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Streamflow and Reservoir Predictions for Summer 2018

Updated: Jan 12, 2022

  • Cool, wet weather from mid-February to mid-April turned an average water supply into one that is decidedly above average.

  • As of May 9, the Henry’s Fork reservoir system is 94% full and filling rapidly.

  • Based on early-April conditions, summertime water supply in the Henry’s Fork watershed is forecast to be above average.

  • More storage water will be delivered from Island Park Reservoir this summer than in 2017, but higher inflows will compensate, resulting in a very high probability of better-than-average carryover at the end of the irrigation season.

April Precipitation 156% of Average

Precipitation during the month of April was above average at all 12 weather stations in the watershed and was 156% of average for the watershed as a whole. Below-average temperatures early in the month were offset by above-average temperatures late in the month, and the month ended up within 1 degree F of average.

Climate summary for the month of April 2018.

Cool temperatures through the middle of April capped a nine-week run of average to below-average temperatures, allowing snow to continue to accumulate until the middle of the month and preventing high runoff until late April.

Graph of temperature departure from average so far this water year.

Snow-water-equivalent (SWE) peaked in late March at the two low-elevation SnoTel sites in the northern part of the watershed but peaked between April 15 and April 27 at the other 7 SnoTel sites. Over the whole watershed, SWE peaked on April 18, 8 days later than average. Peak SWE accumulation was 117% of average, and the month of April ended with 112% of average SWE on the ground.

SWE Summary for the month of April 2018.

As of May 9, 74% of this year’s peak SWE remains on the ground, compared with an average of 82%. However, because 2018’s peak started out higher than average, current SWE is still 106% of average despite high melt rates over the past week.

Watershed-averaged SWE through May 9.

All in all, a long stretch of cool, wet weather from mid-February through mid-April turned an average water supply into one that was decidedly above average at the end of the month. The current warm spell is melting snow rapidly, and snow remaining on the ground will probably equal average in a few more days. The following graph shows that natural streamflow was a little above average all winter and didn’t really start to reflect snowmelt until late April. Note how much higher flow was in March and early April of 2017, when runoff began very early.

Total natural streamflow in the Henry's Fork watershed through May 8.

Predictions for Summer 2018

Last year I developed a numerical computer model to simulate water supply and reservoir operations in the Henry’s Fork watershed. I made several refinements this year to make it more realistic and flexible (see details in next section). One of the new components I added this year was a variable low-flow target in the Henry’s Fork at St. Anthony.

Importance of low-flow target at St. Anthony

During irrigation season, just enough water is delivered from Island Park Reservoir to meet irrigation demand in the Henry’s Fork watershed. The downstream-most diversion point on the Henry’s Fork is the Consolidated Farmers Canal, which is downstream of St. Anthony. This canal does not have a full diversion structure spanning the entire width of the river, so at least 500 cfs must be delivered to that point for the canal to divert its full right. Excess remains in the river downstream. Experience on the part of water users and managers has shown that if flow in the Henry’s Fork at the St. Anthony gage is around 1,000 cfs, sufficient water makes it past the intervening three diversions to allow diversion at the Consolidated Farmers canal. Although we do not have solid biological data, the general sentiment among anglers and fisheries biologists is that 1,000 cfs is enough to sustain the fishery downstream of St. Anthony, although more water would certainly result in more fish habitat in the lower Henry’s Fork. However, increasing the low-flow target at St. Anthony increases required delivery from Island Park Reservoir, to the detriment of fishing experience, water quality, subsequent winter flow and trout recruitment in the reach between Island Park Dam and Riverside.

So, I used the model this year to quantify for the first time the relationship between summertime flow at St. Anthony and Island Park Reservoir storage carryover, and hence, next year’s winter flow.

Primary model results: St. Anthony target set at 1,000 cfs

Overall, natural streamflow is predicted to be above average in the upper Henry’s Fork (upstream of Ashton) and quite a bit higher than last year. Fall River will contribute above-average flow but not nearly as much as last year. Flow in Teton River will be near average. Runoff timing will be a few days earlier than average across the board, a little later than last year in the upper Henry’s Fork and a little earlier in Fall River and Teton River. The end result is that natural flow in the Teton River will fall below average in the middle of summer, so more water will need to be delivered from the Henry’s Fork through the Crosscut Canal than in 2017. This will require more delivery out of Island Park Reservoir than last year, at the same St. Anthony flow target of 1,000 cfs that was used in 2017.

The following graphs each illustrate a key model result. In all graphs, the solid blue line is the predicted value (the average over all of the random simulations—see below), and the gray shaded area depicts the values that will occur with 90% probability, given the April-1 conditions on which the model was based.

Inflow to Island Park Reservoir

is predicted to be higher than average early in the spring because of earlier-than-average snowmelt, lower than average from May through mid-July for the same reason, and average to slightly above average during the late summer, reflecting good baseflows after two years of above-average snowpack. For the same reason, inflow this year is predicted to be about 200 cfs higher during July, August, and September than what was observed last year. This also reflects the second consecutive year of above-average snowpack in the springs that feed the upper Henry’s Fork.

Streamflow in Fall River at Chester

is expected to be above average during the spring because of above-average snowpack and early runoff timing, and average to slightly above average most of the summer. Late-summer flow is expected to be a little lower than last year, but overall, natural flow in Fall River will be sufficient to meet irrigation demand on Fall River and leave 300-500 cfs in the river down to the Henry’s Fork confluence.

Natural streamflow in the Teton River

will have similar runoff timing to that in Fall River but will drop below average by late June and stay there most of the summer.

Delivery of water to the Teton River

through the Crosscut Canal is expected to begin in late June. Predicted delivery will be higher than that in 2017 during the early part of the summer and lower during the late summer. However, uncertainty is high, and higher-than-average delivery is possible through July and August.

Streamflow in Henry’s Fork at St. Anthony

generally reflects runoff in Fall River during the spring and early summer. From June onward, flow is predicted to be near average. At worst, the low-flow target—in this case 1,000 cfs—will determine system operations from late June through mid-September, but under the wettest scenarios, streamflow will remain near 2,000 cfs all summer without requiring any storage delivery. The expected flow is somewhere in between, generally around 1,200-1,500 cfs, which happens when the low-flow target constrains operations for only a short time period during the summer.