ee 
. . 
THE RAIN-FALL IN THE GREAT VALLEY. 13 
Valley there is not a single stream which can be called permanent, flow- 
img down the Coast Mountains from one end of the San Joaquin Valley to 
the other. All disappear soon after reaching the edge of the foot-hills, and 
in summer they hardly carry any water at all, even within the mountains, 
Nearly the same is true for the Sacramento Valley ; Puta, Cache, and Stoney 
Creeks, however, are streams of some size ; but even these, as the dry season 
advances, spread themselves out and become lost before reaching the Sacra- 
mento. On the western slope of the Coast Ranges the precipitation is far 
from regular, it being naturally greatly influenced by the lay of the land. 
It is said that the mean of nine years of observations at Pilarcitos Dam, 
about fifteen miles south of San Francisco, gives an average of fifty-eight 
inches of rain, while at the city it is only twenty-three inches and a half.* 
The amount of rain which falls on the east side of the Coast Ranges at the 
extreme southern part of the Great Valley must be exceedingly small. 
Statistics are wanting ; but, judging from the character of the country, it is 
practically a rainless region. Going south from San Francisco, even on the 
west slope of the Coast Mountains, there is a marked decrease in the amount 
of precipitation, the quantity diminishing until we reach Lower California, 
where it is almost null. At San Diego it has diminished to an average 
of about ten inches, and is very irregular. From here south, the climate 
assumes a tropical character, and the rain-fall, which is exceedingly small, 
occurs during the summer season. On the Peninsula of Lower California 
the average for a long term of years would probably not exceed three or 
four inches. 
In the Great Valley itself the rain-fall on the whole is small, except in the 
immediate vicinity of the opening into the Bay of San Francisco. There is 
undoubtedly a marked increase in the amount on any line of cross-section 
in going from the west towards the east, the influence of the Coast Moun- 
tains in cutting off the precipitation being of course less felt as we recede 
from the range. The west side of the San Joaquin and Tulare Valleys, 
when not artificially irrigated, is but little better than a desert. Crops may 
be raised on the eastern side of that river over considerable portions of the 
region in favorable years; but how often such years occur, on the average, 
we have no means of determining. The average amount of rain being small, 
it needs but a moderate addition to this, provided it falls at a favorable 
season, when the crops are just in the right state of forwardness, to enable 
* Report of the United States Irrigation Commissioners. 
