32 



FIELD OPERATIONS OF THE BUREAU OF SOILS, 19V1. 



onl}' a little above the channel of the stream. An nndei-lyino^ 

 stratum of gravel and sand permits of the ready movement of under- 

 ground water, and there is generally a high water tal)le. The poor 

 drainage of the bottoms is further accentuated by seepage from irri- 

 gation upon adjacent terraces or uplands and by the underground 

 flow from tributary creeks and drainage ways. The latter is of im- 

 portance in numerous places, but is especially pronounced at the place 

 where the McClellan Wash enters the Gila River bottoms. Drainage 

 is also poorly developed locally in the shallow and imperfect drain- 

 age ways of the desert. Here the gradient is low, and the run-oif 

 and percolation much retarded. The greater surface slope and more 

 pervious subsoils of the bottoms make the drainage more perfect than 

 in the case of the low, flat desert areas of old valley-filling soils, with 

 their slight slope and dense compacted subsoil. The high water 

 table of the bottoms is often relieved by open ditches and sometimes 

 by tile drains. Little attempt has been made to provide drainage 

 for the broad flats of the desert, cultivation being confined to the 

 better drained soils. 



ALKALI. 



The Middle Gila Valley area is situated in an arid region of 

 excessive evaporation, where soluble salts are readily accumulated. 

 More than 50 per cent of the area is more or less affected by alkali 

 accumulation. The following table shows the results of an analysis 

 of a composite sample of alkali crust taken in various parts of the 

 survey : 



Chemical analysis of alkali crust. 



[Parts per 100,000.] 



The above re.sults show that sodium chloride, sodium sulphate, and 

 sodium carbonate predominate in the alkali of this area. Sodium 

 chloride, or common salt, is by far the most abundant, composing a 

 little more than 25 per cent of the average crust. The sodium sul- 

 phate constitutes a little over 12 per cent, while the proportion of 

 sodium carbonate, commonly known as black alkali, is less than 5 per 



