Idaho ESPA Could Soon Include Surrounding Tributaries

by: Sidnee Hill

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The Idaho Department of Water Resources (IDWR) is proposing to begin expanding the borders of the Eastern Snake Plain Aquifer Area of Common Ground Water Supply (ESPA) to include the five surrounding tributary basins. “This is an issue of fairness," IDWR Director Mathew Weaver explained at a public hearing on March 24 in Moore, Idaho. “One of the fundamental issues on the Eastern Snake Plain is that those groundwater users [currently in the ESPA] have been mitigating for impacts from the tributary basins and that has been a fundamental unfairness … They know they’re being forced to participate when others that have a physical impact on the river but are on the other side of that imaginary administrative boundary have not.” The five new tributaries proposed in the current expansion include the Big Lost, Little Lost, American Falls, Portneuf and Raft tributaries.

Ultimately, IDWR’s plan will be to annex all the tributary basins not currently under a management plan into the ESPA over the next 10 years. Weaver’s decision to begin with only these five is attributed to a combination of administrative water districts already in place and their impacts on the Snake River. “We are on a multiyear effort here,” the director stated, “to create water districts in the tributary basins and to bring those basins into administration.” According to data provided by the IDWR, the 22 tributary basins surrounding the current boundary of the ESPA are responsible for 10%, or 287,000 acre-feet of average annual consumptive groundwater use within the Snake River Aquifer.

Authority to expand the ESPA administrative boundary into these tributary basins was given to the director of IDWR in the 2024 legislative session under Idaho Code 42-233c(2). As outlined in this code, public hearings were held in each of these tributary basins, and public comments were received via email and postal until April 7.

The announcement of this proposed expansion comes at a time of deep concern and confusion regarding the management of the ESPA. One year ago in April 2024, IDWR announced a forecast shortfall of 74,100 acre-feet of water availability to senior surface right irrigators due to the decreasing level of the Snake River Aquifer. At this same time, the ESPA’s groundwater districts, regional organizations of groundwater pumpers, were found to be out of compliance with their mitigation plans. Mitigation plans are multifaceted water reduction and conservation plans adopted by groundwater pumpers and approved by IDWR to buy safe harbor from curtailment in shortfall years such as 2024.

Subsequently, thousands of groundwater rights were curtailed for five days until a one-year emergency settlement was reached between the Surface Water Coalition and the Groundwater District. In February 2025, a four-year settlement between nine groundwater districts and the Surface Water Coalition (the 2024 Stipulated Mitigation Plan) was approved by IDWR and is the latest effort to precariously balance irrigation water needs against Idaho’s constitutionally protected prior appropriation water rights system.

The management of the ESPA has been a decades-long discussion and research focus. It began in the mid-1980s when the Idaho Supreme Court ruled that groundwater pumpers with junior water rights in the ESPA have a physical impact on senior surface water right holders who currently farm cumulatively over 544,135 acres across the Magic Valley. Further, the Idaho Supreme Court dictated that the director must manage both the surface and groundwater of connected systems together, or conjunctively. While Idaho is not the only state that practices conjunctive water management, it is rare and comes with its own unique set of challenges – measuring the movement and levels of below-ground bodies of water being chief among those challenges.

Currently, the IDWR measures the impact of groundwater irrigation use on the Snake River with their ESPA 2.2 model. While Weaver affirmed in the March 24 public hearing at Moore that the IDWR is utilizing one of the most robust collections of groundwater research and data for their model and decision, many of the groundwater users would like to see more regionally specific research to verify if their Snake River impacts match what is represented in the ESPA 2.2 model. Such a study is already underway in the Big Lost Basin and is expected to be completed in December 2025.

Matthew Anders, the technical services bureau chief at IDWR, was on hand at each public hearing to answer questions about the complexities of underground hydrologic systems. He explained that it is not just about the physical movement of the water molecules through the aquifer to the Snake River, though it certainly includes that as well. “It's also about pressure,” he explained. “As more water is included in the aquifer, it adds pressure that facilitates water movement into the near Blackfoot to Minidoka reach of the Snake River, which is the critical water supply to the Surface Water Coalition.”

Urban development across the ESPA was another hot topic item at the public hearing, with farmers expressing their concern that the impact of domestic groundwater consumptive use was not being fairly considered. Brian Patton, deputy director of IDWR, mentioned that approximately 3% to 5% of the total groundwater consumptive use in the ESPA is attributed to domestic use. Domestic groundwater use refers to single-family residences where the total daily groundwater use is less than 13,000 gallons per day on a total lawn irrigation of less than 0.5 acre. Currently, Idaho statute exempts domestic groundwater use from IDWR’s ESPA adjudication, and until legislation is passed, domestic and stockwater use cannot fall under their ESPA administration.

The final looming question at each public hearing was, “What next?” If included into the ESPA common ground water supply, ground water users in these annexed tributaries will be part of IDWR’s ESPA adjudication and subject to the water calls by senior surface water right holders. To prevent curtailment based on these water calls, groundwater users will be given a grace period to “buy safe harbor” by joining or creating an IDWR-approved mitigation plan. In the past, most ESPA ground pumpers have decided to form or join groundwater districts for cost savings and to ease the administrative burden of mitigation. However, joining a groundwater district is not a requirement.

The director will review public comments made at each public hearing, as well as those submitted afterwards, before making the final decision on whether the five proposed tributary basins will be annexed into the ESPA. While a deadline for the decision has not been announced, Weaver indicated he hopes to have a statement issued before the end of the year.