264 



ENGINEERING ON THE FARM 



Over-irrigation. Where water is abundant there is a 

 tendency to use an over-supply on the crops. Canal leak- 

 age and over-irrigation create a tendency for tracts of land 



After U. S. Reclamation Service 



Fig. 220. The Arrowrock Dam, near Boise, Idaho 



otherwise arid to become swampy. The seepage from 

 over-irrigation and from large supply ditches passes through 

 pervious strata to the lower lands, causing them to become 

 swampy. Continuous irrigation frequently causes lands to 

 become impregnated with alkali. In order to relieve the 

 land of surplus moisture or an accumulation of alkali, exten- 

 sive drainage systems have been installed. Where the land 

 is underdrained, the application of the water on the surface 

 dissolves the alkali and carries it through the soil into the 

 drains and washes it away. The drainage of irrigated lands 

 is based upon different conditions from the drainage of lands 

 saturated with rainfall for the reason that the water supply 

 is coming entirely through the underground strata and is 



