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.

Now that outflow from Island Park Reservoir is stable at its mid-winter value, our water-supply attention turns to accumulation of water in the snowpack and predictions for the upcoming spring. This blog will set the stage for assessing SWE and what it means for management of Island Park Reservoir and streamflows this spring. I will provide the usual reservoir update at the end of the blog, for those of you who have been following that since the fall.

Current Snow-Water-Equivalent (SWE)

Overview of the relevent snow-survey sites

The Natural Resources Conservation Service (NRCS) maintains a large network of snow-survey sites throughout the western U.S. The sites that transmit daily data via telemetry are the most useful to us and are called “SnoTel” sites. You can explore the entire NRCS SnoTel network through a really cool interactive map, linked here. The nine SnoTel sites that indicate water supply in the Henry’s Fork Watershed (Upper Henry’s Fork, Fall River, and Teton River subwatersheds) are shown in the map and accompanying table below.

Map showing snow-survey site in the Henry's Fork watershed.

Map (above) and key (below) to SnoTel sites that indicate water supply in the three subwatersheds that lie within the Henry’s Fork Watershed.

Table of SnoTel sites.

Spatial patterns–across the West and close to home

So far this winter, the trajactory of snowstorms in the western United States has resulted in above-average snowfall in the southern Cascades, Great Basin, Wasatch, southern portion of the Snake River basin, and the Colorado Rockies and below-average snowfall in the northern Cascades, northern and central Idaho, and Montana. This is readily visible in a screen-shot of the interactive map linked above.

Map of western U.S. showing snow-water-equivalent.

The upper Henry’s Fork watershed (Henry’s Lake to Ashton) lies right at the transition between above-average snowfall to our south and below-average snowfall to our north. This is evident in today’s SWE figures, which appear in the table below.

Table of SWE and precipitation values.

Snow-water-equivalent this morning is only 91% of the 30-year median in the upper Henry’s Fork watershed but increases to 123% of median in the Fall River watershed and 130% in the Teton River watershed. This increase in SWE with distance south from the Montana-Idaho border reflects larger spatial patterns across the west.

Then why did I read that snowpack is above average in the Henry’s Fork watershed?

The standard snowpack products–tables and maps provided by NRCS and other agencies such as the US Forest Service and US Bureau of Reclamation–do not divide the Henry’s Fork watershed into its three component subwatersheds but rather report a single watershed-wide average. Furthermore, these standard products, which end up in newspapers and on popular web sites, do not include the Crab Creek SnoTel site because it lies well outside of the Henry’s Fork watershed. Our analysis has shown that this relatively low-elevation site is one of the best predictors of water supply in the upper Henry’s Fork watershed, and its omission ovestimates water supply to Island Park Reservoir, which, of course, is the centerpiece of water management and fishing conditions in the Henry’s Fork. Click here to read more about the importance of the Crab Creek site and how we calculate water supply from SnoTel sites.

This year, SWE is above average in the Fall River and Teton River watersheds but below average in the upper Henry’s Fork watershed–the lowest being the two stations in the Centennial Range: Crab Creek and White Elephant. Omitting the Crab Creek site and including Fall River and the Teton River into the average provides a rosy picture of water supply over the entire watershed but greatly overestimates the water supply to Island Park Reservoir and to the springs that make up the majority of the river’s flow upstream of Ashton. The table above provides a more accurate picture of water supply in each of our three subwatersheds.

I encourage you to explore the large number of ways you can interact with SnoTel and precipitation data. In addition to the interactive map link, here are a few more links to get you started:

Main NRCS Water and Climate home page

Daily Idaho SWE and Precipitation Summary

Main SnoTel data page

What is the current streamflow forecast for the spring?

