HFF Blog

Reservoir Remains Nearly Full Despite Dry July

Photo of clouds over Fall River
  • The month of July was a little warmer than average but very dry.
  • Natural flow and diversion were both around 90% of average for most of the month.
  • Moisture from May and June rain has kept total diversion for the season about 100,000 ac-ft below average.
  • As of August 3, Island Park Reservoir is still 82% full, compared with an average of 70% full at this time of year.

The great spring 2018 sediment flush continues to impact water quality and the fishing experience

Main message: The great spring sediment flush we observed earlier this year not only impacted fishing during Memorial Day weekend—by contributing to high flows and turbidity—but has continued to impact the fishing experience from Box Canyon through Riverside this season in two plainly obvious ways: depressed hatches and higher than average amounts of uprooted floating plant material. In addition, the rainy and cloudy weather that contributed to the spring flush may also have contributed to later hatch timing of some species earlier this summer.

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.

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