HFF Blog

Henry's Fork Reservoir Storage and Streamflow at Peaks

Photo of Rainbow Trout.

Week of June 5 highlights:

  • Island Park Reservoir, Grassy Lake and Henrys Lake have all filled.
  • Natural streamflow in the watershed is around 9,500 cfs and at its final peak for the season.
  • Total watershed-wide irrigation diversion is around 3,000 cfs, right at average for this time of year.
  • Outflow from Island Park Reservoir is being set to keep the reservoir full. Only small changes are anticipated over the next week, and outflows should remain around 950 cfs plus or minus 150 cfs, depending on rain.
  • Delivery of water from Island Park Reservoir in excess of inflow is not expected to be needed until early July.

2017 Box Canyon Rainbow Trout Population Estimate

Every year the Idaho Department of Fish and Game (IDFG) conducts multi-pass electrofishing surveys on various river reaches in the Upper Snake River region. These surveys provide valuable information on abundance, age-class structure, fish size, and species composition within the fishery. Most recently, IDFG finished its annual survey in the Box Canyon reach of the Henry’s Fork. This post will highlight and explain some of the results of that survey.


Invasive Species

Two weeks ago salmonflies started showing up on the lower Henry’s Fork. Once the word got out, fisherman flooded from near and far for the opportunity to catch big fish on big dries.  More than once I heard the classic “you don’t need to bring a boat, you can walk down the river on all the other ones” line. While the river was busy, it wasn’t that bad. After seeing lines of cars and trailers at Ora, Vernon, Chester, etc.

Water Supply Still Good on June 1

Photo of Rainbow Trout.
  • Precipitation in May was about 50% of average, but temperatures were near average, preserving most of the high-elevation snow all the way through the month.
  • Snow remaining in the watershed at elevations above 7,000 feet was almost twice the median amount on June 1.
  • Warm temperatures are leading to increased streamflows in the Henry's Lake basin as well as in Fall River and Teton River.
  • Fall River and Teton River will peak in early June but supply enoughy water to meet irrigation demand through the month of June
  • Island Park Reservoir is 100% full, and outflow will roughly follow natural inflow as it gradually drops from 850 cfs now to around 500 cfs by July 1.


Record Cold Slows Runoff, Buys Another Week of Good Flow Later in Summer

Photo of Rainbow Trout.

A three-day winter storm in the middle of the week set record cold temperatures for the date and greatly slowed snowmelt, saving that snow to melt later in the spring and summer. Elevations above 6,500 feet gained up to inch of new snow-water-equivalent this week, including the White Elephant site on the side of Mt. Sawtelle. As a result, current snowpack at the subwatershed scale is currently between 122% and 147% of median for the date. Although streamflow at most locations is below average this morning, warm temperatures forecast for the next 10 days will resume melt, leading to season-peak streamflows into Henry's Lake and in Fall River and Teton River around June 1. Henry's Lake and Island Park Reservoir will both fill around that time.

Above-average Runoff Benefiting Stream Habitat Across the Watershed

Photo of Rainbow Trout.

A well-above-average May snowpack has begun melting over the past week, resulting in above-average streamflow throught the watershed. Minimal amounts of water are being stored in the watershed's reservoirs, and irrigation diversion is only around 20% of total water supply. As a result, streamflows are very near their natural values--essentially the same flow the rivers would have in absence of reservoirs and diversions. These high, natural flows--occurring within the range of long-term average timing--are currently benefitting physical and ecological processes in stream channels and riparian areas throughout the watershed, mobilizing and removing fine sediment from the stream bottom, creating new habitat, and ensuring reproduction of cottonwood trees. After four years of drought, the sight of snowmelt filling rivers and floodplains is a welcome change.

Water Year 2017 Looking Better Every Day

Photo of Rainbow Trout.

With a very wet April now in the books, the 2017 water year is certain to be above average in terms of water supply in both the Henry's Fork watershed and in the upper Snake River basin as a whole. Highlights are:

  • April precipitation in the Henry's Fork watershed was 162% of the 1982-2016 mean and 261% of last year's value.
  • May 1 snow-water-equivalent in the HF watershed was 146% of the 1981-2010 median and 228% of last year's value.
  • April 30 natural streamflow in the Snake River at Milner was 171% of the 1988-2014 mean and 130% of last year's value.
  • Total diversion on April 30 in the upper Snake River basin was 82% of the 1988-2014 and only 72% of last year's value.

More supply + less demand = higher streamflows.

April Showers Greatly Improve Water Outlook

Photo of Rainbow Trout.

Four weeks ago, water-supply outlook for the upper Henry's Fork watershed was average at best because of an extended period of warm, dry weather in March that had already melted much of the low- and mid-elevation snowpack. However, April has turned out to be one of the wettest on record, greatly improving water supply and increasing the probability that relatively little Island Park Reservoir storage water will be needed to meet irrigation demand this summer. With three days left to go in the month, Island Park has received 4.5 inches of precipitation in April, compared with the 1981-2016 average of 2.3 inches and a maximum over that period of 4.2 inches in 1993. However, temperatures have remained above long-term averages throughout the month.

Buffalo River Fish Ladder: April 2017 Update

Henry’s Fork Rainbow Trout migrating to spawn in the Buffalo River have to pass through the fish ladder at the Buffalo River hydroelectric facility to access upstream spawning habitat. At the end of the fish ladder we, HFF, operate a fish trap from early February through the middle of June. Three times a week we check the Buffalo River fish trap and collect data on species, length, sex, and life histories via passive integrated transponders (PIT) tags if one is present.

Summer 2017 Streamflow Predictions: Much Better than 2016

Photo of brown trout.

To provide streamflow information for all river stakeholders, we have constructed a computer simulation model of the Henry’s Fork watershed stream, reservoir, and irrigation system. Using early-April water-supply conditions and long-term temperature trends as inputs, we expect streamflow conditions to be generally better than average and much better than last year across the watershed. Focusing specifically on Island Park Reservoir and the river immediately downstream, we predict:

  • Streamflow during the second half of June at Island Park Dam will be roughly equal to the river’s natural flow. With 90% probability, this natural streamflow will range between 400 and 750 cfs.
  • Irrigation delivery from Island Park Reservoir will begin around July 1 and peak in mid-July. With 95% probability, releases from the reservoir during July will be lower than 1,400 cfs, and with over 95% probability will be much lower than 2016 releases between the middle of June and the first week of August.
  • With 95% probability, Island Park Reservoir contents at the end of the September will remain above 58,000 ac-ft (43% full), very close to the long-term average.
  • Because of lower outflows and higher reservoir contents, turbidity (how “dirty” the water appears) in the river immediately downstream of Island Park Dam is expected to remain lower than the high values observed in 2016 during the mid-July cyanobacteria bloom and late-summer reservoir drawdown.


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