272 GROVE KARL GILBERT— DAVIS [Uraoin; iviJ!m 



of rivers on such plains, had built up the land surface near their courses to a significantly 

 higher level than that of the shallow "lateral basins" in the plain between the rivers. 

 Farms near the mountains were buried under barren shrouds of sand and gravel ; towns near the 

 rivers had to build levees to protect themselves from the advancing deposits; the danger of 

 floods was heightened with the aggradation of the river channels; the deltas in the bay heads 

 were extended and the bays near their heads were shoaled. 



The amount of debris artificially added to the Sierran rivers increased rapidly from the 

 beginning of hydraulic mining in 1850 to its peak in 1884. Then with the injunction against 

 hydraulic mining, it decreased still more rapidly, and in 1900 was less than in 1860. As a result 

 of the decrease, the rivers proceeded to scour out their canyons, some of which are now laid 

 bare down to bedrock again; but the scour from the canyons caused aggradation to continue 

 for a time on the piedmont fans ; then when the canyon scour was nearly completed, the fans 

 were in turn attacked by their rivers and trenched to depths of 20 or more feet, and thus some 

 of their detritus was washed forward to the "valley rivers"; however, a much larger fraction 

 of the hydraulic debris still remains in the wide sectors of the aggraded fans to the right and left 

 of the new-cut trenches than remains in the narrow canyons within the mountains. The debris 

 from the fan trenches continued the aggradation of the "valley rivers" for a time, but now they 

 also are degrading their middle courses; aggradation continues at present only in the lower 

 courses and on the deltas. 



The downstream movement of a great body of debris is thus analogous to the downstream movement of a 

 great body of storm water, the apex of the flood travelling in the direction of the currents .... It travels in a 

 wave, and the wave grows longer and flatter as it goes. 



In the case of the Yuba River, an important branch of the Sacramento system — 



the apex of the debris flood, leaving the mines in 1S83, passed the mouth of the mountain canyon in about the 

 year 1900 and the mouth of the Yuba River [in Feather River, a main branch of the Sacramento] in about 1905. 



The capacity of the "valley rivers " to wash along the debris that they receive has been increased 

 in their middle courses by the construction of levees which restrain them to a moderate width 

 at time of flood, and in their lower courses by the reclamation of bay-head delta marshes across 

 which the rivers are now confined by dikes to relatively narrow channels of rapid flow. 



DEPOSITS IN SAN FRANCISCO BAT 



Who could have expected, when the investigation of the hydraulic-mining debris was 

 begun, that it would have to include a study of the changes in San Francisco Bay and in the 

 tidal bar outside of the Golden Gate! Yet by following the advance of the debris farther and 

 farther along its course, Gilbert showed most clearly that, contemporaneously with, and there- 

 fore presumably in consequence of, the deposition of debris in the head branches of San Fran- 

 cisco Bay, a shoreward shift of the bar has taken place. His argument proceeds as follows: 

 The facts concerning the bar are first assembled. It is semicircular in plan, curving with a 

 radius of 5 miles around a center in Golden Gate; it is formed of detritus detached from the 

 ocean coast, close along which it is drifted by waves and currents, but on reaching the Gate it is 

 held offshore by the ebbing tide. The depth of water on the bar is now 6 fathoms ; in the Gate, 

 50 fathoms. Possible changes in the bar are next inquired into. If there were no ebb tide, the 

 bar would stretch directly across the entrance to the Gate as a shoal or beach, in fine with the 

 coast beaches to the north and south, and San Francisco Bay would be closed; hence if the 

 present tidal currents in the Gate are weakened, the curve of the bar ought to diminish in radius 

 and the depth of the bar ought to decrease. The relation of tidal currents to bay-head debris 

 is then examined. In so far as the debris merely shoals the bay it has no effect on tidal currents 

 in the Golden Gate; but in so far as it raises the bay bottom above low-tide level in deltas and 

 mud flats, it must weaken the tidal currents; and in so far as delta marshes are reclaimed for 

 agriculture by building dikes to exclude high-tide overflow, the tidal currents at the Gate must 

 be still further weakened. 



