HFF Blog

Water Supply Still Strong Well Into July

Photo of Rainbow Trout.
  • Total natural streamflow in the Henry's Fork watershed is 122% of average and 2,500 cfs greater than irrigation diversion, which remains slightly below average.
  • No water has been delivered out of Island Park Reservoir yet this season, compared with 40,566 ac-ft (30% of reservoir capacity) last year at this time.
  • Island Park Reservoir outflow will remain roughly equal to inflow until release is needed to meet irrigation demand, which now looks to be another week out.

More Showers Keep Water Supply Above Average

Photo of Rainbow Trout.
  • Most locations around the watershed received around one-half inch of rain this week.
  • Natural streamflow supply in the watershed is 7,000 cfs, well above average for the date and well above diversion rate, which is 3,000 cfs.
  • Delivery of storage water from Island Park Reservoir will not be needed until at least July 5.

Rain Boosts Water Supply as Peak Irrigation Season Arrives

Photo of Rainbow Trout.
  • Over two inches of rain fell in parts of the watershed last week, providing another peak in streamflow in Fall River and Teton River.
  • Total watershed streamflow remains around 8,000 cfs, compared with irrigation diversion of 3,000 cfs.
  • Island Park Reservoir will remain full until delivery is needed to meet irrigation demand, which now looks to be at least two weeks away.
  • Outflow from Island Park Reservoir is being reduced frequently to keep pace with natural streamflow recession in the upper Henry's Fork.

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.


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