HFF Blog

Upper Henry's Fork snowpack remains below average

Photo of Rainbow Trout with snow in background.

Eight days ago, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 91% of the long-term median. One snowy day and seven dry days later, Upper Henry's Fork SWE is still at 91% of median. SWE is 120% of median in the Fall River subwatershed (down from 123% eight days ago) and 124% of median in the Teton River subwatershed (down from 130% eight days ago). Current forecasts call for near-average precipitation over the next five days, followed by below-average precipitation for several weeks after that.

Snowpack in Upper Henry's Fork Watershed: Below average and lower than its neighbors

Photo of Rainbow Trout with snow in background.

Summary: Snowpack in the Upper Henry's Fork Watershed is Below Average

Half-way through the snow accumulation season, the amount of water in the snowpack (snow-water-equivalent or SWE) in the upper Henry's Fork watershed is only 91% of the 30-year median. As a result, we are anticipating another year of below-average streamflow in the Henry's Fork watershed upstream of Ashton. The upper Henry's Fork is the only watershed in the entire upper Snake River basin to have received below-average SWE so far this winter. Even the watersheds immediately to the south--Fall River and the Teton River--have received above-average snowfall. When these sub-watersheds are averaged in with the upper Henry's Fork, as is done all of the map-based and tabular products distributed by water-management agencies, the outlook for the Henry's Fork watershed as a whole appears much better than it is in the upper watershed, which is the most relevant to management of Island Park Reservoir and streamflows between Island Park and Ashton.

Fish of the Month: Year Two

Photo of Rainbow Trout.

Last year at this time, I reported on year one of my 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 Thursday December 15, 2016, I successfully added month 24 to the current streak.

Island Park Reservoir update 12/18/2016

Photo pf rainbow trout

Conditions at Island Park Reservoir as of 12/17/2016

  • Reservoir contents: 72,446 acre-feet (53.6% of capacity)
  • Mean outflow since 12/10/2016: 164 cfs by USGS gage; 161 cfs by USBR gate setting
  • Mean streamflow through Box Canyon since 12/10/2016: 354 cfs
  • Inflow: 0 cfs from Henrys Lake + 360 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since 12/10/2016: 386 ac-ft per day
  • Total storage since 9/13: 51,965 ac-ft (starting value was 20,481 ac-ft)

Water Supply Overview

Turbidity persistence test: conclusions from the 2016 data

This fall HFF initiated a new water-quality monitoring study designed to answer the following question: How far downstream of Island Park Dam (IP Dam) do high turbidity levels persist? I introduced this study in my blog post made on September 9, 2016. Please find details of the background and motivation for this study, as well as a map of the sampling sites, in the previous blog post. The current blog post summarizes what we found during our 2016 data collection season.

Island Park Reservoir update 11/10/2016

Photo of rainbow trout.

Conditions at Island Park Reservoir as of 11/10/2016

  • Reservoir contents: 54,480 acre-feet (40.3% of capacity)
  • Outflow: 100 cfs, according to recently adjusting USGS rating
  • Estimated streamflow downstream of the Buffalo River: 290 cfs
  • Inflow: 0 cfs from Henrys Lake + 375 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since Henry’s Lake Dam outflow was reduced: 518 ac-ft per day
  • Total storage since 9/13: 33,999 ac-ft (starting value was 20,481 ac-ft)


Wet weather continues

Photo of Fall River.

Conditions at Island Park Reservoir as of Friday morning, 10/28/2016

  • Reservoir contents: 45,142 acre-feet (33.4% of capacity)
  • Outflow: 110 cfs by USBR gates; 75 cfs by USGS gage
  • Estimated streamflow downstream of the Buffalo River: 300 cfs
  • Inflow: 60 cfs from Henrys Lake + 430 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since my last report on 10/16: 687 ac-ft per day
  • Total storage since 9/13: 24,661 ac-ft (starting value was 20,481 ac-ft)

What do macroinvertebrates tell us about the Henry’s Fork?

Photo of salmonfly nymph.


  1. Aquatic macroinvertebrates—insects and other creatures that live in the stream bottom—feed trout and are the basis of fly-fishing on the Henry’s Fork, but they are also important indicators of aquatic habitat quality.
  2. HFF is two years into a long-term program of monitoring aquatic macroinvertebrates at Flatrock, Last Chance, Osborne Bridge, Ashton, and St. Anthony.
  3. Primary conclusions from comparison of 2015 and 2016 data are:
    1. Abundance of macroinvertebrates averages about 47,000 individuals per square meter of stream bottom across all sites.
    2. Mayflies, stoneflies and caddisflies dominate the invertebrate assemblage at Flatrock, Last Chance, and Ashton, but are outnumbered by other organisms at Osborne and St. Anthony.
    3. Aquatic habitat quality ranges from excellent at Flatrock to good at St. Anthony, decreasing with distance downstream from the headwaters.
    4. The only statistically significant differences between 2015 and 2016 occurred at Osborne Bridge, where total abundance of invertebrates decreased—primarily because of a decrease in non-insects—and where habitat-quality index increased, reflecting an increase in percentage of mayflies, stoneflies, and caddisflies.
    5. Most of the mayflies and stoneflies important to fly anglers were found at all five sites. These were pale morning dun, flav, blue-winged olive, green drake, trico, brown drake, and yellow sally.


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