Dr. Rob Van Kirk's blog

New Year Starts with Good Water Supply

Photo of Rainbow Trout.

December precipitation was only 51% of average across the Henry's Fork watershed. However, water year-to-date precipitation and snow-water-equivalent are at 87% and 89% of average, respectively. And, thanks to above-average precipitation during water year 2017, the upper Snake River reservoir system is 87% full, and winter streamflow is above average. Long-range forecasts call for average to above-average precipitation for the remainder of the winter, so the overall water-supply outlook remains good as we head into the New Year.

Fish of the Month: Year Three

Photo of Rainbow Trout.

Each of the last two years at this time, I have reported on the resurrection of “fish of the month,” a tradition that Henry’s Fork Anglers guide Tom Grimes and I started many years ago. The goal is to catch at least one wild trout every month of the year, on a fly, in our local Idaho, Montana, and Wyoming waters. My longest fish-of-the-month streak lasted 55 months, from July 2004 through January 2009. On December 4, I added month 36 to the current streak.

Water Year 2018 off to a Good Start

Photo of Rainbow Trout.

Six weeks into the new water year, most measures of water supply in the Henry's Fork watershed are average or better:

  1. Snow accumulation in the mountains is well above average for early November.
  2. Watershed-wide natural flow has been above average since October 1.
  3. Island Park Reservoir is already at its April-1 average content.
  4. Medium- and long-term weather forecasts call for above-average precipitation.

Water Year 2017: Precipitation 138% of average, streamflow 109% of average

Photo of Rainbow Trout.

Water year 2017 was above average in both precipitation and streamflow. Averaged over the whole watershed, precipitation was 138% of average, led by the Fall River watershed, at 144% of average. Due to the cumulative effect of four years of drought, especially in the groundwater-dominated subwatersheds, natural streamflow was only 109% of average, led by the Teton River at 134% of average. Streamflow in the upper Henry's Fork, which is dominated by groundwater, was only 91% of average.

Natural Streamflow at Long-Term Average; Minimal Storage Needed

Photo of Rainbow Trout.
  • Watershed-total natural streamflow is at its long-term average of around 3,000 cfs and receding slowly.
  • Watershed-total diversion has dropped around 1,000 cfs from its maximum back in early July and is currently a little over 2,500 cfs.
  • Storage delivery has been minimal this season; outflow from Island Park has dropped from its peak of 1,080 cfs on July 23 to 640 cfs now.
  • Island Park Reservoir is still 90% full, compared with 33% on this date in 2016.

Streamflow Still Above Average on Fall and Teton Rivers

Photo of Rainbow Trout.
  • Natural streamflow is still above average in Fall River and Teton River but below average in the upper Henry's Fork.
  • Total watershed natural flow is 3,400 cfs, right on average for this time of year. Total watershed diversion is also average, at 3,300 cfs.
  • Delivery of storage water from Island Park Reservoir began last week; current flow out of Island Park is 1,060 cfs, compared with the long-term average of 1,300 cfs for the date and last year's value of 1,420 cfs.
  • Island Park Reservoir is currently 96% full, compared with 49% full at this time last year.

Water Supply Still Strong Well Into July

Photo of Rainbow Trout.
  • Total natural streamflow in the Henry's Fork watershed is 122% of average and 2,500 cfs greater than irrigation diversion, which remains slightly below average.
  • No water has been delivered out of Island Park Reservoir yet this season, compared with 40,566 ac-ft (30% of reservoir capacity) last year at this time.
  • Island Park Reservoir outflow will remain roughly equal to inflow until release is needed to meet irrigation demand, which now looks to be another week out.

Rain Boosts Water Supply as Peak Irrigation Season Arrives

Photo of Rainbow Trout.
  • Over two inches of rain fell in parts of the watershed last week, providing another peak in streamflow in Fall River and Teton River.
  • Total watershed streamflow remains around 8,000 cfs, compared with irrigation diversion of 3,000 cfs.
  • Island Park Reservoir will remain full until delivery is needed to meet irrigation demand, which now looks to be at least two weeks away.
  • Outflow from Island Park Reservoir is being reduced frequently to keep pace with natural streamflow recession in the upper Henry's Fork.

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