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Water Year 2022 Summary: Island Park Reservoir Management


  • Winter inflow to Island Park Reservoir was the second lowest in the 1934-2022 record, but the outflow of 220 cfs was higher than that in 14 other years since 1978.

  • Island Park Reservoir filled physically in mid-May, and subsequent runoff events due to heavy rain required commensurate outflow increases to keep the reservoir full until needed to meet irrigation demand.

  • Reservoir draft started on June 28 and ended on September 21—both around 5 days later than average.

  • The reservoir reached its minimum for the year on September 20 at 45% full, compared with 45% full on average and 41% full last year.

  • Over the past five years, a variety of water-management changes have resulted in an average increase in reservoir carryover of 25,000 ac-ft (48%) and in winter outflow from Island Park Dam of 116 cfs (38% through the 2022 winter).


Keeping as much water in Island Park Reservoir has multiple benefits for irrigation, hydropower, water quality, and fisheries and so is the highest water-management priority in the watershed, subject to meeting irrigation demand. Inflow to the reservoir is a major component of management, and it was well below average in water year 2022, particularly during the winter. In fact, total October-March watershed inflow between Henry’s Lake and Island Park—essentially the water available to fill Island Park Reservoir—was second lowest since the Island Park stream gage was first installed in 1934. The only year with lower fall/winter natural flow was 1935. As a result, total inflow to Island Park Reservoir was average during 2022 only for a few days during late October, when outflow from Henry’s Lake was still contributing and rain added some water directly onto the reservoir surface.

Because of below-average winter precipitation, reservoir gain from direct precipitation on the reservoir surface was around 6,000 ac-ft, a little below the long-term average. However, heavy rain in the spring added over 2,000 ac-ft, and net gain from precipitation (less evaporation) over the water year ended up at 7,000 ac-ft, about 1,000 ac-ft greater than average. Although typical net gain from precipitation on the reservoir surface is only 5% of total reservoir capacity, direct precipitation allows slightly higher outflow during the winter than if reservoir fill were dependent on stream inflow alone. During the winter of water year 2022 (December 1, 2021-February 28, 2022), direct precipitation added an average of 26 cfs to reservoir inflow. The contribution of direct precipitation depends on reservoir surface area, which is greater when reservoir volume is greater, one reason to fill the reservoir as much as possible prior to the high-precipitation months of December-February.

Winter outflow from Island Park Dam averaged 220 cfs, ranking 31st out of the 45 years in the “modern” management period (1978-present), compared to 370 cfs in the winter of 2021. The lower value in 2022 was due both to lower physical reservoir carryover (minimum reservoir volume at the end of irrigation season) in 2021 and to near record-low reservoir inflow during the 2021-2022 winter. The 1978-2021 average winter outflow is 360 cfs. More on this aspect of Island Park Reservoir management below.

Due to cool springtime temperatures, ice cover remained on the reservoir until early May. The peak of snowmelt upstream of the reservoir occurred a little earlier than ice-off, requiring a brief outflow increase to prevent ice interference with spillway infrastructure prior to ice-off. Outflow was quickly reduced once ice melted and maintained at around 250 cfs until the reservoir filled in mid-May. The reservoir was then held constant until draft was needed to meet irrigation demand downstream. This required that outflow matched inflow between mid-May and mid-June.

In particular, two rapid increases in outflow—both from around 400 cfs to 1200 cfs—were required during the two biggest rain events to keep the reservoir constant. These increases were followed closely by rapid and large decreases. Although these large fluctuations in flow were not desirable from a fishing standpoint, they reflected natural precipitation-driven watershed runoff events. The graph of total streamflow in Box Canyon and the graph of regulated vs. natural flow at Island Park both show that outflow peaks during the spring very closely matched the river’s natural flow. In other words, even in absence of regulation at Island Park and Henry’s Lake dams, the river’s flow would have been essentially the same during this time period as we experienced. Because Island Park Dam is not authorized or used for flood control, and given the numerous reasons to keep it as full as possible year-round, the objectives of springtime reservoir management are to fill it as far ahead of potential irrigation need as possible and pass natural runoff events through the reservoir after that.

Although Island Park Reservoir would have physically filled by the end of May even without rain, the other two reservoirs in the watershed probably would not have filled. Based on April-1 conditions, Grassy Lake had nearly no chance of filling, and there was a pretty good probability that Henry’s Lake would also not have filled. May and June rain, which was concentrated in the northern half of the watershed where those two reservoirs are located, filled both of them before reservoir draft was needed to meet irrigation demand. After spending the entire winter well below average, inflow to Henry’s Lake during June was above average, in part because of rain and in part because of late snowmelt. As mentioned in the climate section, spring and summer rain in 2022 slightly improved drought in the short term, and the Henry’s Lake basin and adjacent areas of the northern end of the watershed showed the greatest improvement during the water year.

