HFF Blog

Water Year 2018: Average supply but Above-average Reservoir Carryover

Photo of Brown Trout.
  • Water-year 2018 ended up close to average, at 102% of average precipitation and 105% of average streamflow.
  • Upper Henry's Fork subwatershed was below average in precipitation and streamflow, vs. above-average values in Fall and Teton rivers.
  • However, Upper Henry's Fork water supply improved from 70% of average in 2016 to 91% in 2017 and 94% in 2018, indicating recovery of deep aquifers from 2013-2016 drought.
  • May and June rain compensated for early snowmelt and resulted in below-average irrigation diversion.
  • Despite very dry conditions and below-average streamflow during July, August and September, Island Park Reservoir ended the water year at 73% full, compared with 43% full on average, thanks to careful and precise water management.

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.

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