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Photo of moonset on snowy morning
  • January climate stats: 1 degree F above average temperature, 89% of average precipitation, and 83% of average snow water equivalent (SWE) accumulation.
  • February 1 water-year totals: 85% of average precipitation and 77% of average SWE.
  • February has better than even odds of being colder than average, and has started out very wet.
  • Island Park Reservoir has been steady at 88-89% full for the past two months. Outflow has averaged 520 cfs.

January wetter than December, but still dry

The month of January was near average in temperature, at 1 degree F warmer. Although two periods of moderate to heavy precipitation occurred during the month, monthly precipitation was only 89% of average. New snow water equivalent (SWE) accumulation for the month was 83% of average. However, January was quite a bit wetter than December, so even these modest monthly values moved water-year totals up a little. Accumulated precipitation increased from 84% of average to 85% of average over the month, and SWE increased from 71% of average to 77% of average.

Table of climate data

Table of SWE data

Precipitation was not uniformly distributed across the watershed during January. Valley areas and the upper Henry’s Fork received a disproportionate share of precipitation. The upper Henry’s Fork subwatershed received 99% of its monthly average precipitation and 102% of average SWE. The Fall River headwaters came out at the bottom of the geographic regions, receiving 72% of average precipitation and only 67% of average SWE. Meanwhile, precipitation in the valleys was 125% of average. Ashton received 2.32 inches of water equivalent for the month, a whopping 160% of average. All of that water fell as snow, albeit wet at times, and it added substantially to a snowpack in town that was already pretty substantial going into the month.

Graph of precipitation as a percent of average

Graph of snow water equivalent

February starts out extremely wet

Even though January was on the dry side, the two graphs above show how much improvement was made to precipitation and SWE totals between February 1 and February 5. The watershed received 90% of its monthly total precipitation in the first 5 days of the month, moving water-year precipitation up to 96% of average and total SWE accumulation up to 91% of average. As of February 6, forecasts call for below-average temperatures and above-average precipitation for the next two weeks. Overall February is expected to be cold and wet. At this point, it won’t take much more precipitation for February to end up above average.

Island Park Reservoir 89% full; outflow over 500 cfs

Island Park Reservoir stayed constant throughout the month at 88-89% full, the same place it has been since the beginning of December. Outflow from the reservoir has been in the range of 500-560 cfs all winter, with an average of around 520 cfs. For reference, last winter’s average outflow was 504 cfs. In order to keep the reservoir from getting too full while ice-covered, outflow has been a little higher even than the river’s natural flow, as recent precipitation has added substantially to total reservoir inflow. Essentially, with the reservoir already at its desired April-1 contents, the extra water added by direct precipitation has contributed to outflow. Buffalo River flow has been close to average all winter, and the result has been flow through Box Canyon that is equal to the river’s natural flow–what would be in the river in absence of Henry’s Lake and Island Park dams.

Graph of Island Park Reservoir volume

Graph of outflow from Island Park Reservoir

Graph of flow through Box Canyon

What does this mean for the 2019 fishing season?

With almost 8 weeks still remaining in the snow accumulation season, the range of possible peak SWE values is pretty large. Worst-case scenario is that we get no more precipitation the rest of the winter and end up at 60% of average SWE. This would put the 2019 snowpack close to the minimum experienced over the last 30 years. Of course, that won’t happen, given that moderate precipitation is forecast with high confidence 3-5 days from now. Based on long-term outlooks, there is a pretty good chance of receiving average precipitation for the rest of the winter, which would result in peak SWE of about 93%. We would need 120% of average precipitation over the next 8 weeks to reach average in early April. Although well within the range of possibilities, 120% of average precipitation for the rest of the winter is not likely. At this point, I predict that peak SWE will end up around 90% of average, which is pretty good considering that SWE was below 70% of average in early January.

When and where this year’s snowpack contributes to streamflow depends on how it is distributed across the watershed and how fast it melts. Right now, the upper Henry’s Fork and Teton subwatersheds seem likely to end up with SWE near average, but peak SWE in Fall River will almost certainly be below average. Based on these predictions and on current baseflows (amount of water in streams during the late winter, when groundwater is the primary source), summertime streamflow is likely to be around average in the upper Henry’s Fork and Teton River and around 85% of average in Fall River. Depending on spring and early summer rain, these conditions would suggest summertime streamflow in Fall River that is only about 80-90% of its 2018 value and delivery of storage from Island Park Reservoir that is a little higher than it was in 2018. However, I do not anticipate extended periods of outflow from Island Park Reservoir any higher than around 1,300 cfs.

Regardless of what the weather is like between now and irrigation season, two things are certain at this point. First, because Island Park Reservoir is so close to full, outflow from the reservoir during periods of snowmelt and rain-fed runoff this spring will be roughly the same as inflow, resulting in natural springtime freshet flows in the river downstream of the dam. If rain events are heavy, like they were in 2018, some of these runoff flows could be pretty high–1,600 cfs or greater–and could result in rapid flow changes in response to changes in the weather. The second thing that is certain right now is that trout recruitment in the river downstream of Island Park Dam will be very good this year and next year, due to well above-average winter flow both last year and this year. Whether we get a third consecutive year of high winter flows will depend on the 2019 snowpack, spring and summer rain, and how much more water can be saved in Island Park Reservoir as HFF and its partners continue to expand irrigation demand-reduction and precision water management programs.





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