The Eastern Snake Plain Aquifer: Curtailment and Settlements

If you live far from the Snake River plain, the story still matters. The same legal tools, a call, a methodology, curtailment, mitigation, are in the Conjunctive Management Rules, which apply wherever surface water and ground water are connected. The plain is where they are tested hardest.

The aquifer and the river

IDWR's public FAQ on its 2024 order describes the Eastern Snake Plain Aquifer (ESPA) as roughly Lake Erie-sized, supplying drinking, irrigation, business and industrial water across the Magic Valley and eastern Idaho. The same FAQ says more than 5,500 wells draw from it, and that the aquifer has been declining since the 1950s and is overdrawn by about 200,000 acre-feet a year. It also says that in 2023 aquifer levels and spring discharge were at record lows. A 2018 news report on the Water Board's recharge program gave the same overdraft figure. These are IDWR's statements and one news report, not independent measurements, but they match each other. USGS and the Water Board publish the underlying water level data.

The rules: Conjunctive Management Rules, IDAPA 37.03.11

The state's procedure for a delivery call that involves ground water is set in IDWR's rules, adopted under the ground water authority in Idaho Code 42-237a. That statute lets the director supervise and control ground water rights, find areas that share a common ground water supply, and incorporate such an area into a water district. It states that water in a well is not "available" to fill a right if the withdrawal would affect, contrary to the policy of the act, the present or future use of a prior surface or ground water right.

Rule 41.03 has a dated consequence worth keeping straight. Rule 41 governs designated ground water management areas, and its September 1 date applies there. The Surface Water Coalition call runs under the delivery call procedure of Rule 40 and the court-approved methodology, and its curtailment orders have come in season, in May 2024 and May 2026. Read each order to see which rule and which date it relies on.

A delivery call, from 2005 to 2026

IDWR's summary of the Surface Water Coalition (SWC) delivery call says canal companies and irrigation districts that deliver Snake River water to about 550,000 acres formed the SWC in 2005 and filed a delivery call against junior ground water users, arguing that ESPA pumping was depleting the river. Junior users formed the Idaho Ground Water Appropriators, Inc. (IGWA) with member ground water districts. IDWR says those junior users irrigate nearly 1,000,000 acres. A court-approved methodology began evaluating water supply and crop needs each year to measure injury to the SWC. IDWR says it has been revised six times, most recently in 2023.

The legal record sits in IDWR docket CM-DC-2010-001, which holds the orders from 2010 to the present.

The 2026 season, as documented

The July order, as IDWR's text states it, found that snowpack and runoff were low (April to July forecast at 70 percent of average, June forecast at 50 percent), temperatures were above average and spring precipitation below average. Crop water need for April 1 to June 30 was 657,039 acre-feet, 137 percent of the 2000 to 2025 average. The shortfall by entity in the July order:

The December 2025 and earlier history helps place 2026. IDWR's overview records that the director issued a curtailment order on May 30, 2024 for rights junior to March 31, 1954, finding a shortfall of 74,100 acre-feet for Twin Falls Canal Company, and that curtailment orders followed in 2025, including a December 2025 order curtailing rights junior to August 15, 1952. The 2026 cutoff of October 11, 1900 reaches much further back, because, per the July 2026 order, it matches the most senior SWC natural flow right predicted to be injured, Twin Falls Canal Company right 1-209. A priority date earlier in time is senior; curtailing "to" a date means rights junior to it, meaning later, are stopped.

The June 30, 2015 settlement

The Surface Water Coalition members (A&B, American Falls Reservoir District No. 2, Burley, Milner, Minidoka, North Side Canal Company and Twin Falls Canal Company) and the IGWA members signed a settlement agreement dated June 30, 2015. IDWR's FAQ says it was brokered by then Lieutenant Governor Scott Bedke, and that IDWR approved it as the 2016 Stipulated Mitigation Plan. Terms from the agreement text:

• Reduced pumping. Starting in 2016, annual ground water diversions are to drop by 240,000 acre-feet. Each ground water or irrigation district carries a share, or offsets it through equivalent private recharge (section 3.a).
• Measurement. Flow meters are to be on all remaining unmeasured ground water diversions by the start of the 2018 irrigation season, phased in over three years in an order the parties set (section 3.d).
• Benchmarks. Ground water level benchmarks are staged for April 2020, April 2023 and April 2026, aiming at a goal equal to the 1991 to 2001 average aquifer levels (section 3.e). If benchmarks are missed, adaptive measures such as added recharge or further cuts apply.
• Storage water. 50,000 acre-feet a year of storage water from private leases (section 3.b), and a minimum of 110,000 acre-feet of near-term water for 2015 (section 2.a).
• Recharge. The parties will support a state-sponsored managed recharge program averaging 250,000 acre-feet per year across the ESPA (section 3.f).
• Conversions. IGWA will pursue ground water to surface water conversions above American Falls, with the Blackfoot area as a preferred site, with 2015 conversion funding capped at $1.1 million (sections 2.a.iv and 3.h).
• Term. The agreement is perpetual and binds successors (sections 7 and 8).

