HFF Blog

High turbidity and sediment event during Memorial Day weekend

The main message: Spring of 2018 brought the highest runoff event in 7 years to the upper Henry’s Fork watershed! Our network of water quality monitors showed that these flows were strong enough to provide a major springtime sediment flush--a natural rhythm of our local hydrology that provides significant benefit to trout and aquatic insect habitat. These favorably high natural flows came in two periods during April and May of this year.

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.

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.

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