Dr. Rob Van Kirk's blog

Water Year 2019: 3rd year of high reservoir carryover

Photo of Brown Trout


  • Climate: Record-breaking February snow, springtime temperature 2 degrees below average, and heavy precipitation in September. Mean temperature for water year 2019 was equal to the 1989-2018 average, but mean April-June temperature was 2 degrees F below average.
  • Natural streamflow: 98% of average: 97% in upper Henry’s, 103% in Fall River, and 96% in Teton River. Snowmelt in Fall and Teton rivers lasted into July. Upper Henry’s Fork water supply was above the 1930-2019 average for the first time since 2012.
  • Irrigation Management: Draft of Island Park Reservoir started 9 days later than average and ended 4 days earlier than average. Over the period of draft, streamflow in the Henry’s Fork at St. Anthony averaged 1,070 cfs, compared with this year’s target of 1,000 cfs.
  • Island Park Reservoir: Ended the water year 73% full, compared with 46% full on average. Outflow during the upcoming winter is expected to be 500-550 cfs for the third consecutive year.
  • Predictive Model Performance: Based only on April 1 conditions, my predictions for most key hydrologic parameters were within 10% of actual values except those directly affected by spring and summer weather. The model over-predicted natural flow by 8.1% and Island Park Reservoir carryover by 6.8%.  

Water Supply Above Average for Third Consecutive Year

Photo of Rainbow Trout.
  • Record precipitation in February moved snowpack from 80% of average on February 1 to 110% of average on April 1.
  • Watershed-wide natural streamflow for April through September is forecast to be 105% of average, compared with 104% of average in 2018.
  • Delivery of water from Island Park Reservoir is predicted to begin around July 1 and remain in the range of 800-1200 cfs during July and August.
  • Because of very good inflow, the reservoir is expected to end the irrigation season around 80% full, compared with 72% full in 2018.
  • Higher water levels in Island Park Reservoir have numerous, scientifically documented benefits to the Henry's Fork fishery, prompting HFF to continue to expand the  number of methods we use to limit delivery of water from the reservoir.

Water Supply Improves in January

Photo of moonset on snowy morning
  • January climate stats: 1 degree F above average temperature, 89% of average precipitation, and 83% of average snow water equivalent (SWE) accumulation.
  • February 1 water-year totals: 85% of average precipitation and 77% of average SWE.
  • February has better than even odds of being colder than average, and has started out very wet.
  • Island Park Reservoir has been steady at 88-89% full for the past two months. Outflow has averaged 520 cfs.

Fish of the Month: Year Four

Photo of Brown Trout.

In January of 2015, I started a new run of "fish of the month," a tradition I started years ago with long-time friend Tom Grimes, who is a guide at Henry's Fork Anglers. The idea is to catch a wild trout or whitefish every month of the year in our local waters, the streams and lakes of Montana, Wyoming, and Idaho in the Yellowstone region. My previous record was 55 months, from July 2004 to January 2009. Five academic years spent in California broke that streak, but I'm four years into the current one. How did 2018 turn out?

After dry December, snowpack only 70% of average

Photo of Brown Trout.
  • Precipitation for the month of December was only 57% of average.
  • As of January 1, water-year precipitation was 84% of average, and snow water equivalent was 71% of average.
  • However, because of above-average precipitation from September 2016 through June 2018, natural streamflow is near average throughout the watershed.
  • Winter flow out of Island Park Dam has averaged 516 cfs since December 1, compared with the 1978-2018 average of 351 cfs.

Winter Starts Dry, but Flow at IP Dam is 143% of Average

Photo of Brown Trout.
  • As of December 10, water-year precipitation is 87% of average, and snow-water-equivalent is only 75% of average.
  • Precipitation deficit since July 1 is over 5 inches.
  • Natural streamflow has been around 95% of average since early October.
  • Outflow from Island Park Dam has averaged 502 cfs since December 1, 143% of average.

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.


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