PROBLEMS FROM DEVELOPMENT 89 



lamette Valley, which is well supplied with surface water 

 and does not draw heavily upon its ground-water reservoir. 

 Current draft from wells is estimated to be at the rate of 

 about 60,000 acre-feet a year. 



The principal area of ground-water replenishment is in 

 the south half of the valley. About one-third of the annual 

 precipitation usually falls in December and January. In an 

 average winter about 500,000 acre-feet goes into the ground- 

 water reservoir. The draft from wells takes only a small part 

 of this, and the rest is discharged to streams and by evapo- 

 transpifation during the year. This discharge is accom- 

 panied by a gradual recession of water levels in wells from 

 April until the following December. 



Snake River Plain, Idaho. 32 The ground-water reservoir 

 of the Snake River Plain is geologically composite. At many 

 places it is in basalt, which commonly is so permeable that 

 in the natural regimen it was drained readily by the Snake 

 River. That river is deeply entrenched in the plain at most 

 places below Minidoka Dam, and the water table before 

 irrigation development was hundreds of feet below the land 

 surface. Above Minidoka, in some reaches, the river was 

 perched above the regional water table. 



The use of the Snake River for irrigation on the Snake 

 River Plain has had a variety of effects upon storage in the 

 ground-water reservoir, depending upon the permeability 



s 2 References: Stearns, H. T., Lynn Crandall, and W. G. Steward, Geology 

 and Ground-water Resources of the Snake River Plain in 

 Southeastern Idaho, U.S. Geol. Survey Water Supply Paper 

 774, 1938, 268 pp. 



Stearns, H. T., L. L. Bryan, and Lynn Crandall, Geology and 

 Water Resources of the Mud Lake Region, Idaho, U.S. Geol. 

 Survey Water Supply Paper 818, 1939, 122 pp. 



Nace, R. L., "Preliminary Report on Ground Water in Mini- 

 doka County, Idaho, with Special Reference to the North 

 Side Pumping Division of the Minidoka Project," October 

 1948, 71 pp.; in U.S. Bur. Reel. Proj. Planning Rept. 1- 

 5.53.1-1, Mimeo., April 1949. 



