Somach Simmons & Dunn | Attorneys at Law
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Disputes between senior surface water users and junior ground water users in eastern Idaho are nothing new. As covered in prior SSD alerts from June 2024, November 2024, and August 2025, few years pass without some form of drama involving the conjunctive management of surface water and ground water in the Eastern Snake Plain Aquifer (ESPA).
This year is no exception. This alert provides an update on recent developments and the challenges that water users and the Idaho Department of Water Resources (IDWR) face in 2026.
Surface Water Coalition
The seven entities comprising the Surface Water Coalition (SWC) hold senior water rights in the Snake River near the City of Twin Falls with priority dates from circa 1900. Since 2005, the SWC’s ongoing “delivery call” has resulted in IDWR regulating junior ground water rights in an effort to prevent injury to the SWC’s water rights. Since 2010, IDWR has done this by applying its “Methodology Order” that predicts/determines the SWC members’ annual supplies and demands at the beginning, middle, and end of each irrigation season; if there is a “demand shortfall” at any time, then IDWR requires some or all ground water users—whose priority dates range from the early 1900s to the 1990s—to either mitigate their injurious impacts or face curtailment.
Ground Water Districts
The vast majority of ESPA ground water use is by irrigators that are represented by the several Ground Water Districts (GWDs) spanning the Plain. The Idaho Legislature authorized the establishment of GWDs in the mid-1990s to collectively mitigate their members’ pumping impacts to senior surface water rights. Since 2009, many ESPA GWDs (shown below) have been subject to various approved “mitigation plans” that provide protection from curtailment.
Map showing the SWC members (bottom left) and the original GWDs to the 2024 Plan (throughout).
2023: IDWR Switches to Transient Modeling
Following a hearing in spring of 2023, IDWR issued a revised Methodology Order styled the “Sixth Methodology Order.” Many of the changes were controversial, but perhaps the most controversial was IDWR’s switch from using “steady state” model simulations to using “transient” model simulations to determine curtailment dates. “Steady state” modeling assumes the ESPA is in a state of quasi-equilibrium and Snake River reach gains will increase over time from continuous curtailment; conversely, “transient” modeling assumes Snake River reach gains will increase within a given irrigation season based on short-term curtailment. IDWR purportedly made the switch to prevent or minimize injury, by maximizing reach gain increases, in years where the SWC has a demand shortfall.
To illustrate the impact of this change: before 2023, ground water users with rights senior to 1977 were never on the curtailment list, even if the predicted in-season demand shortfall was as great as 170,000 acre-feet (AF). Since 2023, all ground water users across the ESPA have become susceptible to curtailment, including those with 1901 water rights, even when the predicted shortfall is less than 100,000 AF.
2024: IDWR Approves Stipulated Mitigation Plan and Expands ACGW
In spring of 2024, the Idaho Legislature authorized IDWR to expand the “area of common ground water” (ACGW) boundary, thereby authorizing IDWR to curtail ground water pumping from the basins tributary to the Snake River, as shown below.
Map showing expansion of ACGW, with the two Lost River basins at the top-center.
In fall of 2024, IDWR approved a new stipulated mitigation plan for the existing ESPA GWDs that replaced a prior plan approved in 2016. This “2024 Plan” has an initial four-year term that ends after 2027. In exchange for receiving protection from curtailment, the plan requires the GWDs to conserve ground water by an average of 205,000 AF per year (AFY) and secure (for potential delivery to the SWC) 75,000 AF of storage water each year.
Paragraph 10 of the 2024 Plan authorizes the GWDs from the tributary basins made susceptible to IDWR’s curtailment jurisdiction—through the expansion of the ACGW boundary—to join the plan as a party. New GWDs would be responsible for a share of the storage water obligation (which remains fixed at 75,000 AF) and a share of the conservation obligation (which would increase above 205,000 AF).
2025: A&B Challenges As-Applied Methodology Order
In summer of 2025, IDWR issued a mid-season as-applied Methodology Order that predicted the SWC’s injury to be 75,300 AF. Because transient model simulations predicted that curtailment of all ground water users for the rest of the season would not fully offset the predicted injury, for the first time, every ESPA ground water user was subject to mitigation requirements or curtailment.
