academy of sciences] HYDRAULIC MINING DfiBRIS 273 



The actual conditions are then determined. The quantity of detritus washed into the bay- 

 head branches between 1849 and 1914 from natural sources of supply in all parts of the drainage 

 basin as well as from hydraulic mining in the Sierra is estimated at 1,146,000,000 cubic yards — 

 a quantity almost as great as the total outwash from the mines between 1850 and 1884 — and 

 this has sufficed to fill some 13,000 acres of bay-branch heads above low-tide level. The diminua- 

 tion of the bay area thus brought about is calculated to reduce the tidal volume in the Golden 

 Gate by 2.5 per cent of its measure in 1S50, while the reduction due to the reclamation of tide 

 marshes must amount to an additional 1.5 per cent. The total reduction of the tidal currents 

 from their former value must therefore be about 4 per cent, and it is thought that such a reduction 

 should have a recognizable effect in the shifting of the semicircular tidal bar. The argument 

 closes by an examination of the position of the bar as shown by Coast Survey charts in 1855, 1873, 

 and 1900. The discovery is thus made that in each interval between surveys the crest of the 

 bar migrated landward, the total change being nearly or quite 1,000 feet; but no loss of depth 

 was demonstrated. As the expectable loss of depth would be only about half a foot for a shoreward 

 shift of 1,000 feet, the failure to detect so small a change is not held to invalidate the conclu- 

 sion that the recent inward migration of the bar is actually due in large part to deposition of 

 hydraulic-mining and other debris in the bayheads, and that the remaining part is due to rec- 

 lamation of the marshlands. Could there be imagined a more beautiful enchainment of argu- 

 mentation, or a more convincing demonstration of the principle announced in the Henry 

 Mountains report years before, regarding the "interdependence of drainage lines"! 



The conclusion thus reached is very properly taken as the basis for an outlook into the 

 future. Further change from hydraulic-mining debris is not expected, because such mining 

 is not likely to be permitted again unless the tailings are impounded in the mountains; 

 but further change in consequence of reclamation of delta marshes is quite another matter, for 

 this process still continues and bids fair to exclude the tides from all the marsh areas ; and when 

 that is done, the tidal discharge at the Golden Gate will be reduced to about two-thirds of its 

 original amount. The bar may then be shifted shoreward by a considerable distance with a 

 troublesome decrease of depth. The question is thus raised whether a local community on the 

 delta marshes should make a large addition to agricultural wealth at the cost of a moderate 

 injury to a great commercial harbor; here agriculture, which was successful in its conflict with 

 mining, will become antagonistic to commerce ; and on this point Gilbert remarks : 



That agriculture in its entirety is the industry of first importance is recognized by all, but it does not follow 

 that commerce should yield to it at every point of interference. Each particular case of conflicting interest 

 involves an economic problem in relative values and should be adjudged on that basis. 



QUANTITATIVE PHYSIOGRAPHY 



It must appear from the foregoing paragraphs that the report here summarized represents 

 one of the most remarkable physiographic studies ever accomplished. In its quantitative 

 aspects it is probably unparalleled. The amount of work that it involved was enormous ; every 

 step taken was carefully analyzed. Although the observational work on which it was based 

 was in large measure finished before Gilbert's illness in 1909, most of the facts concerning San 

 Francisco Bay appear to have been determined at a later date; all the statements regarding 

 the transporting action of streams- were also critically reviewed in the light of the laboratory 

 experiments, after the period of incapacity through illness. One of the principles concerning 

 transportation most frequently quoted is to the effect that although a river fully charged with 

 debris can not continue to carry its load if its slope be decreased while other conditions 

 remain unchanged, it can be so if its width be at the same time artificially reduced ; in other words 

 a graded, or even an aggrading stream may be changed into a degrading stream by narrow- 

 ing its channel. 



The quantitative values that appear all through the report were determined with great 

 care. They are not precise, but they may be accepted as close to the truth. For example, 

 the debris deposited by the "valley rivers "since 1850 is considered in two parts, one here called 

 normal and coming from the ordinary processes of erosion; the other here called artificial, coming 



