HFF Blog

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.

Current water temperatures through the Ranch and Pinehaven reaches

Headlines: Water temperatures through the Ranch have been on the warm side. Temperature has climbed above 71˚F for a couple of hours each day over the last week. However, daily maximum temperatures have not been deadly for rainbow trout, and only a few daily averages have been at the lowest levels that would stress fish.  This is the same pattern we saw in both 2015 and 2016, when streamflows were about 800 cfs higher than current flows. Thus, releasing additional water from IP dam does not result in significantly cooler summertime water temperatures, but we do know that increasing flow out of Island Park Dam decreases water clarity and will reduce winter flows, which reduces survival of juvenile trout next winter and hence recruitment of 2-year old fish into the population in 2019.

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.

More Showers Keep Water Supply Above Average

Photo of Rainbow Trout.
  • Most locations around the watershed received around one-half inch of rain this week.
  • Natural streamflow supply in the watershed is 7,000 cfs, well above average for the date and well above diversion rate, which is 3,000 cfs.
  • Delivery of storage water from Island Park Reservoir will not be needed until at least July 5.

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.

Henry's Fork Reservoir Storage and Streamflow at Peaks

Photo of Rainbow Trout.

Week of June 5 highlights:

  • Island Park Reservoir, Grassy Lake and Henrys Lake have all filled.
  • Natural streamflow in the watershed is around 9,500 cfs and at its final peak for the season.
  • Total watershed-wide irrigation diversion is around 3,000 cfs, right at average for this time of year.
  • Outflow from Island Park Reservoir is being set to keep the reservoir full. Only small changes are anticipated over the next week, and outflows should remain around 950 cfs plus or minus 150 cfs, depending on rain.
  • Delivery of water from Island Park Reservoir in excess of inflow is not expected to be needed until early July.

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