PROBLEMS FROM DEVELOPMENT 53 



acre-feet and has exceeded 100,000 acre-feet in most years 

 since 1924. From 1916 to 1934, the average water level in 

 wells dropped about 1 10 feet; the pumping draft exceeded 

 replenishment in nearly every year, and it was estimated 

 that more than 800,000 acre-feet of water was removed from 

 storage in the ground-water reservoir. About 230,000 acre- 

 feet of this water is considered to have been derived by slow 

 drainage of sil ts and clays, with resulting compaction of those 

 beds to the extent that the land surface was depressed over 

 an area of about 200 square miles, and the subsidence 

 reached a maximum of more than 5 feet in the city of San 

 Jose. 



As early as 1920, water levels were lowered below sea level 

 by pumping, thus providing conditions favorable to en- 

 croachment of salt water from San Francisco Bay. Gener- 

 ally the clay blanket near the bay is sufficiently impervious 

 to protect the underlying ground-water reservoir, but salt 

 water has entered in places where the blanket is punctured. 

 Abandoned wells have been the chief avenues of such en- 

 croachment, but some salt water evidently has entered 

 through the gravels along stream channels. In some areas 

 saline water has moved inland more than 3 miles from the 

 tidelands of the bay. 



The ground-water situation was sufficiently harassing by 

 1934 so that the Santa Clara Valley Water Conservation Dis- 

 trict was formed for the purpose of harvesting waste flood- 

 waters and recharging them into the ground- water reservoir. 

 Records indicated that the runoff wasted to the bay in an 

 average year was about 126,000 acre-feet, or more than half 

 the total stream inflow to the valley. Dams to hold back flood- 

 waters, percolating ponds, baffles in stream beds, diversion 

 canals, and spreading grounds were constructed in 1935 and 

 1936. In the first 5 years of the District's operation, the natu- 

 ral percolation was reported to have been augmented nearly 

 80 per cent by these artificial aids, but more than half the 

 stream flow was still lost to the ocean, chiefly in 1938 when 



