HFF Blog

Snowpack melting rapidly: below-average water supply likely by mid-summer

Photo of brown trout.

On March 12, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 122% of the 30-year median. Unfortunately, the last two weeks brought record high temperatures, and we have already lost a large amount of snow. Today, SWE in the Upper Henry's Fork is only 104% of median. SWE is 127% of median in the Fall River subwatershed (down from its peak of 137%) and 125% of median in the Teton River subwatersed (down from its peak of 138%). Although some of the early snowmelt has been stored in Henry's Lake, Grassy Lake, and Island Park Reservoir--and in local and regional aquifers--the vast majority of recent record-high streamflows has flowed out of the upper Snake River basin. This water will not be available later in the summer when we need it most. Check back next week for detailed predictions of streamflow for the upcoming spring and summer.

Timing of snowmelt: why is it important and what do we know about it?

Photo of brown trout.
  • Timing of snowmelt runoff affects river ecology, trout behavior, fishing experience, and mid-summer streamflow in natural and regulated river reaches.
  • Runoff timing is most strongly influenced by April-June temperatures (warmer = earlier runoff) and April 1 snow-water-equivalent (SWE; more snow = later runoff).
  • Since 1930, runoff timing in the upper Henry’s Fork has displayed a parabolic (upside-down “U”) trend over time, with the latest runoff occurring in the 1970s.
  • Date of 30-day peak streamflow in the upper Henry’s Fork in 2015 and 2016 ranked 2nd and 9th earliest, respectively, in the last 87 years. Earliest runoff on record occurred in 1934.
  • Over the past 30 years, mean April-June temperature at the watershed’s snow survey sites has warmed at 1.29°F per decade. All other factors being equal, this moves the 30-day peak flow window 5.4 days earlier each decade. Global climate models and data predict continued warming over the next few decades, so we can expect continued earlier runoff.
  • Runoff timing is poorly predicted by conditions in the month of March, so it is too early to make precise predictions of runoff timing in 2017. However, rapid loss of low- and mid-elevation snow since March 10 is greatly reducing the amount of snow on the ground. Thus, we are sure that runoff will be earlier than I would have predicted two weeks ago. Streamflow conditions will most likely be better than last year but not nearly as good as they could have been had the robust early-March snowpack stuck around until the beginning of April.

Spring-time streamflows predicted to be higher than last year

Photo of brown trout.

Based on snowpack and baseflow conditions as of March 1, predicted natural streamflows for the upcoming April 1 - June 30 time period are:

  • Henry's Lake: 121% of average
  • Henry's Lake to Island Park Dam: 91% of average
  • Henry's Fork upstream of Ashton: 88% of average
  • Fall River: 115% of average
  • Teton River at St. Anthony: 139% of average

Full results are shown in the table below. The text explains methods and inteprets the output.

Table of streamflow predictions

Upper Henry's Fork snowpack reaches 120% of median

Photo of brown trout.

On February 7, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 99% of the 30-year median. The intervening month has brought heavy precipitation to the entire watershed, raising the Upper Henry's Fork SWE to 120% of median. SWE is 131% of median in the Fall River subwatershed (up from 121% on February 7) and 135% of median in the Teton River subwatersed (up from 133% on February 7). All three subwatersheds have already reached their median peak SWE for the whole season.

Upper Henry's Fork snowpack reaches 99% of median

Photo of brown trout.

Ten days ago, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 89% of the 30-year median. As forecast 10 days ago, storms over the past few days have favored the northern part of the watershed for a change, raising the Upper Henry's Fork SWE to 99% of median. SWE is 121% of median in the Fall River subwatershed (up from 113% nine days ago) and 130% of median in the Teton River subwatersed (up from 123% nine days ago). Forecast calls for four more days of wet weather, followed by a week-long dry period.

Upper Henry's Fork snowpack falls below 90% of median

Photo of Rainbow Trout with snow in background.

Nine days ago, snow-water-equivalent (SWE) in the Upper Henry's Fork subwatershed was 91% of the long-term median. Following another large storm that dropped heavy snowfall across the southern part of the state but relatively little across the north, Upper Henry's Fork SWE has dropped to 89% of median. SWE is 113% of median in the Fall River subwatershed (down from 120% nine days ago) and 123% of median in the Teton River subwatersed (compared with 124% nine days ago). Models hint at a more northerly track for the next round of precipitation, which would help the upper Henry's Fork.

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.

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