WATER SUPPLY OF THE S ALTON BASIN. 105 



by means of water conducted from near Yuma, as above described. 

 Fortunately, the Colorado River water is of remarkably good quality, 

 although this stream flows for hundreds of miles through arid regions 

 and many of its tributaries drain highly alkaline deserts. An exten- 

 sive set of analyses was made by Prof. R. H. Forbes for the period 

 from January 10, 1900, to January 24, 1901," during which time the 

 content in soluble salt of the river water at Yuma varied from 21 to 

 1 25 parts per 100,000, or from 0. 021 to 0. 125 per cent. During the low 

 stages of the river in winter, early spring, and late in summer, the 

 alkali content runs about 90 parts per 100,000. For two months (from 

 May 25 to July 27, in 1900), during the flood caused by the melting of 

 the snows of the Rocky Mountains, less than 27 parts of soluble salt 

 in 100,000 were observed. On the other hand, during a smaller sud- 

 den rise in October, due to torrential downpours on the Arizona water- 

 shed, the alkali content rose markedly, averaging 105 parts per 100,000 

 from September 26 to November 19. This decided increase in the 

 soluble salt content of the water was doubtless occasioned by the wash- 

 ing of salts out of the desert soil into the Arizona rivers and its subse- 

 quent drainage into the Colorado River. During the year 1900 the 

 Colorado River water contained less than 100 parts of salts per 100,000 

 of water during 315 days and more than 100 parts per 100,000 during 

 only 50 days. 



During the growing and fruiting season of the date palm, from 

 April 15 to September 15, inclusive, when four-fifths of the water 

 needful for the whole 3^ear,must be applied, the soluble salt content 

 ranges from 0.021 to 0.068 per cent, or from 21 to 68 parts in 100,000 

 of water; while for two months during the flood, when water is most 

 abundant for irrigation purposes and consequently most easily spared 

 for washing alkali out of the soil, the alkali content is only about 27 

 parts per 100,000, or 0.027 per cent. 



A considerable part of the soluble salts held in solution consists of 

 harmless (if not beneficial) gypsum, which varies but slightly during 

 the year, making up from 5.6 to 8.6 parts per 100,000, which would 

 reduce the harmful alkali content during the summer months to about 

 14 to 60 parts per 100,000, and to 20 parts per 100,000 during the two 

 months of flood in May, June, and July. Such small amounts of 

 alkali in irrigation water are without harmful influence. 



The relatively high purity of the Colorado River water is shown 

 best by a comparison with that used to irrigate the flourishing date 

 gardens of the Sahara. At Biskra the amount of soluble salt varies 

 from 75 to 235 parts per 100,000, and is highest in summer, when 

 the palms need most water. At Chegga, Algeria, the soluble salt 



Forbes, B. H. The River Irrigating Waters of Arizona Their Character and 

 Effects. Bui. No. 44, Arizona Agricultural Experiment Station, Tucson, 1902, p. 202. 



