HFF Blog

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.

Summer Water Supply Predicted to be Above Average

Photo of Rainbow Trout.
  • April 1 snow-water-equivalent was 106% of average in the Henry's Fork watershed.
  • Based on April 1 conditions, summer water supply is predicted to be 108% of average for the whole watershed:
    • 99% of average in the Henry's Fork upstream of Ashton
    • 117% of average in Fall River
    • 114% of average in Teton River
  • Snowmelt is starting at its normal time this spring due to seasonable temperatures.

Henry's Fork Dodges a Rain-on-Snow Bullet

Photo of Rainbow Trout.

Despite a forecast for conditions that could have resulted in large loss of snowpack yesterday, atmospheric conditions lined up just right to not only avoid the loss but actually gain a very large amount of snow-water-equivalent (SWE). Read on for the details, as well as for an example from the spring of 2010 that illustrates a very large rain-on-snow event.

February Ends Cold and Wet; Water Supply Improves

Photo of Rainbow Trout.
  • Over the month of February, water-year precipitation improved from 93% of average to 98% of average, and snow-water equivalent improved from 96% of average to 101% of average.
  • Mean outflow from Island Park Dam over the December-February time period most critical for survival of juvenile trout was 509 cfs, 147% of average and the highest since the winter of 2011-2012.
  • Streamflow throughout the watershed remained above average at most locations.

A Warm Winter

 

It’s no secret that 2018 has been much warmer than we’re used to (in January, 5°F warmer than normal across the whole watershed and as high as 7°F above average in Island Park). At the same time, HFF has been reporting SWE (snow-water equivalent) numbers at 96% of average at the end of January (as high as 103% of average this past week), above average streamflow, and near average precipitation across the watershed (not to mention 111% of average Jan. precipitation at Island Park). If you’re looking out your window in Island Park right now, you’re probably wondering, “How is that possible?”

Water Supply Outlook Improves in January

Photo of Rainbow Trout.
  • Over the month of January, water-year precipitation increased from 87% of average to 93% of average, and snow-water-equivalent increased from 89% of average to 96% of average.
  • Streamflow throughout the watershed was generally above average, including outflow from Island Park Reservoir, which, at 508 cfs, was 134% of average and the highest since the winter of 2011-2012.
  • The only negative aspect of the current water situation is that January was much warmer than average, limiting snow accumulation at lower elevations and setting up the snowpack to melt early and rapidly.

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