🏛️ Issues and Advocacy Training
Courses › Water, Land and Agriculture

Lesson 3 of 9 · about 18 minutes

One Water: Conjunctive Management and the Eastern Snake Plain Aquifer

You'll be able to

  • Explain why pumping a well can shrink a river miles away
  • Read a curtailment order's priority date and tell whether a given well is affected
  • Describe the 2015 and 2024 settlements and what a mitigation plan does
  • State the positions of senior surface users, junior pumpers and cities fairly

The Eastern Snake Plain Aquifer is a huge body of water held in fractured volcanic rock under southern Idaho. Water moves through it toward springs and river reaches on the Snake River. Pump a well on the plain and, over months or years, some of that water would otherwise have come out of a spring or fed the river. Idaho's answer is conjunctive management: treat the wells and the river as one supply and administer them by priority across both.

That sounds tidy. It took decades of litigation to make it work, and it still produces a fight every spring.

Why the river feels the well

Land surface (the plain) Aquifer in fractured basalt Junior well Canal seepage and recharge (in) groundwater flow Springs Snake River
Water entering the aquifer (seepage, irrigation, recharge) and water leaving it (wells, springs) are connected. A well's effect on a spring is delayed and spread out, which is why IDWR uses a groundwater model to estimate it.

Much of the water that feeds those springs started as canal seepage and over-applied irrigation water a century ago. As farms switched from flooding to sprinklers and more wells were drilled after the 1950s, less water went in and more came out. Spring flows and aquifer levels dropped. That is the physical story under the legal one.

The delivery call

In 2005, a group of canal companies and irrigation districts that hold old surface water rights formed the Surface Water Coalition and filed a delivery call: a demand that the state curtail junior groundwater pumpers whose wells were shrinking the river flows the coalition's senior rights depend on. IDWR says the coalition's members irrigate about 550,000 acres. The Idaho Ground Water Appropriators formed to represent junior pumpers serving almost 1,000,000 acres.

In 2010 a court-approved Methodology took effect to measure injury each year. It has been revised several times; IDWR lists the Sixth Final Order, dated July 19, 2023, as the current version. Each spring the director forecasts the river supply, estimates crop demand, and calculates an in-season demand shortfall. Then the model works out which junior wells must be shut off, by priority date, to make up the shortfall. The director revisits the numbers in July.

The annual cycle, recent seasons

SeasonForecast shortfallCurtailment date (unmitigated rights junior to)Notes
202474,100 acre-feet (April)March 31, 1954 (May 30 order)Settlement reached June 20; curtailment avoided for participants
202563,000 acre-feet (April); 75,300 predicted in JulyAugust 28, 1955 (May); October 11, 1900 (July 25 order)Users given 15 days to join a mitigation district; a December 8, 2025 reasonable carryover order curtailed unmitigated rights junior to August 15, 1952
2026181,600 acre-feet (April)October 11, 1900 (May 14 order)Forecast based on April to July inflow at about 70 percent of average; July 21 order kept curtailment in force

Status as of October 2026, from IDWR's delivery call page (last updated October 14, 2026): status summaries for the May 14 order were posted through September 29, 2026, along with a summary of enforcement actions. Check the page for later orders before you cite these numbers.

Worked example: is this well curtailed?

Three invented wells sit inside the ESPA area of common ground water supply in 2026. The curtailment cutoff is October 11, 1900.

WellPriority dateIn an approved plan?Result
Marrow Creek Farms No. 1April 2, 1898NoSenior to the cutoff. Not curtailed.
Larkin Bros. irrigation wellMay 9, 1961Yes, through its groundwater districtJunior, but protected while the district complies
Dunmore feedlot wellJune 3, 1948NoJunior. Curtailed.

The 1900 date looks strange because almost no wells are that old. That is the point. A cutoff that early means essentially every unmitigated well in the area is subject to curtailment. In practice the order works as a strong push to be inside a plan.

Putting the shortfall in familiar units. 181,600 acre-feet times 325,851 gallons per acre-foot = 59,174,541,600 gallons, or about 59 billion. Spread over a 100-day peak season, that is 181,600 divided by (1.983 acre-feet per cfs-day times 100 days) = about 916 cfs of continuous flow. (Lesson 4 shows where 1.983 comes from.)

Mitigation plans and the settlements

A mitigation plan lets junior pumpers keep pumping if they offset their injury: by cutting use, delivering storage water to seniors, recharging the aquifer, or some mix. IDWR approves each one. In 2026, IDWR listed A&B Irrigation District, the Coalition of Cities, Southwest Irrigation District and the Water Mitigation Coalition as covered by approved plans, along with participants in the 2024 settlement.

The 2015 settlement between the coalition and groundwater users set a goal of conserving 240,000 acre-feet a year. It did not clearly define the baseline or whether averaging was allowed, and it did not cover cities or other non-member pumpers. Lawsuits followed.

