Dr. Rob Van Kirk's blog

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.

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.

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.

Heavy Rain Produces High Streamflow: How high is high?

Photo of Henry's Fork in Box Canyon
  • May 22-24 precipitation totals were over 1 inch at most locations; water-year precipitation jumped from 105% of average to 109%.
  • Snowmelt continues at average rates, and SWE remains at 102% of average.
  • Watershed-total natural flow has increased to its highest level so far this year and higher than last year’s peak.
  • Inflow to Island Park Reservoir is around 1,600 cfs, and outflow is currently just a hair over inflow, allowing the reservoir to drop very slowly. Current reservoir content is a little higher than full pool.

Hydrology and Water-Management Course: Year 14

Photo of Henry's Fork.

In 2005, when I was a professor at Idaho State University, the Henry's Fork Foundation Board of Directors asked me to present an overview of hydrology and water management in the upper Snake River basin. Since then, the hydrology and water-management short course has taken on a life of its own, and I give this presentation in some form or another a few times each year. Every time I give the presentation, I update it with new information, particularly as related to climate change and the rapidly changing work of water management and admininstration.

Streamflow and Reservoir Predictions for Summer 2018

Photo of Fall River.
  • Cool, wet weather from mid-February to mid-April turned an average water supply into one that is decidedly above average.
  • As of May 9, the Henry’s Fork reservoir system is 94% full and filling rapidly.
  • Based on early-April conditions, summertime water supply in the Henry’s Fork watershed is forecast to be above average.
  • More storage water will be delivered from Island Park Reservoir this summer than in 2017, but higher inflows will compensate, resulting in a very high probability of better-than-average carryover at the end of the irrigation season.

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