Yuba River and passes near 

 Towles station, on the Central Pacific Railroad. 



This stratum is generally softer and more easily eroded than adjoining strata. 

 The canyons of the various forks are therefore upon lighter grades through it than 

 immediateh' above and below, and the canyons are generally wider. This softer 

 material also controls the loci of longer and more deeply eroded tributaries, which 

 afford approaches to the main canyons for roads and trails. 



This stratum is of further interest sines it is the dividing line between the 

 auriferous strata in the watershed. To the west of it the mines are more extensive 

 and more reliable, the occurrences of gold-bearing rock to the east being irregular 

 and difficult to trace. 



The topographic features of the drainage basin of Yuba River and those of 

 adjacent basins can not be understood without a brief outline of certain occurrences 

 in the geological history of the region. 



The Sierras as a mountain range antedate Tertiary time. The drainage lines 

 and features of much of the early topography north of the Tuolumne River were 

 blotted out during the Tertiar}' era by an overflow of lava, basaltic in some instances, 

 but principally mud lava in vast sheets, carrying bowlders of all sizes and shapes, 

 coVjbie.s, and gravel. The features established by this overflow can now be traced 

 only partially, as far into and through these sheets Glacial and recent erosions have 

 carved a new set of channels and established a new set of topographic features. 

 Upon the disappearance of the Glacial period modern rivers carved out new channels 

 upon lower lines than the early livers and intersecting these latter at variable angles. 

 Being lower, they have afforded the outfall for the hydraulic mining opei'ationa 



