482 TwF.xTv-Eir.HTH Annual Report 



of the four wells are projected on the section. Downstream from 

 this locality a short distance is the Rillito grroundwater-fall, where 

 the water table sinks abruptly from the shallow depth of 18 feet to 

 a depth of 120 feet, in a distance of less than one mile. 



Settlement in the Lower Santa Cruz, or Casa Grande, Valley, 

 has proceeded rapidly, and much groundwater development has' 

 taken place. It is to be hoped that the Gila River floodwaters will 

 soon be stored and made available, for the groundwater supply 

 alone cannot be expected to water the entire area of land already 

 patented or entered upon. 



THE LOWER GILA VALLEY 



Mention was made in the last annual report of the discovery of 

 good water-bearing gravels in the Antelope Valley, which is the 

 section of the Gila Valley from Mohawk to Dome. The good 

 gravels underlie a thick bed of silt and fine sand, in which many 

 costly atteiupts to develop irrigation supplies have proven failures. 

 The logical inference of the deeper drilling was that the Recent 

 valley fill was similarly constituted throughout the whole extent 

 of the Lower Gila Valley. This hypothesis has been confirmed by 

 recent drilling in the valley east of Yuma, though in this location 

 the overlying beds of silt are found to be considerably thicker than 

 near Wellton. 



1'he aggregate potential groundwater supply thus revealed is 

 very extensive and will permit of important development. It is 

 likely, however, that the good gravel stratum is confined to the 

 bottomlands and is not continuous beneath the mesa lands to the 

 north and to the south of the valley. 



Two precautions should be pointed out at the beginning of 

 development of these groundwaters. First, the Gila Valley, in both, 

 soil and water, contains much alkali. The amount present is vari- 

 able, and in some places is beyond the limit of toleration of ordinary, 

 plants.. Consequently, the first well drilled in a new locality should 

 be considered a test well, and an analysis of the water should be 

 obtained before further expenditures are made. In some places 

 where surface water in the river may be had by means of canals or 

 by pumping for a considerable part of the year, groundwater with 

 75, possibly with as high as 100. parts of salt ]ier 100.000 may be 

 used during the months when the river is dry. 



The other precaution was referred to in the last report. A defi- 



