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Water Supply and Water Quality Predictions for Summer 2026

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Photo of confluence of Warm River and Henry's Fork

The photo above captures the unprecedented situation we face at the beginning of April. The photo, of the confluence of Warm River and the Henry’s Fork, was taken on February 7 and shows the shocking lack of snow we experienced in the watershed all winter as a result of record-breaking warm temperatures.

How will the warm winter and lack of snow affect water supply, water quantity, and fisheries? The highlights are below. If you want to read the full 21-page report and see the numbers and graphs, click here.

  • Mean October–March temperature was 5.6°F above average and 2.0° warmer than 2015, which was second warmest in the 1989–2026 record.
  • Natural flow between Henry’s Lake and Island Park Dam was 4.5% higher over the winter than I predicted on October 1, due to much of the winter’s precipitation falling as rain instead of snow.
  • Winter flow out of Island Park Dam averaged 215 cfs, compared with the 1989–2025 average of 349 cfs and an expectation of 170 cfs based on water supply.
  • Snow water equivalent (SWE) peaked on March 16, 27 days earlier than average, at 67% of average, 3rd lowest on record.
  • Natural streamflow is predicted to be the lowest in the 1978–2026 record and 62% of the 2001–2025 average: 67% in upper Henry’s Fork, 65% in Fall River, and 60% in Teton River.
  • Watershed natural flow is expected to peak in late May, around 2–3 weeks earlier than average.
  • Island Park Reservoir will fill in early May, but draft is expected to start before June 9. Outflow could reach 1,200 cfs by mid-June.
  • End-of-season reservoir volume is predicted to be 12,279 ac-ft (9% full), which would be 6th lowest on record but still higher than expected based on water supply.
  • Turbidity, water temperature, and sediment load at Island Park Dam have a high probability of being the highest since we started monitoring in 2014, with onset of high turbidity in mid-June.
  • We expect no acute effects of this year’s low water supply on fish populations, but high reservoir draft and subsequent low winter flow will result in decreased populations in the river reaches immediately upstream and downstream of Island Park Reservoir 2–4 years from now.
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An aerial view of Island Park Dam. The reservoir sits behind it and the river flows below it.
Water Quality
Christina Morrisett

Summer 2026 Water Quality Forecast for Island Park

This article was originally published in Island Park News on May 29, 2026 Ice melted on Island Park Reservoir on March 31, 2026—four weeks earlier than normal and the earliest the Henry’s Fork Foundation (HFF) has observed in 12 years of monitoring. Such an early ice melt was not unexpected given the region’s near-record warm and dry winter. But ice melting a month early has implications for water quality in Island Park Reservoir and the

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Christina Morrisett May 28, 2026
Aerial photo of Island Park Reservoir. The dam is in the bottom left corner. The Centennial Mountain Range is in the top.
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Warmer and drier conditions have slowly degraded water quality in Island Park Reservoir over the last 50 years.  In the 1970s and 1980s, federal reports and university research indicate that the reservoir had relatively good water quality. Island Park Reservoir was mesotrophic—with mostly clear water and occasionally low dissolved oxygen at the reservoir’s deepest depths during the summertime. But, as time went on, water quality began to decline. Historical satellite imagery analyzed by the U.S.

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Christina Morrisett May 22, 2026
A photo taken of Island Park Reservoir while standing on the dam and looking north. The sky is cloudy, Mt Sawtelle sits with little snow in the background, and the reservoir is open water (save for a few small sheets of ice in the distance).
Water Quality
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Reservoir Ices Off, Growing Season Turns On

Photo of an ice-free Island Park Reservoir taken on March 31, 2026 On March 31, 2026, with no accompanying commentary, HFF scientist Christina Morrisett stood on top of Island Park Dam and texted colleagues Rob Van Kirk and Jack McLaren photos of open water on Island Park Reservoir. Rob, eager to see that the reservoir reality matched what he was seeing in the real-time river data downstream, texted back, “Yep! That’s close enough to ice

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Christina Morrisett May 7, 2026
Photo of confluence of Warm River and Henry's Fork
Water Quality
Rob Van Kirk

Water Supply and Water Quality Predictions for Summer 2026

The photo above captures the unprecedented situation we face at the beginning of April. The photo, of the confluence of Warm River and the Henry’s Fork, was taken on February 7 and shows the shocking lack of snow we experienced in the watershed all winter as a result of record-breaking warm temperatures. How will the warm winter and lack of snow affect water supply, water quantity, and fisheries? The highlights are below. If you want

Read More »
Rob Van Kirk April 10, 2026