Because of the cool, wet spring, reservoir draft began on June 28, five days later than average and 18 days later than in 2021. Reservoir draft ended on September 21, six days later than average. Thus, the period of reservoir draft was average, but the entire period of draft was shifted 5-6 days later than average. Reservoir draft was again very well predicted by my “600-cfs rule,” which is based on the observation that reservoir draft is generally needed to meet irrigation demand when natural-flow supply drops to within 600 cfs of total diversion. By that rule, reservoir draft would not have been needed for a day or two in August following widespread rain, but irrigation-system adjustments are not logistically feasible on such short time scales in the middle of the summer. Peak reservoir outflow occurred from July 7 to July 26; mean flow over that time period was 1351 cfs, compared with 1168 cfs on average.

As I will discuss in much more detail in the installment on system management once the irrigation year is complete, basin-wide needs required additional draft of Island Park Reservoir from August 24 to September 19 to send water to American Falls Reservoir. Based on lower-Henry’s Fork streamflow targets and management, around 12,700 ac-ft was sent out of the Henry’s Fork watershed beyond what was needed to meet within-watershed irrigation demand and streamflow targets.

Despite the additional draft, minimum reservoir volume was 45% full, on September 20, compared with 41% full in 2021 and 45% full on average. Usually, a little reservoir fill is achieved in late September by lowering outflow. In 2021, late-September fill increased reservoir volume to 45% full by the end of the water year. Average September fill is around 2800 ac-ft, which results in a reservoir level of 47% full to begin the new water year. This year, hot dry weather persisted through most of the month of September and into October, preventing fill between the end date of reservoir draft and the end of the month. The reservoir ended the water year at 45% full.

To interpret the effectiveness of water conservation and management programs implemented by FMID, HFF and other partners over the past few years, physical reservoir carryover must be considered relative to the major variables that determine it. Statistically, the three most important predictors, in order of importance, are natural flow (positive effect), streamflow in the lower Henry’s Fork (negative effect), and total diversion (negative effect). Together these three variables explain 85% of the year-to-year variability in carryover. Implementation of current water-management programs first started in water year 2018. Additional research, monitoring, strategies, and irrigation infrastructure were added in subsequent years.

To objectively assess the effects of these programs, I fit a statistical model of physical reservoir carryover in Island Park to 1978-2017 data and then compared outcomes in 2018-2022 to those expected based on that statistical model. This approach essentially asks the question, “What would reservoir carryover and subsequent winter flow have looked like under 1978-2017 management, given the water years we have experienced since then?” Over the past five years, reservoir carryover has averaged nearly 25,000 ac-ft greater than expected, amounting to an average 48% increase in carryover.

Keep in mind that the expectation has already accounted for the effects of streamflow in the lower Henry’s Fork. So, higher reservoir carryover is not simply a function of reducing lower-Henry’s Fork streamflow. Because of the need to send water to American Falls this year, streamflow in the lower Henry’s Fork during the period of reservoir draft was greater than it has been since 2014. Nonetheless, reservoir carryover this year beat expectations by 43,590 ac-ft, 2.5 times the expected value of 17,200 ac-ft. This indicates that improved carryover in Island Park is due to a variety of water-management and conservation practices across the whole watershed, as I will discuss in more detail after the irrigation year ends.

Increased reservoir carryover results in increased winter outflow, all other factors being equal. Winter flow from 2019 to 2022 (following water years 2018 through 2021) beat expectations every year and averaged 116 cfs (38%) higher than expected over the four years. That increase in winter flow is worth around 580 additional age-2 rainbow trout added in the Box Canyon population, roughly an 18% increase in the trout population.

Inflow to Island Park Reservoir is the single largest factor other than reservoir carryover that determines winter outflow. Thus, achieving near-average carryover in what turned out to be the driest water year in the 1978-2022 record will not be enough to guarantee average winter flow during the upcoming winter. Although conditions in the upper Henry’s Fork subwatershed have improved a little over the past water year, inflow to Island Park Reservoir over the upcoming winter is forecast to be the 5th lowest in the 90-year record, behind 1935, 2022, 2016, and 2004. As a result, expected winter flow based on 1978-2017 data is 99 cfs. Given very similar reservoir carryover as last year and only slightly better watershed conditions, forecast winter flow this year is essentially the same as last year’s, around 220 cfs. If that forecast holds up, the average improvement in winter flow over the past five years will stay around 116 cfs, but the percent improvement will increase to 44% as a result of beating the upcoming winter’s expectation by over a factor of two.

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