E. W. HILGARD — THE CIENEGAS OF CALIFORNIA. 125 



If we imagine the structure that must result from such a mode of accumulation of 

 a debris-fan, the spi iradic appearance and peculiar localization of the cienegas 1 1 >eing 

 the points at which the water fed into the coin' at the mouth of the canon is forced 

 near tu the suface either by a cross ridge or by the termination of a water-bearing 



cobble-'bed underlain by an impervious layer) is easy to understand. But it is also 

 obvious that the continued supply of water from the stream into the various old 

 channels of the debris cone must depend upon the maintenance of the open gravel 

 surface at the apex of the cone. When this is wholly or partially closed, whether 

 by natural or artificial processes, then, the source of supply being stopped, the 

 springs or artesian wells dependent upon it must diminish or cease to flow. Such 

 variations and stoppages have already been experienced at several points, and as 

 they may prove very costly, if not disastrous, to heavy investments already made, 

 it is quite important that the need of keeping the area of infiltration open for the 

 winter floods should be fully understood by the populations concerned. When 

 this is attended to it is obvious that we have here natural storage reservoirs for Hood 

 waters, annually replenished and likely to be fully refilled each season, no matter 

 how heavy may have been the drafts made upon them during the preceding irri- 

 gation season. 



The most extensive example of del iris-fan storage of flood waters thus far known 

 to me occurs in the upper San Bernardino valley, at the head of which two large 

 streams — the Santa Ana river and Mill creek — emerge from narrow canons, at 

 whose outlets there are truly phenomenal accumulations of huge bowlders, which 

 in time of Hood are tossed about by the torrents with a thundering noise sometimes 

 audible miles away. Here are many square miles of open cobble surface, into 

 which flood waters can be and are absorbed with the greatest ease, although in the 

 usual channels of the summer flow the bottom is made sensibly waterproof by finer 

 sediments. Costly tunnels have been driven through these cobble-beds under the 

 impression that large amounts of water could be thus collected; but while the con- 

 stant drip proves the perviousness and absorbent nature of the deposit, that very 

 circumstance prevents the gathering together of any very large supply of water 

 in the relatively insignificant areas of the artificial drifts. 



From the head of the d6bris-fan of Mill creek to its base, near the town of San 

 Bernardino, the distance i> between 12 and 14 miles, according to the initial point 

 chosen; the fall of the surface within the same distance is between 600 and 700 

 feet. The average width of the valley is about 10 miles, and artesian borings have 

 shown the gravels and cobble to be nearly a thousand feet in thickness within a 

 mile of the southeastern edge. This enormous gravel mass, filled with water from 

 the floods of the two streams, forms a natural reservoir of such magnitude that the 

 drafts thus far made upon it by the numerous boreholes sunk in the lower valley have 

 failed to show an v such degree of mutual interdependence as is usually observed in 



wills situated short distances apart — a fact which I have ascertained by experi- 

 mental measurements made under proper conditions. This relative independence 

 of the flow of contiguous wells also indicates that the water-bearing stratum con- 

 sists of gravel so large and so open that the water mass may be considered as exert- 

 ing its pressure rather freely in all directions; yet on reopening a closed well there 



always exists a material accumulation of pressure, which takes several hours to 



recede In its normal amount . 



besides the artificial outlets mentioned, however, there is a number of natural 

 outlets on the slope of this greal gravel reservoir. The mosl conspicuous is the 



