Photo of Rainbow Trout.

December precipitation was only 51% of average across the Henry’s Fork watershed. However, water year-to-date precipitation and snow-water-equivalent are at 87% and 89% of average, respectively. And, thanks to above-average precipitation during water year 2017, the upper Snake River reservoir system is 87% full, and winter streamflow is above average. Long-range forecasts call for average to above-average precipitation for the remainder of the winter, so the overall water-supply outlook remains good as we head into the New Year.

December 2017: Slightly warm and very dry

December was a little warm but very dry throughout the Henry’s Fork watershed. Thanks to extended periods of high-pressure temperature inversions, valley stations were 0-2 degrees F cooler than average, while high elevations were 2-4 degrees warmer. Averaged over the watershed, December turned out to be 2 degrees warmer than average.

Meanwhile, precipitation for the month was only 52% of average over the watershed, ranging from 48% of average in the Teton subwatershed to 67% in the valleys. New SWE accumulation for the month was 51% of average, ranging from 45% of average in Fall River subwatershed to 55% of average in the Upper Henry’s Fork subwatershed. However, thanks to a wet November, water year-to-date totals aren’t as bad: precipitation is 87% of average, and SWE is 89% of average.

Table of climate statistics.

Summary of climate statistics for December 2017.

Graph of SWE as a percent of average.

Snow-water-equivalent (SWE) as a percent of average.

Reservoirs Nearly Full

Thanks to above-average precipitation during water-year 2017, very little reservoir storage was needed to meet irrigation demand during the summer of 2017. The upper Snake Reservoir system is currently 87% full, a figure that is reflected in the Henry’s Fork watershed. Island Park Reservoir is 88% full, and total storage across the three reservoirs in the Henry’s Fork watershed is 89% of capacity. Outflow from Henry’s Lake so far this water year has very closely matched inflow; Henry’s Lake has the same volume now as it did on October 1. Outflow from Island Park Reservoir has averaged just a little under 500 cfs since October 1, the highest autumn/winter flow since the winter of 2011-2012. The reservoir has slowly gained volume over that time period, primarily due to direct precipitation on the nearly-full reservoir surface.

Graph of Henry's Lake volume

Graph of Henry’s Lake volume since October 1, 2017. 

Graph of inflow to IP Reservoir.

Graph of net inflow to Island Park Reservoir. Note contribution of direct precipitation during November and mid-December.

Graph of Island Park Reservoir volume.

Graph of Island Park Reservoir volume since October 1, 2017. Note accelerated reservoir fill during November and mid-December as a result of direct precipitation.

Graph of Island Park Reservoir outflow.

Outflow from Island Park Reservoir.

Streamflow Above Average

Natural streamflow remained strong over the month of the month of December, at 118% of average in the Teton River and 91% of average in the upper Henry’s Fork (watershed upstream of Ashton). Stream gages on Fall River were affected by ice for most of the month, but above-average streamflow in the Henry’s Fork at St. Anthony reflects above-average contribution from Fall River, given that flow in the Henry’s Fork at Ashton has been very close to the long-term average. The available data suggest that watershed-total natural flow was above average during December.

Graph of streamflow in HF at St. Anthony.

Graph of regulated streamflow in the Henry’s Fork at St. Anthony.

One of the most important indicators of long-term recovery from the four-year drought of 2013-2016 is natural inflow between Henry’s Lake and Island Park Reservoir, because this primarily reflects inflow from Big Springs and other groundwater sources. This natural inflow, referred to as “Henry’s Lake-to-Island Park reach gain,” was 111% of average over the month of December.

Graph of inflow between Henry's Lake and IP Reservoir.

Graph of natural inflow between Henry’s Lake and Island Park Reservoir.

Water-Supply Outlook Favorable

Despite below-average snowpack to date, the overall water-supply outlook remains favorable as we head into the New Year. Henry’s Lake and Island Park Reservoir will fill easily without any reduction in outflow, even if the remainder of the winter is dry. Stream baseflows remain strong from above-average precipitation during 2017 and will carry over into the spring of 2018. At this point, even average precipitation over the remainder of the winter will provide average to above-average streamflow during the spring and early summer. Beyond that, timing of runoff and irrigation demand will determine need for reservoir storage, which, in turn, will affect water supply and streamflow next year at this time.

New Data Guide Available

Thanks to a new map created by HFF Research Associate Bryce Oldemeyer, we now have a comprehensive guide to the data sources and individual weather and streamflow stations used in my daily water reports and blogs. The map appears below. The data guide is linked here. To receive my daily water-supply reports via email, send an email to

Map if HF watershed, showing stations used in our water reports.