academy op sciences] HYDRAULIC MINING DEBRIS 271 



I have been up to Bear river and driven a buggy horse three days — trying to find what the river is doing 

 witli the mining debris, and looking for a dam-site for debris storage. Not much result and the work was rather 

 trying to me. The weather changed from hot to hotter — 110° in the shade the last day — and I was too exhausted 

 to continue. I met the sea breeze at Suisun and it was delicious to feel chilly once more. 



It appears to have been during these excursions in the uplands of the Sierra that Gilbert's 

 attention was turned, and this time successfully, to a problem which, under the title, "The 

 convex profile of bad-land divides," was left unsolved in his report on the Henry Mountains 

 30 years before. In certain areas of the uplands where the auriferous gravels had been hydrau- 

 licked off, a body of decomposed granite was revealed which had been given a sort of bad-land 

 form by the action of the weather; its close-spaced stream lines were sharply incised, but its 

 little divides were pronouncedly convex, and their convexity was explained as the result of the 

 creeping of a thin surface soil. He gave the explanation thus found a general application : 



Convex hill tops are found alike in forested regions, prairies and deserts. . . . Soil creep is omnipresent 

 and appears to be competent. ' 



In contrast to its immediate predecessor, the second report is unusually readable. It 

 abounds in entertaining matter tersely expressed; it is exceptionally instructive by reason of 

 the frequent introduction of quantitative results; it is beautifully illustrated with halftone 

 views, very pertinent to the text ; it is full of wise and practical counsel which should lead to 

 conservational measures in the near future ; and it rises to a dramatic climax, for its farthest- 

 reaching conclusion is that hydraulic minin g debris should be controlled in the mountains 

 because its unrestricted outwashwill in time decrease the depth of water on the tidal bar outside 

 the Golden Gate, although the bar is nearly 30 miles from the river mouths at the head branches 

 of San Francisco Bay, and from 100 to 200 miles from the mines; and a decrease of depth on 

 the bar would diminish the volume of San Francisco commerce. Some of the leading topics 

 are here briefly summarized. 



GRAVELS OUTWASHED UPON THE VALLEY PLAIN 



The report assumes a knowledge of the auriferous gravels on the Sierran uplands: They 

 are heavy, stream-laid piedmont deposits of a former time, which aggraded the lower ground 

 along the western border of what was then the Sierran belt after the mountains of an earlier 

 cycle of erosion had been worn down to moderate or small relief, and before the relatively recent 

 and rapid, slanting uplift of the present range, in response to which the rivers have as yet incised 

 only narrow and steep-sided canyons in the uplifted mass. The gravels have a thickness of 

 from 100 to 300 feet, and to-day constitute high-level "flats" or open upland "valleys," 

 adjoined by rounded hills and drained by comparatively sluggish streams which, on their way 

 to join the incised rivers, cascade through short gorges and so descend into the canyons 500 

 or 1,000 feet below the uplands. Before the advent of the gold seekers, the rivers flowed down 

 their canyons on beds of solid rock and coarse boulders ; their normal load of finer debris was then 

 less than their capacity. But after the introduction of hydraulic mining, chiefly on the tribu- 

 taries of the Sacramento, the "tailings" were washed from the gravels of the uplands into the 

 canyons, where much of the overload was deposited. The river beds were thereby aggraded 

 and the narrow canyon bottoms were built up in gravelly flood plains, thus slightly diminishing 

 the fall of the upper courses, but. increasing the river fall toward the mountain border. On 

 emerging from the mountains the rivers had decreased power and the debris that was not left in 

 the canyons was there in part added to the preexistent piedmont alluvial fans, which constitute 

 a modern analogue to the upland gravels and which grew in some cases to a thickness of 50 or 

 more feet; and in part washed forward to the larger "valley rivers" of the fluviatile plain, and 

 swept down these rivers to the head branches of San Francisco Bay. Aggradation became the 

 order of the day, and as the total 35-year output of the hydraulic washings, or "mines," to 

 streams of the Sacramento system alone is estimated at 1,295,000,000 cubic yards — about seven 

 times the volume of excavation in the Panama Canal — the aggradation had very visible and 

 threatening effects; all the more so because the rivers of the fluviatile plain, following the habits 



' The convexity of hilltops. Journ. Geo!., ivii, 1909, 344-350. 