The 2024 settlement replaced that structure. As reported by Ag Proud, the terms include:

  • Groundwater districts conserve 205,000 acre-feet a year on average, measured monthly and averaged over four years.
  • Districts deliver up to 75,000 acre-feet of storage water a year to the coalition, depending on the director's injury findings (the 2015 deal set a fixed 50,000).
  • Compliance is assigned district by district and farm by farm, so one party's failure does not expose everyone else.
  • Renewable four-year terms from January 1, 2024, with the first exit opportunity in 2027.
  • A joint push to raise the state's managed recharge target from 250,000 to 350,000 acre-feet a year.

Worked example: a district's four-year average

Suppose an invented district's share of the 205,000 acre-foot obligation is 10 percent: 20,500 acre-feet a year, or 82,000 over four years.

Year 1 it conserves 15,000. Year 2, 18,000. Year 3, 22,000. Total so far: 55,000.

Year 4 it needs 82,000 minus 55,000 = 27,000 acre-feet, about a third more than its yearly average. A dry fourth year makes that harder. That is the trap to watch for in 2027, the first exit window.

Recharge

Managed recharge diverts surplus river water, usually in winter and spring, into canals, basins or injection wells so it sinks into the aquifer. In January 2025 the Water Resource Board reported a 10-year average of about 251,000 acre-feet a year, just above the old 250,000 goal. The 2025 Legislature then directed the board to raise the goal to 350,000 acre-feet a year on average. The gap between the old average and the new goal is about 99,000 acre-feet a year.

Recharge wells drew scrutiny. In January 2025 the board paused new funding approvals for injection wells; in March 2025 it lifted the pause after water users called the wells a vital part of the 2024 settlement. New applications must show injected water will not endanger nearby drinking water wells, under IDWR's underground injection program.

The positions, stated fairly

WhoCore argumentStrongest evidence they cite
Senior surface water usersTheir rights came first. The constitution and statute protect priority. Pumping took water their rights already claimed.Declines in spring flows and aquifer levels; model results linking pumping to reach gains and losses
Junior groundwater usersCurtailment shuts off productive farms and towns to deliver a small fraction of water to seniors, years later. Statute also protects full economic development of groundwater.Model lag times; the language of Idaho Code 42-226; economic studies of lost farm output
CitiesMunicipal wells serve homes and jobs. Cities negotiated their own mitigation plan to avoid being caught in a farm fight.Coalition of Cities plan; domestic preference in the constitution (with compensation)

Where the evidence is clear: the aquifer and the river are connected, pumping does reduce flows over time, and priority is the law. Where honest people still disagree: how big the injury is in any year, how much uncertainty the model carries, and whether the settlement's conservation is real or just counted well.

Common mix-ups

  • Claim: The state is "turning off farmers' water" across the board. What the evidence shows: Curtailment applies only to junior ground water rights not covered by an approved mitigation plan. Most pumpers are in plans.
  • Claim: Curtailing a well instantly puts that water in the river. What the evidence shows: Effects are delayed and spread out. IDWR uses ESPAM 2.2 to estimate them.
  • Claim: More efficient irrigation always helps the aquifer. What the evidence shows: Less seepage and runoff can mean less recharge. Lesson 4 shows the arithmetic.

Reporting angle

  • Watch for two orders every year: the April forecast and the early July revision. Note the shortfall in acre-feet and the curtailment date each time.
  • Ask your local groundwater district for its share of the 205,000 acre-foot obligation and its year-by-year conservation. Then do the four-year math.
  • Read the enforcement summaries IDWR posts. They show whether curtailment was enforced or just ordered.
  • Get the IWRB recharge totals for the season and compare them to the 350,000 acre-foot goal.

Research angle

Check yourself

1. In 2026, IDWR's curtailment order applied to unmitigated ground water rights junior to what date?

2. A well with a priority date of June 3, 1948 sits in the ESPA area and its owner is not in any mitigation plan. Under the 2026 order, what happens?

3. Under the 2024 settlement, groundwater districts committed to conserve how much water per year on average?

4. In 2025 the Legislature directed the Water Resource Board to raise the ESPA managed recharge goal to what average?

5. What is a mitigation plan in this setting?

6. The April 2026 forecast projected a 181,600 acre-foot shortfall. About how many gallons is that?

Terms in this lesson

Conjunctive management
Administering connected surface water and groundwater as one supply, by priority across both.
Delivery call
A request by senior right holders that the state curtail junior users who are injuring their rights.
Curtailment
IDWR's term for reducing or restricting use of surface or groundwater based on priority date.
Mitigation plan
A director-approved plan in which junior ground water users prevent or compensate for injury to senior rights, protecting them from curtailment while they comply.
Managed aquifer recharge
Deliberately putting surplus surface water into the ground, through canals, basins or wells, to raise aquifer storage.
ESPAM
The Eastern Snake Plain Aquifer Model. IDWR used version 2.2 in 2026 to estimate how pumping affects river reaches and springs.

Full glossary

Sources