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Island Park Reservoir Outflow Plan (Updated 9/16)

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Drone image of Island Park Dam taken on September 9, 2026
Drone photo taken on September 9, 2026
The Henry’s Fork Drought Management Planning Committee met on Wednesday, September 16 2026 to develop an updated reservoir management strategy. Downstream of the Henry’s Fork watershed, lower-valley irrigation demand over the rest of the season is anticipated to be slightly lower than projected last week, so Island Park Reservoir and Grassy Lake (in the Fall River drainage) do not need to deliver any more water out of the watershed. A small amount will be needed to meet within-watershed demand for the next week or so. The Committee decided on the following plan:

Island Park Outflow reductions

  • 250 cfs reduction Thursday morning
  • 250 cfs reduction Friday morning
  • 250 cfs reduction Saturday morning
  • FMID will assess within-watershed needs on Monday afternoon

Resulting Island Park outflow

  • 930 cfs by noon Thursday
  • 680 cfs by noon Friday
  • 430 cfs by noon Saturday

Resulting Box Canyon flow (downstream of Buffalo River)

  • 1090 cfs by noon Thursday
  • 840 cfs by noon Friday
  • 590 cfs by noon Saturday

Island Park Reservoir volume

The reservoir will drop to roughly 16,000 ac-ft (12% full) by Saturday afternoon and stay at that level at least through Monday afternoon. The next reduction, likely on Tuesday morning, will allow the reservoir to start filling again.

Turbidity

Turbidity at the dam will continue to increase at least until the reservoir stabilizes on Saturday afternoon. However, because flow will be lower, suspended material will settle out faster than it currently is, so turbidity will start to improve on the lower Ranch/Pinehaven area and downstream by Sunday, with more improvement after each flow reduction. However, outflow from the dam will probably not be clear for several weeks.
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