The agreement, then, is a private settlement made enforceable by approval of the director as a mitigation plan. It is not a statute. Districts that join it are covered under Rule 42.02. IDWR's 2024 FAQ says some districts instead sought to mitigate under the 2009 Storage Water Delivery Plan. In 2024, nine entities were found in compliance and four were not: Bingham, Bonneville-Jefferson, Jefferson-Clark and Carey Valley Ground Water Districts, and IDWR curtailed users in the non-compliant districts from the May 30, 2024 order. The FAQ itself is slightly inconsistent about how many districts met the 2016 plan, so use the order for the exact list.

Managed recharge

The Idaho Water Resource Board runs a managed recharge program to add water to the ESPA. A 2018 news report says the Board pays canal companies and irrigation districts, per acre-foot, to move water from the Snake, Big Wood and Little Wood rivers through canals and settling basins where it seeps into the ground, under the Board's winter water rights that must be used by mid-April or the start of irrigation season. That report said the Board announced on February 19, 2018 that it had passed 250,000 acre-feet for the winter, with 317,000 acre-feet recharged the previous year, and a goal of an average of 250,000 acre-feet a year. Recharge depends on surplus water, so it is large in wet winters and small in dry ones.

IDWR's Water Board page notes an Environmental Resources Technical Working Group that reviews effects of recharge on aquatic life, recreation and water quality, with meetings from October 2018 through a scheduled October 28, 2026 meeting. Look for volume data on the Board's managed recharge data hub. I did not find current annual recharge totals on the page I checked, so I give none.

Expanding the area: 2024 to 2026

The area of common ground water supply has grown. IDWR's summary says a 2024 law expanded the ESPA area effective July 1, 2024, adding about 1,400 water rights. IDWR orders then brought the Little Lost River Basin (November 26, 2025), the Raft River Basin (December 15, 2025) and the Big Lost River Basin (January 13, 2026) into the area. On February 2, 2026 the director found 1,005 ground water rights in the expanded area could be subject to administration in the SWC call, judged the injury to be delayed or long range, and phased in administration: parts of the Big Lost (up to Mackay) and Little Lost (at Howe) are subject in 2026, and the rest, with the Raft River Basin, not until April 1, 2027. The phase-in follows the five-year limit idea in Rule 40.01. If you irrigate from a well in those basins, read the February 2 order and Attachment A.

Questions and answers

What is a "call"?

A call is a request by a senior user for the state to regulate juniors so the senior's right can be filled. In the SWC case the senior users are canal companies and districts, and the juniors are ground water pumpers.

Who decides whether injury is occurring?

The director of IDWR, using the methodology and the Rule 42 factors, and the order can be reviewed in court. IDWR's page says the methodology was court approved in 2010.

If I am curtailed, what are my choices?

Join a ground water district or other entity covered by an approved mitigation plan, or stop diverting. For listed rights in a May 2026 order, some had 15 days to join a district. The order itself gives the terms, so read your order.

Does curtailment affect domestic wells?

The orders I reviewed concern listed irrigation and other rights. I did not verify how exempt domestic wells are treated in them. Check the order's list and ask the regional office.

Why curtail to 1900 when the shortfall is bigger than the yield?

Because curtailing to a date is a way of regulating by priority. The model in the April order predicted 94,000 acre-feet from curtailing to October 11, 1900, against a 181,600 acre-foot shortfall. The order explains the date as matching the most senior injured right. Whether the remaining gap is met by storage, mitigation or is left unfilled is in the order.

What is a water district and a watermaster?

A water district is an organized area where a watermaster distributes water by priority. Lesson 02 explains the structure. The Conjunctive Management Rules route a delivery call through the watermaster.

Where does the money for recharge come from?

The Water Board's program is funded through state appropriations and other sources. I did not verify current amounts; check the Board's budget documents.

Is the settlement permanent?

The agreement says it is perpetual and binds successors. It has benchmarks in 2020, 2023 and 2026 and adaptive measures if they are missed. Watch for orders and filings on whether the April 2026 benchmarks were met; I did not verify the result.

Hard questions

"Why should a farmer who followed the rules lose water to someone with an older right?"

Prior appropriation gives the older right first call on the water. Idaho's constitution and statutes build the system that way, and everyone who bought land with a junior right bought it knowing the date. The counterpoint is practical: many junior users developed their land under state permits, invested in wells and pivots, and employ people. The Conjunctive Management Rules weigh both through the Rule 42 factors and phase-ins, and the settlement is the parties' attempt to avoid curtailment by negotiated cuts.

"Does the aquifer really connect to the river, or is that a theory?"

IDWR and the courts treat it as established, and the state's computer model of the aquifer is used in the orders. Some junior users have challenged specific modeling choices and the methodology in court. Read the court opinions in the docket for the exact arguments. This lesson does not take a side in those disputes.

"If the shortfall is hundreds of thousands of acre-feet and curtailment yields less, who bears the gap?"

The senior users bear the part that is not met, and they may use storage and carryover under their own plans. The 2026 numbers show it: a 741,100 acre-foot shortfall and a projected 78,500 acre-feet from continued curtailment over the last quarter of the season. Whether the system is fair is a question of values. The arithmetic is in the order.

"Will cities and rural subdivisions be pulled in as the area expands?"

The area of common ground water supply has been enlarged by statute and orders, and administration in some new areas was phased in to April 2027. Whether a particular city or subdivision right is subject depends on its priority date, location and mitigation coverage. Domestic exempt uses are discussed in lesson 05. Ask the IDWR regional office about a specific parcel before assuming either answer.