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Predicted 2020 Water Supply Near Average

Updated: Jan 12, 2022

  • A cold, wet March brought snow water equivalent up to 97% of average, after it spent most of the winter down around 90% of average.

  • Watershed-wide natural streamflow for April through September is forecast to be 97% of average, compared with 99% of average in 2019.

  • Delivery of water from Island Park Reservoir is predicted to begin around July 1, 12 days earlier than in 2019, and stay in the range of 800-1100 cfs during July and August.

  • The reservoir is expected to end the irrigation season around 77% full, compared with 73% full in 2019.

  • Higher water levels in Island Park Reservoir have numerous, scientifically documented benefits to the Henry's Fork fishery, prompting HFF to continue to expand the number of methods we use to limit delivery of water from the reservoir.

Cool, Wet March Sets Up Favorable Water Supply

Wet, cold weather at the end of March capped off a month of above-average precipitation. Watershed-average precipitation in March was 4.17 inches, 113% of average. New accumulation of snow water equivalent (SWE) was even better, at 5.8 inches, 135% of average. These totals increased water-year precipitation from 86% of average at the beginning of the month to 91% of average at the end. SWE increased from 90% of average to 97% of average over the month. Although mean temperature for the month was right at average, temperatures over the last two weeks of the month were well below average, preserving snowpack already on the ground in the middle of the month and adding more snow at the end of the month during a time when snow starts to melt at the lower elevations. As of April 2, SWE is at its peak at all nine SnoTel sites, whereas on average, SWE at Island Park peaks on March 26 and that at Pine Creek Pass peaks on March 30. Thus, March set the table for a healthy, fresh snowpack going into spring.

Precipitation as a percent of average, by subwatershed.

Watershed-averaged temperature relative to average.

Snow water equivalent as a percent of average, by subwatershed.

Watershed-averaged snow water equivalent.

Predictions for the Upcoming Spring and Summer

In 2017, I developed a probabilistic ("stochastic") model of the entire Henry's Fork stream, reservoir, and irrigation system for the purposes of predicting streamflow and reservoir content for the upcoming irrigation season. The model is based on hydrologic conditions as of April 1. The most important output of the model is content of Island Park Reservoir at the end of September, what we term as "reservoir carryover." In 2017, the model predicted carryover to within 1%. In 2018 and 2019, the model underpredicted carryover by about 7%, primarily because precipitation during mid- to late-summer was low in both years, and more reservoir storage was needed late in the summer than expected. However, the observed values were well within the margin of statistical error in both years.

The 2020 version of the model includes new methods for simulating diversion and what are called "reach gains," the amount of water that stream reaches in the lower watershed gain from or lose to groundwater. Extensive analysis of diversion and reach gains in the Henry's Fork watershed show that both declined substantially around the year 2000 as a result of conversion from flood to sprinkler irrigation (full report available here). Thus, the 2020 version of the model randomly selects analog years to model diversion from 2001-2019, using April-1 SWE and stream baseflows (the amount of streamflow during the fall and winter) as predictors. It turns out that contrary to expectations, diversion is higher during wet years than during dry years, simply because more water is available to divert during wet years. Reach gains show no particular pattern other than a sharp decrease in the early 2000s, so gains were selected randomly from among those observed during the 2004-2019 irrigation seasons.

Natural streamflow

The model starts with predictions of natural streamflow over the April-September period. Over the whole watershed, natural flow is expected to be 97% of average, compared with 99% of average in 2019 and 104% of average in 2018. However, whereas natural flow was fairly uniformly distributed across the three subwatersheds in 2019, a relatively larger percentage will come from the Teton subwatershed in 2020. April-1 SWE this year was above average only in the Teton subwatershed. With 90% probability, natural flow will be at least 81% of average. This means that under a worst-case scenario, streamflow will be at least 81% of average, which is still within the range that will lead to near-average carryover in the reservoir and near-average outflow the following winter. For comparison, April-September natural streamflow was 71% of average in 2016, 69% of average in 2015, and 74% of average in 2013, some of the more recent dry years.

Predicted natural streamflow in the three subwatersheds of the Henry's Fork watershed for spring and summer of 2020.

Streamflow in lower watershed

Even though Teton River streamflow is predicted to be higher than in 2019, recall that the spring of 2019 was very cool, and snowmelt was later than in recent years, more typical of that in the late 1990s. This year's peak streamflow is predicted to occur about four days earlier than last year. The result is that Teton River flow this year is expected to be a little higher than last year's flow during spring, a little lower during July, and generally equal to or a little higher late in the summer. As seen in the table above, streamflow on Fall River is expected to be lower than last year. Along with prediction of peak streamflow around three days earlier than last year, we can expect streamflow in Fall River to be lower than that in 2019 throughout the spring and summer. Late in the summer, streamflow in Fall River is expected to be close to what we saw in 2019.

Graph of predicted streamflow in Teton River, compared with 2019 and the long-term average.

Graph of predicted streamflow in Fall River, compared with 2019 and the long-term average.

The single most important component of the Henry's Fork system that determines flow releases from Island Park Dam during the summer is need to deliver water from the Henry's Fork to the Teton River via the Crosscut Canal to meet irrigation needs on the lower Teton. The Crosscut Canal diverts water from the Henry's Fork at Chester Dam and delivers it to the Teton River just north of the town of Newdale. Need for delivery of water through the Crosscut Canal is expected to start around July 1, whereas substantial delivery through the Crosscut Canal was not needed until around July 10 last year. This, along with lower natural flow in Fall River, will trigger need for draft of Island Park storage around July 1, versus July 12 last year.

Graph of predicted delivery of water to the Teton River through the Crosscut Canal.

Draft of Island Park Reservoir is needed to meet diversion needs upstream of St. Anthony and keep streamflow in the Henry's Fork at St. Anthony at its irrigation-season target flow, which has been set at 1,000 cfs each of the past two years. I have used 1,000 cfs in the model, even though the Henry's Fork Drought Management Planning Committee may choose to use a different target this year. Generally, 1,000 cfs at St. Anthony is sufficient to meet irrigation needs downstream plus keep around 350 cfs in the river. At worst, the St. Anthony target will begin to require draft of Island Park around June 20 but most likely not until around July 1. Regardless, streamflow at St. Anthony will hover near or just a little above 1,000 cfs for all of July and the first half of August. In the driest model scenarios, the St. Anthony target will determine system operations until the end of September, as happened in 2018.

Graph of predicted streamflow at St. Anthony, compared with 2019 and the long-term average.

Streamflow in upper Watershed

Inflow to Island Park Reservoir is expected to be quite a bit lower than it was in 2019 during the spring and early summer and a little higher during August, which turned out to be dry during 2019. Heavy rain during early September increased streamflow watershed wide in 2019, and thus 2020 inflow to Island Park Reservoir is predicted to be lower than it was in 2019 late in the season. In general, streamflow at Ashton will be similar to that in 2019 during the spring but higher at the beginning of July due to increased outflow from Island Park Reservoir to meet irrigation demand.

Graph of predicted inflow to Island Park Reservoir in 2020, compared with 2019 and the long-term average.