A&B Irrigation District (A&B), the sole SWC member that also holds ground water rights, challenged the order. A&B, who supported the switch to transient modeling in 2023, suddenly had to mitigate the impacts resulting from its pumping under a very large 1948 ground water right (1,100 cubic feet per second). A&B, who IDWR determined was responsible for providing 5,039 AF of mitigation water, argued that IDWR should employ a different method to quantify ground water users’ proportionate shares of a predicted demand shortfall. In January 2026, however, an IDWR hearing officer dismissed A&B’s challenge based on res judicata because A&B had previously (and unsuccessfully) litigated the issue at the spring 2023 hearing. IDWR’s Director affirmed that ruling on April 17, 2026.
2026 Steps 1-3 Order
On April 16, IDWR issued its initial 2026 as-applied Methodology Order. It predicted a combined demand shortfall of 181,600 AF to the Twin Falls Canal Company (TFCC) and American Falls Reservoir District No. 2 (AFRD#2) and accordingly set a curtailment date of 1900, therefore subjecting all ground water users to mitigation requirements or curtailment. This is a huge demand shortfall and reflects the dry conditions and low snowpack.
The Perils of Adding New GWDs to the 2024 Plan
On April 13, the original GWDs asked IDWR to approve the addition of the Big Lost and Little Lost GWDs to the 2024 Plan. The SWC opposed. The stated reason was disagreement over the technical data relied upon to establish the new GWDs’ conservation obligations. As events developed, it became clear the actual disagreement revolved around A&B’s effort to secure approval of its own stipulated mitigation plan. To understand this, we take a brief digression to highlight A&B’s story.
A&B
A&B is a Bureau of Reclamation project with the “A” unit operating on surface water from the Snake River and the “B” unit relying on ground water from its 1948 (and other) water rights. On behalf of its ground water users, A&B secured a mitigation plan in 2015 that tied its mitigation obligation to the “injury” determination under the Methodology Order. When the Methodology Order used “steady state” modeling to determine curtailment dates, A&B’s obligations were infrequent and small; since adopting “transient” modeling, however, A&B has been subject to significant mitigation requirements, like this year. A&B has approached its problem in two ways: first, by challenging the method by which IDWR quantifies ground water entities’ proportionate shares of a demand shortfall, as described above. Because A&B was unsuccessful in this effort, its 2015 mitigation plan is still in place and, under the initial 2026 as-applied Methodology Order, it currently owes 10,800 AF of storage water this year.
A&B’s second effort appeared in the form of a “new” stipulated mitigation plan, filed on March 24, that would fix A&B’s mitigation obligation at 120 AF in 2026-2027 rather than tie the obligation to predicted demand shortfalls under the Sixth Methodology Order rubric. This would allow A&B to mitigate on the SWC’s terms rather than IDWR’s terms. The GWDs have opposed the request, arguing that there is no valid basis for A&B to effectively get a free pass from mitigating, especially compared to the GWDs’ obligations under the 2024 Plan.
A&B and the Addition of the “Lost” River Basin GWDs to the 2024 Plan
Following the GWDs’ April 13 request to add the Lost River Basin GWDs, the SWC moved to dismiss it and asked IDWR to immediately curtail the new GWDs because they were not subject to an approved mitigation plan. At the hearing convened by the Director on April 15, the SWC offered that it would not oppose a stay on curtailment of the Lost River Basin GWDs so long as the GWDs did not oppose a stay on curtailment of A&B while its new stipulated mitigation plan was subject to a contested case, which seemed to suggest the stated concerns over technical data were merely pretextual. The GWDs rejected this offer, and the Director issued a ruling that ordered curtailment of the Lost River Basin GWDs while the alleged data concerns were resolved by a technical working group.
This ruling was a shock to Lost River Basin users that were expecting to be able to turn on their pumps once the irrigation season commenced on April 1 since they agreed to join the 2024 Plan as contemplated. However, cooler heads prevailed because, on April 17, the GWDs and SWC reached a stipulation to stay curtailment in the Lost River Basins while they worked cooperatively to resolve disputes over the technical data and the GWDs’ opposition to A&B’s stipulation, among other things. The two sides jointly requested that IDWR approve the stipulation and stay curtailment orders in those basins. IDWR granted the request that same day.
The squabbles detailed above are ongoing, although seem to be on a positive trajectory. Unfortunately, due to water supply conditions, all water users—members of the SWC and GWDs alike—will continue to face challenges in 2026.
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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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