Dr. Rob Van Kirk's 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.

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.

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.

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