Water year 2017 was above average in both precipitation and streamflow. Averaged over the whole watershed, precipitation was 138% of average, led by the Fall River watershed, at 144% of average. Due to the cumulative effect of four years of drought, especially in the groundwater-dominated subwatersheds, natural streamflow was only 109% of average, led by the Teton River at 134% of average. Streamflow in the upper Henry’s Fork, which is dominated by groundwater, was only 91% of average.

Water Year 2017 Climate: Wet but Warm

Photo of rainbow over Henry's Fork.

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Graph of water-year precipitation as a percent of average.

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Table of water-year total precipitation

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Graph of water year temperature.

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Table of water-year temperatures.

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Photo of rain clouds over Henry's Fork.

Water Year 2017 Streamflow 9% Above Average

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Graph of natural streamflow in Henry's Fork watershed.

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Table summarizing natural streamflow.

2017 Diversion Near Average but Storage Delivery Below Average

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Graph of total diversion in Henry's Fork watershed.

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Graph of outflow from Island Park Reservoir.

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Graph of Island Park Reservoir volume

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Table of end-of-season volume in Island Park Reservoir.

Looking Ahead

With Island Park Reservoir already very near its April-1 average, outflow from the reservoir over the remainder of the fall and winter will be very close to inflow. Projected gain in the river between Henry’s Lake and Island Park will average 415 cfs over the fall and winter. Currently, Henry’s Lake is delivering about 50 cfs to Island Park Reservoir, but that will probably decrease to somewhere between 0 and 20 cfs later in the fall. Thus, projected outflow from Island Park Reservoir during the middle part of the winter is between 400 cfs and 450 cfs, which will be the highest outflows since the winter of 2011-2012.