Based on current SWE, the U.S. Bureau of Reclamation predicts streamflow in the upper Henry’s Fork watershed at 85% of average. Based on the current 91% of average SWE and current streamflow in the spring-fed tributaries to the upper Henry’s Fork (77% of average), my prediction is consistent with Reclamation’s. At the beginning of February, when a larger fraction of the winter’s snowpack is on the ground, I will produce projected hydrographs and reservoir levels that will have much more certainty than they do right now. However, predicting the amount of water that will end up in the reservoir and the river is less than half of the story. In recent years, timing of runoff has proven to have a greater influence over management of Island Park Reservoir and streamflows in the river downstream. See following links for how these predictions evolved during the winter and spring of 2016, which at this point in the winter looked very similar to this year in the upper watershed:

How a reasonably good February outlook deteriorated by March

April 1 conditions and predictions

Effect of early snowmelt on streamflow

Take-home messages

The take-home messages here are:

  • that a lot can change between now and April and that
  • timing of snowmelt is critical to spring-time reservoir and river conditions.


Current Reservoir and Streamflow Conditions

Conditions at Island Park Reservoir as of 1/9/2017

  • Reservoir contents: 80,829 acre-feet (59.8% of capacity)
  • Mean outflow since 12/10/2016: 164 cfs by USGS gage; 162 cfs by USBR gate setting
  • Mean streamflow through Box Canyon since 12/10/2016: 354 cfs
  • Current inflow: 0 cfs from Henrys Lake + 380 cfs reach gain from Henrys Lake to IP
  • Mean storage rate since 12/10/2016: 369 ac-ft per day
  • Total storage since 9/13: 60,348 ac-ft (starting value was 20,481 ac-ft)

Conditions at Henrys Lake as of 1/9/2017

  • Reservoir contents: 80,995 acre-feet (90.0% of capacity)
  • Mean inflow since December 10: 39 cfs
  • Mean storage rate since 12/10/2016: 77 ac-ft per day
  • Total storage since 10/21: 6,134 ac-ft (starting value was 74,861 ac-ft)

Streamflow overview

Reach gain between Henry’s Lake and Island Park continues to track slightly below my late-summer prediction except during and immediately following large precipitation events (see graph below). Since December 10, inflow to Island Park Reservoir has averaged 352 cfs, compared with a long-term average of 452 cfs and with my earlier prediction of 357 cfs. As a result, Island Park Reservoir is filling slightly slower than my earlier projections. At current fill rate relative to average, April 1 reservoir content will be 110,400 ac-ft, around 1.5% below our April 1 target of 112,100 ac-ft. On the other hand, Henry’s Lake inflow has averaged 39 cfs, compared with a long-term average of 42 cfs, and Henry’s Lake has been filling rapidly. At the current inflow rate relative to average, Henry’s Lake will fill on April 20, allowing early-spring snowmelt from the Henry’s Lake basin to contribute toward filling remaining space in Island Park.

Graph of inflow to Island Park Reservoir

Reach gain between Henry’s Lake and Island Park, September 1 – January 8, 2016, compared with the prediction I made in September, observed values in water-year 2016, and the 1979-2016 average.

Island Park Reservoir Management and Streamflow

As decided at the November 21 meeting of the Island Park Drought Management Planning Committee (DMPC), outflow from Island Park Reservoir was increased during the week of November 28. The weather forecast at the time indicated that the first single-digit lows of the season would occur around December 2. Accordingly, outflow was increased on November 30 from 100 cfs to 150 cfs. A slight upward adjustment was made on December 9 to accommodate an adjustment of the USGS stream stage recorder made on December 7. Since December 10, outflow from the dam has averaged slightly above the DMPC’s winter objective of 160 cfs, and total streamflow downstream of the Buffalo River has averaged 354 cfs, compared with last year’s 299 cfs and the long-term average of 531 cfs (see graph below). At its January 10 meeting, the DMPC agreed to keep outflow from Island Park Reservoir at its current value for the next 6 weeks, even though the reservoir is filling a little more slowly than anticipated and upper Henry’s Fork snowpack is below average.

Graph of flow in Henry's Fork downstream of Buffalo River.

Total streamflow in the Henry’s Fork through Box Canyon (IP Dam outflow + Buffalo River flow) since October 1, 2016, compared with observed values in water-year 2016, and the 1979-2016 average.

Photo of yard and driveway with a lot of snow.

Unfortunately, above-average snowpack in my driveway does not indicate above-average snowpack in Island Park!