30 BULLETIN 502, U. S. DEPAETMENT OF AGEICULTUEE. 



necessary to put in occasional braces to hold tliem. The drainage of 

 this tract could have been better accomplished by immediate drainage 

 of the entire affected area, which includes several acres on the west, 

 but the owner of the latter tract was not ready at that time. While 

 the tract is not yet free from alkali, the surface has become fairly 

 dry and the rushes have disappeared ; however, the road on the west 

 side of the tract still remains impassable. 



EXAMPLE IV. 



Twenty years ago the farm shown in figure 6 was considered one 

 of the best in the vicinity of La Junta, Colo. The gradual rise of the 

 water table and subsequent accumulation of alkali salts have seriously 

 damaged 700 acres of land in this vicinity. Much of it is not farmed 

 at all and none of it yields profitable crops. 



The tract is located in a circular basin which contains approxi- 

 mately 3,000 acres, all of which is irrigated. Just below this tract 

 the basin narrows down to a draw which bears north about If miles 

 to a creek. This draw is the only feasible drainage outlet. There are 

 two irrigation canals running through this district. More than 78 

 second-feet are furnished to the land in this basin. Water runs dur- 

 ing the entire year in one of the canals. The irrigation season is 

 very long, and many people practice winter irrigation, which con- 

 tributes largely to the damaging seepage water. The quantity of 

 water used is no doubt considerably over 4 acre-feet per acre. 



The soil varies from a fine sandy loam to a sandy adobe. Gypsum 

 is very abundant. It occurs in partly disintegrated crystals and 

 flakes, often very nearly pure an 1 imparting a mealy character to 

 the soil. This constituent seems to have been derived largely from 

 decomposition of the shales. A layer of gypsum always was found 

 just above the shale and it seemed to carry water freely. On the 

 south half of this tract, near the surface, is found a. series of alter- 

 nating beds of limestone and calcareous shales. The shales are very 

 compact, and borings showed that while the upper layers, which had 

 partly disintegrated and contained gypsum crystals, usually were 

 moist, they became very hard and dry with depth. The water-carry- 

 ing capacity of these shales may be considered as negligible. 



The idea is prevalent among the people of this section that the 

 trouble is due to loss from the irrigation canals and that the proper 

 method of drainage is to construct an intercepting line just below 

 the canal. While these canals doubtless contribute to the under- 

 ground water, most of it is due to loss from laterals, to failure to take 

 care of waste water, and to the use of excessive quantities of irriga- 

 tion water. The injury due to excessive irrigation lies not only in 

 the swamping of large areas of lower lands, but also in the ruin of 

 large tracts from the accumulation of alkali salts on the surface. 



