HFF Blog

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.

Precipitation Continues to Headline HF Water Situation

Henrys Fork near Marysville, with snow-covered Teton Range in background

Conditions at Island Park Reservoir as of midnight, Monday 10/17/2016

  • Reservoir contents: 37,588 acre-feet (27.8% of capacity)
  • Outflow: 129 cfs by USBR gates; 71 cfs by USGS gage (see below)
  • Inflow: 60 cfs from Henrys Lake + 560 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since last flow adjustment, made October 6: 699 ac-ft per day
  • Total volume of water stored since storage began on September 13: 17,108 ac-ft


Island Park Reservoir Continues to Fill Rapidly

Early October snow at Island Park Dam

Conditions as of 12:00 a.m. Oct. 11, 2016

  • Reservoir contents: 33,384 acre-feet (24.7% of capacity)
  • Outflow: 127 cfs by USBR gates; 76 cfs by USGS gage (see below)
  • Inflow: 60 cfs from Henrys Lake + 425 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since last flow adjustment, made October 6: 695 ac-ft per day
  • Total volume of water stored since storage began on September 13: 12,904 ac-ft


Projected Winter Flow Below Island Park Dam

Rainbow over Henry's Fork River.

Island Park Reservoir update, 10/3/2016

In this blog, I provide:

  1. an update on current conditions at Island Park Reservoir (20% full, filling at 400 ac-ft per day, with 7,500 ac-ft filled already this fall),
  2. a review of management objectives (lower flows to 60-70 cfs by mid-October, while keeping flow at St. Anthony around 850 cfs),
  3. my best estimate of fall and winter operations and winter flows at the dam (120 cfs from the dam + 180 cfs from the Buffalo River after November 20),
  4. analysis of USGS streamflow measurement at Island Park (consistently lower than outflow measured through dam gates as plants have decayed), and
  5. some statistics on how water year 2016 stacked up (only slightly better than 2015 and second lowest natural streamflow since 1979).

Brown Trout Redds

The aspens are yellow, the mornings are frosty, and the Brown trout (Salmo trutta) are staging to spawn. Brown trout, like many of their salmonid cousins, migrate from lakes, oceans, or main-stem river reaches to headwater habitat to dig redds (nests in the gravel) and deposit their eggs­1. For Brown trout in the Upper Snake River region, many don’t have access to headwater habitat due to hydraulic barriers and will spawn in main-stem river reaches instead.

Follow Up to Fall-Winter River Flows and Drought Management Planning Update

On September 19, we posted information on fall and winter management of Island Park Reservoir indicating that outflow out of Island Park Reservoir would be gradually reduced to about 60 cfs by mid-October to allow mid-winter outflows of about 120 cfs (http://henrysfork.org/fall-winter-river-flows-and-drought-management-planning-update).

Gill Lice Study Update #2

In June 2016, the Idaho Department of Fish and Game (IDFG) and Henry’s Fork Foundation (HFF) received reports of the presence of gill lice on trout caught within the Henry’s Fork River system. Shortly after, a collaborative effort between HFF and IDFG was underway and we asked anglers and guides to help collect data on the prevalence, severity, and distribution of the parasite within the Upper Snake River region.

Annual Henry's Fork Watershed Council Tour

Photo of council members viewing cloud-seeding equipment tower.

The Henry’s Fork Watershed Council held its annual field tour on August 9. About 25 participants met in Ashton to begin the tour, but more people joined later in the day.

The tour started with a trip to one of Idaho Power’s remote cloud-seeding sites in Island Park, where meteorologist Derek Blestrud explained the science and goals of the cloud-seeding program. Full implementation of ground- and aircraft-based operations in the upper Snake River basin is expected to increase annual streamflow by 400,000 acre-feet.


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