Projected Springtime Management of Island Park Reservoir

The Island Park Drought Management Planning Committee met on April 8 to discuss projected water supply and management of Island Park Reservoir this spring. Organizations represented at this meeting were Fremont-Madison Irrigation District, North Fork Reservoir Company, Fall River Rural Electric Cooperative, Idaho Department of Fish and Game, The Nature Conservancy, U.S. Bureau of Reclamation, Water District 01, and Henry's Fork Foundation.

The short version of the meeting outcome is that spring-time runoff in the upper Henry's Fork Basin is expected to be only around 70% of average, a value close to what I have been predicting for several months now (see my two previous blog posts on this subject at and Assuming that the current weather forecasts for a somewhat wet spring hold up, we can expect inflow, outflow, and reservoir contents over the next few months to look something like what is shown in the following graphs. The most important thing to note is that both inflow to and outflow from Island Park Reservoir are expected to be much lower than average. Inflow during mid- to late-June is predicted to be only around 500 cfs, if not even a little lower, so that even if Island Park Reservoir is full, just matching the river's natural flow will result in outflows not much higher than 500 cfs.

If you would like more details on the numbers, the overall management strategy for the spring, and variables that could still change over the next three months, read on.

Current Conditions

  • Inflow (Henry’s Lake to Island Park): 360 cfs
  • Outflow from Island Park Reservoir: 127 cfs
  • Island Park Reservoir Contents: 123,303 ac-ft (91% of average)
  • Reservoir Pool Elevation: 6301.5 ft (0.5 ft. below spillway crest, 1.5 ft. below full pool)

Projected Inflow

Values of the three best predictors of April-June inflow to the watershed between Henrys Lake and Island Park are:

  • April 1 snow-water-equvialent (SWE) at Black Bear Snotel site, on the continental divide between Big Springs and West Yellowstone: 31.9 inches (83% of long-term mean)
  • April 1 SWE at Crab Creek snotel site in the Centennial Range: 13.6 inches (99% of long-term mean)
  • Mean watershed inflow over the preceding October-March period: 347 cfs (75% of long-term mean, and lowest recorded since 1930s). NOTE: In October 2015, I predicted this to be 341 cfs.

Based on these inputs, the predicted mean April-June inflow between Henry's Lake and Island Park is 621 cfs, which is 71% of the 1979-2015 mean and the 33rd percentile over that period. With 95% statistical confidence, the inflow will be between 444 cfs and 868 cfs. This confidence interval is relatively large because springtime precipitation can have a very large influence over actual. The current weather forecast calls for slightly above-average precipitation throughout the April-June period, so actual inflow could be somewhat greater than the 621 cfs we predict. Using a different model, the U.S. Bureau of Reclamation predicts a mean inflow of 610 cfs over the period April through July, including contribution from the Henry's Lake basin. This value is 69% of its long-term mean, very close to our prediction of 71% of average. Having all parties agree on predicted water supply always gets a meeting off to a good start!

Potential April-June Management Scenario

Objectives of this particular scenario

  1. Allow Island Park hydroelectric power plant to begin operating for the spring. It has not been able to operate since last fall.
  2. Provide at least 4,000 acre-feet (0.5 feet) of space in the reservoir during April and May, until after peak runoff has occurred.
  3. Physically fill the reservoir in mid-June.
  4. Avoid large flow changes prior to and during Memorial Day weekend.
  5. Avoid large flow changes during the week surrounding opening day of Harriman State Park.
  6. Maintain outflow of at least 500 cfs from late May through late June.


  1. Henry’s Lake outflow remains negligible for most of this time period, as water is stored there.
  2. Henry's Lake to Island Park inflow averages 621 cfs.
  3. No Island Park storage water is needed to meet irrigation demand prior to the end of June.
  4. Precipitation and runoff timing are average through the period.


  1. Increase outflow to around 200 cfs on Monday, April 11 to allow the hydroelectric plant to begin operating. Operate it at near-minimum capacity for 7-10 days while testing new water-quality monitoring procedures. This will allow reservoir to reach 127,000 ac-ft (water elevation right at the bottom of the one-foot inflatable spillway collar) by April 16.
  2. Around April 16-20, increase outflow as needed to reach 131,000 ac-ft (0.5 ft below full pool) around May 1. Under assumptions above, 500 cfs outflow will achieve this volume.
  3. Around May 1, increase flow as needed to maintain reservoir volume near 131,000 ac-ft through the month of May. This leaves space of 4,000 ac-ft in volume or 0.5 ft in elevation in case there is a large runoff event. Under assumptions above, outflow would be around 750-800 cfs during May.
  4. Assess conditions during week of May 23 so that outflow can be set several days prior to Memorial Day weekend and left at a constant flow from May 26 through May 30.
  5. After Memorial Day weekend, reduce outflow slightly to fill remaining reservoir space over first two weeks of June. Under above assumptions, outflow would be 500-700 cfs.
  6. Assess conditions during week of June 6 so that outflow can be held as constant as possible between June 13 and June 20.
  7. After June 20, operate to meet irrigation demand as needed.

