54 THE U. S. GEOLOGICAL SURVEY 



With these limitations in mind, it may be stated that stream 

 measurements covering a period of years are now available 

 for 4,000 points on the streams of the country. The collec- 

 tion of data as to the quality of surface waters has made less 

 progress. Reports including data on the quality of the sur- 

 face waters in nearly every state have been published, though 

 only six reports cover the quality of surface waters of entire 

 states, and these reports consider only the principal streams. 

 In general, it may be said that most of the principal streams 

 of the country have been investigated for their quality, but 

 these streams constitute a very small percentage of the total 

 surface water supplies of the country. 



Ground Waters. The location of ground waters is essen- 

 tially a geologic problem and is determined principally by 

 geologic methods. It involves an examination of the strata 

 in the region explored and the determination of the position, 

 character, and capacity of those that are water-bearing. 

 These geologic methods give results that are in the nature of 

 forecasts. Drilling is necessary to determine with certainty 

 the yield, head, and quality of the waters sought, but until 

 the fiscal year 191 7-1 8, the Survey did no test drilling for 

 this purpose. The appropriation act for that year provided 

 that of the $175,000 appropriated for water resources inves- 

 tigations, $25,000 might "be used to test the existence of 

 artesian and other underground water supplies suitable for 

 irrigation in the arid and semiarid regions by boring wells." 

 It will be noted that this provision includes only supplies suit- 

 able for irrigation. Under this authorization the Survey has 

 begun drilling in a desert area in Nevada where the geologic 

 conditions are believed to be favorable, and has already dis- 

 covered a large supply of water that will be available for 

 irrigation. 



The determination of the quantity of ground water in any 

 given area is difficult. In some areas it is practicable to 

 measure the flow of the ground water by electrical apparatus 

 inserted in test holes or to make estimates based on the ob- 



