Extensive irrigation from the Snake River and its tributaries began in the late 1800s on the eastern Snake River Plain. Gravity irrigation systems typically divert more than twice the amount of water necessary to meet crop requirements (Goodell, 1985). The remainder of the water returned to the Snake River or infiltrated to the aquifer. The incidental recharge from the approximately 900,000 acres of surface water irrigated land resulted in increased elevation of ground water levels. The volume of water stored in the aquifer, as shown in the above graph, increased by about 15 million acre-feet between 1915 and 1955. On the average, 340,000 acre-feet of water were being added to aquifer storage annually during this period. Spring discharge, especially in the Thousand Springs reach, also increased dramatically during this period due to higher water levels. Cumulative discharge in the Thousand Springs reach increased from about 4,800 cfs in 1915 to about 6,800 cfs in 1955. An infrared image of the plain shows the approximate extent of irrigated lands in the early 1990s for areas that appear in red adjacent to the Snake River.
In the mid-1950s, irrigation technology on the eastern Snake River Plain began to change. An increased water use efficiency resulted from completion of additional surface water storage facilities, water conservation programs, and, probably most importantly, increased use of sprinkler irrigation. Surface water diversions for irrigation began decreasing in the early 1970s (see right graph). The increased efficiency of the system led to decreased ground-water recharge that has contributed to the decline of ground water levels and spring discharge. In addition, ground-water withdrawals for irrigation increased dramatically during the last half of the century. About 800,000 acres of ground water irrigated land have been brought into production since the 1950s. At an average estimated irrigation demand of 1.8 acre-feet/acre, the total aquifer withdrawal is about 1.5 million acre-feet/year. The combined effects of decreased recharge from surface water irrigation and increased ground-water withdrawals, along with weather variation, are apparent in the declines in ground-water storage and spring discharge since the mid-1950s. The average rate of decline in ground-water storage between 1975 and 1995 is about 350,000 acre-feet/year. Changes in the collective discharge of springs in the Blackfoot to Neeley and Kimberly to King Hill reaches of the Snake River are shown by a hydrograph.