90 CONSERVATION OF GROUND WATER 



of the rock materials under the irrigated areas. Irrigation 

 caused a considerable enlargement of Mud Lake, 30 miles 

 northwest of Idaho Falls, which in 1900 was an intermittent 

 pond never larger than 100 acres. By 1921, the lake had 

 grown to 17,500 acres, with an additional 10,000 acres of 

 swampy land around it. The lake and shallow ground water 

 that forms it are perched more than 200 feet above the re- 

 gional water table and have been built up largely by per- 

 colation from irrigation on the Egin Bench 30 miles away, 

 which was first irrigated in 1905. In recent years wells have 

 been drilled to put this surplus ground water to use, with 

 such success that the available supplies now may be over- 

 developed. 



Near Twin Falls, water was first diverted for irrigation in 

 1905. Prior to that time, the water table on the south side 

 of the river had probably been more than 200 feet deep 

 under most of the area. It rose as much as 25 feet a year in 

 the first years of irrigation, and, until 1928, it was still rising 

 several feet a year. Early in the 1920's many areas had be- 

 come waterlogged, and drains and tunnels were constructed 

 to keep the water table down. Rights have been established 

 to the use of this drainage water for additional irrigation. 

 It is estimated that about 6 million acre-feet was added to 

 permanent ground- water storage from 1906 to 1928. In con- 

 trast, the water levels in wells north of the river from Milner 

 to Bliss have risen only a few feet since irrigation was begun, 

 although about 600,000 acre-feet of surface water is applied 

 each year. Studies have indicated that the excess on that side 

 percolates westward through permeable rock materials and 

 is discharged from springs in the Hagerman Valley of the 

 Snake River. 



In southern Idaho there are several other places where 

 the rocks underlying the irrigated area are impermeable 

 enough to create drainage problems, including parts of the 

 Boise, Weiser, and Payette Valleys and the Mountain Home 

 area. There are also many areas where the underlying rocks 

 are so permeable that the principal effect of irrigation has 



