HFF Blog

Blog in response to Island Park flow transfer on July 15, 2018

Prepared by Melissa Muradian and Jack McLaren

What happened?

Due to warm water temperatures and low dissolved oxygen in the water at the intake to the Island Park hydroelectric plant, the plant’s aerators have been unable to maintain dissolved oxygen in power plant outflow above the level required by Fall River Electric's operating license. Thus, the plant shut down yesterday morning (July 15), and all outflow from Island Park Reservoir was shifted to the bottom-withdrawal gates on the west side of the dam.

June Closes Out Warm, Wet Spring

Photo of clouds over Fall River
  • Watershed temperature over the month of June was 1 degree F above average.
  • June precipitation was 135% of average, but average natural streamflow at Island Park for the month of June was only 92% of average.
  • As of July 6, watershed-wide natural flow has dropped to 89% of average.
  • Delivery of Island Park Reservoir storage water began on July 3.
  • Watershed temperature over the April-June period was 2 degrees F above average, continuing the 40-year trend of increasing springtime temperature.
  • April-June precipitation was 134% of average.

Buffalo River Fish Ladder: 2018 Summary

Henry’s Fork Rainbow Trout migrating to spawn in the Buffalo River have to pass through the fish ladder at the Buffalo River hydroelectric facility to access upstream habitat. At the end of the fish ladder we, HFF, operate a fish trap from early February through the middle of June. Three times a week we check the Buffalo River fish trap and collect data on species, length, sex, and life histories via passive integrated transponders (PIT) tags if one is present, before passing the fish upstream of the hydroelectric facility.

High turbidity and sediment event during Memorial Day weekend

The main message: Spring of 2018 brought the highest runoff event in 7 years to the upper Henry’s Fork watershed! Our network of water quality monitors showed that these flows were strong enough to provide a major springtime sediment flush--a natural rhythm of our local hydrology that provides significant benefit to trout and aquatic insect habitat. These favorably high natural flows came in two periods during April and May of this year.

May was Warm and Wet; Very Little Snowpack Left

Photo of moose swimming across river
  • May 2018 saw warm temperatures, heavy rain, and above-average snowmelt.
  • Only 30% of this year’s peak snow-water-equivalent (SWE) remains, compared with an average of 43% remaining on June 1.
  • Water-year precipitation stands at 108% of average, but SWE has dropped from 117% of average at its peak in April to 82% of average on June 1.
  • Lack of snow means that streamflow will drop rapidly once the current wet weather ends.

Heavy Rain Produces High Streamflow: How high is high?

Photo of Henry's Fork in Box Canyon
  • May 22-24 precipitation totals were over 1 inch at most locations; water-year precipitation jumped from 105% of average to 109%.
  • Snowmelt continues at average rates, and SWE remains at 102% of average.
  • Watershed-total natural flow has increased to its highest level so far this year and higher than last year’s peak.
  • Inflow to Island Park Reservoir is around 1,600 cfs, and outflow is currently just a hair over inflow, allowing the reservoir to drop very slowly. Current reservoir content is a little higher than full pool.

Hydrology and Water-Management Course: Year 14

Photo of Henry's Fork.

In 2005, when I was a professor at Idaho State University, the Henry's Fork Foundation Board of Directors asked me to present an overview of hydrology and water management in the upper Snake River basin. Since then, the hydrology and water-management short course has taken on a life of its own, and I give this presentation in some form or another a few times each year. Every time I give the presentation, I update it with new information, particularly as related to climate change and the rapidly changing work of water management and admininstration.

Streamflow and Reservoir Predictions for Summer 2018

Photo of Fall River.
  • Cool, wet weather from mid-February to mid-April turned an average water supply into one that is decidedly above average.
  • As of May 9, the Henry’s Fork reservoir system is 94% full and filling rapidly.
  • Based on early-April conditions, summertime water supply in the Henry’s Fork watershed is forecast to be above average.
  • More storage water will be delivered from Island Park Reservoir this summer than in 2017, but higher inflows will compensate, resulting in a very high probability of better-than-average carryover at the end of the irrigation season.

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