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Friday, October 11 • 11:00am - 11:30am
Watering Idaho: The Eastern Snake Plain Aquifer and Conjunctive Administration

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Idaho is a prior appropriation state that administers junior ground water rights conjunctively with senior surface water rights, resulting in the curtailment of junior ground water pumping that causes material injury to senior surface water users. Conjunctive administration has occurred, primarily, within the boundaries of the Eastern Snake Plain Aquifer (“ESPA”).
The ESPA is approximately 170 miles long and 60 miles wide as defined by the United States Geological Survey Professional Paper 1408-F, 1992. IDAPA The ESPA is predominately in fractured Quaternary basalt having an aggregate thickness that may, in some locations, exceed several thousand feet, decreasing to shallow depths in other locations. The ESPA fractured basalt is characterized by high hydraulic conductiveness, typically 1,000 feet/day but ranging from 0.1 feet/day to 100,000 feet/day. The presence of interbedded sediments, a volcanic rift zone, and less permeable basalts in lower hydraulic conductivity in some areas of the aquifer. Notable areas of lower hydraulic conductivity are in the vicinity of Mud Lake and in the Great Rift zone, which extends north to south across the plain from the Craters of the Moon to just west of American Falls Reservoir. These zones of lower hydraulic conductivity impede the transmission of water through the aquifer. In two areas of note, ground water is in direct hydraulic connection with the Snake River: (1) American Falls Reservoir; and (2) the Thousand Springs.
Based on the period 1980-2008, the ESPA receives approximately 7.7 million acre-feet of recharge on an average annual basis through: (1) incidental recharge associated with surface water irrigation on the plain (5.3 million acre-feet); (2) underflow from tributary drainage basins (1.1 million acre-feet), (3) infiltration of precipitation on non-irrigated lands (0.7 million acre-feet); and (4) seepage losses from rivers and streams (0.6 million acre-feet). During this time period, average annual discharge from the ESPA exceeded annual average recharge by approximately 270,000 acre-feet, resulting in declining aquifer levels and declining discharge to hydraulically connected reaches of the Snake River and tributary springs.
Using Modflow, and in collaboration with stakeholders, the Idaho Department of Water Resources (“IDWR”), developed a calibrated ground water model to predict the impacts of pumping from the ESPA and benefits of curtailment at hydraulically connected reaches of the Snake River and tributary springs.
Since 2005, and in response to conjunctive management delivery calls filed by senior surface water irrigators that rely on reach gains in and around the area of American Falls Reservoir, and senior spring users that rely on spring discharge in and around the Thousand Springs area, IDWR has curtailed ground water pumping for the benefit of the senior users. These curtailments have included domestic, commercial, municipal, and industrial ground water rights.
1 Technical information regarding the Eastern Snake Plain Aquifer is sourced from the Idaho Department of Water Resources’ administrative orders in the Surface Water Coalition and Rangen, Inc. conjunctive management delivery calls.


Chris Bromley

McHugh and Bromley
Chris Bromley is a founding member of McHugh Bromley, PLLC, a law firm focusing on water law and administrative law. Through his practice, Chris represents water users in front of the Idaho Department of Water Resources (“IDWR”) in matters involving water rights and water administration... Read More →

Friday October 11, 2019 11:00am - 11:30am MDT