424 EXPERIMENT STATION RECORD. 



"As suggested by the maximum percentages of sediment, the silting up of reservoirs 

 and ditches would be greatest under the Gila River, next greates^t under the Colorado, 

 and least under the Salt River, for the years of observation." 



The percentages by weight of sediment in the total run-off of the 3 rivers for the 

 year periods of observation were, approximately, in the Gila, 2 per cent; in the Colo- 

 rado, 0.30 jier cent; and in the Salt River, 0.21 per cent; but the sediments in Salt 

 River for this time were much less than normal. 



"The sediments of the Colorado are probably in larger part directly erosive in 

 character, coming mainly from the canyon and bad-lands country. The excessive 

 sediments of the Gila &re in large part the sweepings from overstocked grazing 

 ranges, and the detritus from valleys whose erosion is the result of overgrazing. 

 The comparatively small amounts of sediments observed in Salt River are in part 

 due to the reservations which protect the upper Salt River watershed. The color, 

 sjiecific gravity, fineness, flocculence, and mineral nature of river sediments are all 

 traceable to watershed conditions. The Colorado is notable (part of the year) for 

 its dense, highly colored sediments and the Gila for its flocculent, more usually 

 black sediments, sometimes exceedingly rich in organic matter. The Salt River 

 sediments more nearly resemble those of the Gila in i^hysical character, but are 

 percejatibly coarser. 



"The specific gravities of solid sediments were observed to vary in Salt River 

 from 2.249 to 2.703, averaging 2.604; in the Gila River from 2.580 to 2.766, averaging 

 2.672; and in the Colorado River from 2.581 to 2.694, avei'aging 2.652. 



"The bulk of mud in flood waters, as compared with that of solid sediments, was 

 observed to vary, after from 10 to 30 days' settling, in Salt River from 5.6 to 7.5 times 

 as great, averaging 6.2 times; in the Gila River from 5 to 8.3 times as great, averag- 

 ing 6.7 times; and in the Colorado River from 4.5 to 9.5 times as great, averaging 6.2 

 times. These mud volumes are subject to slow contraction, during long periods of 

 time, amounting to 20 per cent and upward. 



"The fertilizing value of these sediments mainly depends upon the nitrogen and 

 organic matter they contain. Potash and phosphoric acid were also estimated, the 

 former being already abundant in our desert soils, and the latter l)eing in slowly 

 available, insoluVde form in the river waters. All fertilizing materials are most 

 abundant in time of flood, greatly decreasing at time of low water. . . . 



" Under the agricultural conditions which obtain in southern Arizona, it is judged 

 that 100 parts of salts in 100,000 of water, or about 2,700 lbs. per acre-foot, is an 

 objectionable but manageable degree of salinity in an irrigating water. On this 

 basis," Salt River, during 369 days, was above the limit 302 days, below 67; the 

 Gila, during 187 days, was 138 days abov^., 59 days below; the Colorado River, 

 during 380 days, was 50 days above, 330 days below. 



"The character of the soluble salts of the Salt, Gila, and Colorado rivers is usually 

 'white alkaline' in excess, consisting mainly of chlorids and sulphates of sodium and 

 calcium. The Salt and Gila rivers only, during some but not all floods, have been 

 observed to contain an excess of sodium carbonate, or black alkali, evidently swept 

 into the drainage from overlying watersheds." 



The conditions favoring the accumulation of alkali in the surface soil and the 

 control of the rise of alkali and the distribution of the alkaline accumulation by 

 flooding, drainage, and deep and thorough cultivation are briefly explained. 



A soil study. IV, The ground water, W. P. Headden {Colorado Sta. Bui 72, 

 pp. 47)-— 'in continuation of previous investigations (E. S. R., 13, p. 830) the author 

 studied the ground water with special reference to soluble salts. Summarizing his 

 results he says that " alkalization in Colorado resolves itself into a question of drain- 

 age" and is aggravated by overirrigation. 



" Crops growing on alkalized soil with the water table quite near the surface were 

 sensitive to droughty conditions. . . . 



