Updated: Jan 12
This is the 5th consecutive year I have predicted upcoming spring and summer streamflow conditions based on April-1 data, including streamflow for the winter just ended, Island Park Reservoir volume, snowpack, and air temperature. Streamflow during the upcoming spring and summer, along with Island Park Reservoir carryover, will certainly be the lowest since 2016. Thus, I made numerous improvements to my predictive model and greatly increased both breadth and depth of model outputs in order to provide the most detailed analysis possible for all watershed stakeholders. Including all of this information in blog format would serve neither me nor you well, so I'm including only bulleted summaries in the blog and putting all of the details, including graphs and tables, in a downloadable report.
Winter Streamflow and Island Park Reservoir Fill
Natural streamflow for October-March was 92% of average, where water year 2020 ended.
Island Park Reservoir filled from 57% full on October 1 to 89% full on April 1, attaining the management target of 120,000 ac-ft.
Outflow from the reservoir averaged 356 cfs during Dec-Feb, slightly below the 1978-2020 mean of 360 cfs.
Dec-Feb streamflow through Box Canyon (IP outflow + Buffalo River) averaged 566 cfs, 104% of average
April-1 water-year precipitation and SWE are both at 83% of average.
April-1 SWE ranks 26th out of the last 33 years of record, 5 spots below 2016.
The month of March was very dry but cool, keeping the meager snowpack largely intact going into April.
Short-term forecasts indicate light precipitation and cool temperatures for mid-April.
Long-term outlooks call for warm, dry conditions from late April into the summer.
Streamflow Predictions for Spring and Summer 2021
Natural streamflow for April-September is predicted to be 82% of average, compared with 90% of average in 2020.
There is a 16% chance that April-September streamflow this year will be as low as it was in 2016.
Runoff timing in Fall River and Teton River is expected to be around 3 days earlier than average but 3-4 days later than it was in 2016.
Mid-summer outflow from Island Park Reservoir is expected to be about like last year, but there is a small chance that it could be as high as 2,000 cfs for several weeks in July.
Predicted Island Park Reservoir carryover is 55% full, compared with an average of 44% full.
However, the middle 50% probability range is 33-75% full, and there is about a 5% chance that the reservoir could end the summer below 10,000 ac-ft (7% full).
Simulation Input/Output and Management Implications
Island Park Reservoir filled by May 29 in all simulations, at a mean April 8 – May 27 outflow of 620 cfs. However, in the driest simulations, mean outflow as low as 316 cfs was needed.
Under all scenarios, reservoir content was 127,744 ac-ft on May 1, around when ice-off is expected.
Operations for Grassy Lake and Henry’s Lake similar to those of past years will result in draft of these reservoirs within typical ranges. The exception is that the model allowed Grassy Lake to draft to 9,000 ac-ft in very dry years, which is lower than last year’s value of 11,000 ac-ft.
Shortage of physical water needed to meet theoretical irrigation demand on Fall River occurred in 24% of all simulations but exceeded 5% of observed mean annual diversion in only 2% of all simulations.
Shortage of physical water on the Teton River occurred in 63% of all simulations. If met with exchange wells, total exchange pumping would exceed the observed 2001-2020 median of 7,700 ac-ft in only 6% of all simulations.
Shortage of physical water to meet theoretical irrigation demand on the Henry’s Fork exceeded 1% of mean annual diversion in 8% of simulations. Some of that shortfall was a modeling artifact that resulted from mismatch of short-term weather patterns apparent in analog diversion years and those apparent in simulated natural-flow supply.
Model precision in predicting September-30 Island Park Reservoir content was very poor, with outcomes nearly uniformly distributed between “empty” (3,000 ac-ft in the model) and full. However, conservative and reasonable estimates lie in the range of 23,000 to 64,000 ac-ft, compared to the long-term median of 62,000 ac-ft.
Expected Fishing Conditions
Hatch timing is expected to be near normal in all reaches of the Henry’s Fork this spring and summer, and fish populations will be high watershed-wide as the result of four consecutive years of good water supply.
While water supply will be the worst since 2016, fishing conditions will not be nearly as bad.
Draft of Island Park Reservoir is expected to begin in mid-June, and outflow is likely to increase from around 600 cfs to 1,000 cfs by early July and stay around 1,500 cfs or possibly higher until mid-August.
Turbidity downstream of the reservoir is expected to be higher than average most of the summer.
Expect small, early streamflow peaks in in Fall River and Teton River, meaning fishable conditions 2-3 weeks earlier than last year.
Mid-summer, low-flow conditions are expected in the lower watershed to begin as early as mid-June and last until September.
Due to precision water management and other conservation efforts, end-of-season content in Island Park Reservoir is expected to be 20,000 ac-ft higher than in past years with similar water supply.