Other Considerations

Under ideal conditions, the operational procedure above will accommodate all stakeholder groups: the power plant will be able to operate, the reservoir will fill to capacity, and anglers will have relatively constant flows in the neighborhood of 500-700 cfs from Memorial Day weekend through the end of June. However, any of a number of variables could change from what we project right now. Accomodating these changes will necessarily change these ideal outflows and reservoir contents.

Furthermore, when critical decisions need to be made as conditions change throughout the next few months, the U.S. Bureau of Reclamation (USBR) has the ultimate authority and responsibility to make these decisions in order to meet its legal obligations. First and foremost, USBR has an obligation to physically fill Island Park Reservoir for Fremont-Madison Irrigation District before that water is needed for irrigation. Second, in the unlikely event of very high runoff, USBR must follow specific rules for capturing a certain amount of that runoff in the reservoir in order to avoid large outflow. Third, USBR is obligated to manage Island Park Reservoir in the context of the entire upper Snake River system, across which it has multiple reservoirs, spaceholders, hydroelectric power plants, flood-control responsibilities, and recreational stakeholders. Even with perfect knowledge of weather, water supply, and irrigation demand, making everyone happy is a tall order. In the face of uncertainty, it is even more difficult. USBR always does an admirable job of balancing the needs of competing stakeholders and will do this best it can at Island Park Reservoir this spring, but as you can see from the left panel of the graph above, we are projecting inflow during mid- to-late June to be only around 500 cfs, so there will be little margin to accomodate unexpected changes in conditions, maintain a full reservoir, and ensure that flows are optimal for fishing and floating.

Listed below are some of the variables that USBR and the other stakeholders will have to accomodate throughout the spring. 


Current weather forecast calls for warm temperatures over the next week, followed by near-normal temperatures. Precipitation is forecast to be above average. This could accelerate runoff and also increase volume, necessitating higher outflows in April and May than those proposed above and potentially lower inflows and outflows in June. However, the overall operational strategy would remain unaffected, unless the threat of a large rain-on-snow event and resulting flood requires more space than proposed here. If snowmelt runoff is greatly accelerated, the reservoir can be filled sooner than June, but that will likely mean inflows lower than the 500 cfs I project during June. To maintain a full reservoir, outflow must equal inflow, and if inflow is low, outflow must necessarily be low, through no fault of water managers. 

Outflow from Henry's Lake

Henry's Lake is still 10,000 ac-ft short of full, and SWE at the White Elephant Snotel site is at average, so near average inflow can be expected over the next few months. At that inflow, Henry's Lake will approach full near the end of May. Additional outflow from Henry's Lake will need to be accommodated with increased outflow at Island Park. The proposed assessment during the week of May 23 would be timed well to accommodate changes at Henry's Lake.

Increased irrigation demand

Given current snowpack basin-wide and forecast for above-average precipitation, delivery from Island Park should not be needed before late June to meet irrigation demand, which is generally accomplished by maintaining flow in the Henry's Fork at St. Anthony at or above 1000 cfs. However, this could change and require additional release from Island Park earlier than average. Recall that in 2015, irrigation demand was very high by mid-June, requiring high delivery from Island Park about three weeks earlier than average. Although we do not anticipate a repeat of last year's extreme conditions, warm, dry weather between now and mid-June could greatly increase projected irrigation demand.

Accrual of storage rights

In the upper Snake River reservoir system, physical water in reservoirs and accural of storage water rights (so-called "paper water") are not always the same thing. For example, right now, Island Park Reservoir is only 12,000 ac-ft short of physical fill, but its water right is 57,321 ac-ft short of fill on paper. About 290,000 ac-ft of system-wide flow, in excess of irrigation demand and water needed to fill Jackson and Henry's Lake accounts, is still needed to fill the American Falls storage account before the Island Park right can even begin to accrue. Over the next few weeks, runoff will likely be just enough meet irrigation demand, so that very little storage accural will occur in any of the reservoir accounts. In May, as high-elevation snow begins to melt, system-wide water supply should exceed demand, allowing the American Falls account to fill. Until that happens, the physical water stored in Island Park Reservoir over the next few weeks will belong to the American Falls account, meaning that it may need to be delivered downstream at some point later in the summer anyway. However, once the American Falls right fills, it is important to Fremont-Madison Irrigation District that Island Park be physically full so that all paper water filling the Island Park account over the remainder of the spring is realized as physical water in the reservoir. Thus, when the American Falls account is close to full, there is more urgency from a water-rights perspective to complete physical fill of Island Park. We will have a better idea of how physical and paper water will align in Island Park by the time we assess conditions again prior to Memorial Day weekend.