Somach Simmons & Dunn | Attorneys at Law
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SSD Environmental Alerts in 2023 and 2024 reported on the Idaho Department of Water Resources’ (IDWR) administration of the Surface Water Coalition’s (SWC) delivery call on Eastern Snake Plain Aquifer (ESPA) groundwater users. This Alert reports on IDWR’s administration of the SWC delivery call in 2025 and the status of an appeal pending before the Idaho Supreme Court regarding the current iteration of the “Methodology Order” that is IDWR’s tool to administer junior ESPA groundwater pumping.
As a refresher, the SWC is a group of canal companies and irrigation districts with senior surface water rights in the Snake River near Twin Falls, Idaho. Since 2005, IDWR has regulated junior ESPA groundwater pumping based on various agency orders related to the SWC delivery call (as well as district court and Idaho Supreme Court decisions). Since 2010, IDWR has regulated ESPA groundwater pumping under the Methodology Order, as amended.
As previously reported, the IDWR Director (Director) issued an order in summer 2024 that curtailed unmitigated ESPA groundwater users with rights junior to March 31, 1954. Until then, water rights senior to March 14, 1971, had never been curtailed due to the SWC delivery call. However, a larger pool of groundwater users has become vulnerable to curtailment because the Fifth Amended Methodology Order (the current iteration, issued in April 2023) requires IDWR to utilize “transient” groundwater modeling simulations—rather than “steady state” simulations—to determine curtailment dates. The rationale is that utilizing transient simulations better redresses any SWC injury, as predicted/determined by the Director, within that irrigation season because curtailing to an early priority date is likely to yield more water to the Snake River for the benefit of the SWC.
On July 25, 2025, the Director issued an order curtailing unmitigated ESPA groundwater users with rights all the way back to October 11, 1900, which is the priority date of Water Right No. 01 209, Twin Falls Canal Company’s (TFCC, an SWC member) right to divert 3,000 cfs of natural flow from the Snake River. The Director determined the early curtailment date is necessary because it would “offset as much of the [predicted injury of 75,300 acre-feet] as possible.” Practically speaking, virtually every groundwater right across the ESPA is now vulnerable to curtailment in dry years unless they obtain a new (or join in an existing) IDWR-approved mitigation plan.
Another new development is IDWR’s proposed expansion of the ESPA “Area of Common Ground Water Supply” (ACGWS) boundary to include basins tributary to the Snake River in which ground water pumping affects the ESPA. In the 2024 session, the Idaho Legislature enacted Idaho Code section 42-233c, which modified the then-current ACGWS boundary and authorized the Director to further modify it. The current ACGWS boundary, as well as the various tributary basins, are shown in the map below.
In March 2025, the Director held public hearings to present information and solicit feedback from stakeholders on IDWR’s proposal to include the Big Lost, Little Lost, American Falls, Portneuf, and Raft Basins in the ESPA ACGWS. The presentations at these hearings explained that all tributary basins collectively account for roughly 14 percent of the consumptive use of groundwater that is hydraulically connected to the ESPA, as shown below.
The five tributary basins mentioned above may be officially added to the ESPA ACGWS before the 2026 irrigation season. If so, ground water users in these basins will join the ESPA ground water users in facing a mitigate-or-be-curtailed dilemma next year.
The curtailment risk that junior ground water users in the ESPA (and soon the tributary basins) find themselves facing can be laid at the feet of the Fifth Amended Methodology Order. In addition to formalizing the Director’s decision to utilize transient modeling, the order also amended (or, alternatively, failed to amend) the method by which IDWR predicts TFCC’s supplies, and the irrigated acreage numbers that IDWR uses when determining TFCC’s demands, in a given year. A group of Cities across the ESPA challenged the Fifth Amended Methodology Order on the basis that the Director failed to use the “best available science and information” when amending the Methodology Order. A key issue in the case is what legal standard applies when a party presents evidence that the “science and information” underlying the amended Methodology Order is not, in fact, the “best available.”
Under Idaho law, the Director’s final order imposing the terms of the Fifth Amended Methodology Order must be first appealed to the district court. The district court affirmed the Director’s determinations in May 2024, despite the order’s deficiencies, and the Cities appealed to the Idaho Supreme Court. The case has been fully briefed and set for oral argument on October 6, 2025 (City of Idaho Falls v. IDWR, #52102). Max Bricker will present argument for the Cities.
The Cities’ appeal, if successful, could impact IDWR’s management of the SWC delivery call by reducing the magnitude of the Director’s injury determinations (by requiring IDWR to use better science and information when predicting/determining SWC’s water supplies/demands), which in turn impacts the Director’s curtailment date determinations. This will help to ensure that ESPA groundwater users, such as the Cities, are not erroneously curtailed due to the SWC delivery call in the near- or long-term.
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Somach Simmons & Dunn represents the City of Pocatello and the American Falls-Aberdeen Ground Water District in legal matters including, but not limited to, those described herein. This alert reflects the observations of the authors and is not intended to limit or otherwise describe the views of our clients. Somach Simmons & Dunn provides the information in its Environmental Law & Policy Alerts and on its website for informational purposes only. This general information is not a substitute for legal advice, and users should consult with legal counsel for specific advice. In addition, using this information or sending electronic mail to Somach Simmons & Dunn or its attorneys does not create an attorney-client relationship with Somach Simmons & Dunn.
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