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November Water Summary and Winter Strategy at Island Park Dam

Updated: Jan 12, 2022


  • The month of November was very warm but well short of the record.

  • Precipitation for the month was 16% below average but is 8% above average for the water year to date.

  • At a mean outflow of 120 cfs for the month, Island Park Reservoir gained 15,120 ac-ft (504 ac-ft/day), about 3% of which was due to direct precipitation on the reservoir surface.

  • Outflow from the reservoir was increased to roughly 200 cfs following yesterday’s meeting of the Henry’s Fork Drought Management Planning Committee.

  • The latest forecasts call for above-average precipitation for the next month.

Temperature. Mean temperature for the month of November was 5 degrees F above the water year 1989-2021 average but still 3 degrees short of the record high, set in November 1999. A record daily high was set on Monday, and additional daily high temperature records are likely to be broken today through Saturday. Mean temperature in October was 1 degree above average, so water year 2022 has definitely started out on the warm side. The latest medium- and long-range outlooks issued yesterday call for above-average temperatures to continue for the foreseeable future.

Precipitation. Although the last 10 days have been very dry, precipitation for the month of November as a whole wasn’t nearly as low as the recent trend would lead one to believe. Watershed-averaged precipitation for the month was 3.19 inches, vs. the average of 3.81 inches. Precipitation for the month of October was 1.10 inches above average, so even with the slightly dry November, precipitation for the water year to date is 8% above average (aka 108% of average). I have not yet reported precipitation or snow water equivalent (SWE) figures relative to average because they don’t mean much early in the water year. However, now that we are two months in, I will start reporting percentages every day.

Snow Water Equivalent. Despite above-average precipitation, warm temperatures so far this fall and winter have prevented commensurate accumulation of SWE. A watershed average (over the 9 SnoTel stations) of 3.0 inches of new SWE was accumulated during the month of November, compared with 4.4 inches on average. Including what was accumulated in October, current SWE is 37% below average (63% of average). However, all nine SnoTel stations are reporting at least 0.7 inch of SWE on the ground this morning. Not surprisingly, the two highest-elevation stations are reporting the highest totals relative to average: 75% of average at Grand Targhee and 78% of average at Black Bear. Current SWE is 2.2 inches below average, and if current weather forecasts hold, that deficit will be overcome in the next two weeks.

Climate outlook. Speaking of forecasts, the short-range forecasts are becoming confident on a big weather pattern change on Monday, when heavy and widespread precipitation returns to the area. Although temperatures are expected to remain above average, they will be cold enough that snow will accumulate above 6,000 feet, where the vast majority of the watershed’s annual snowpack is stored. Temperatures are expected to drop by a good 10 degrees F between Saturday and Monday, roughly from 15 degrees above average to 5 degrees above average. The seven-day precipitation outlook calls for 0.1-0.25 inch of precipitation in the valleys and up to 1.5 inch along the Teton crest. Beyond that, new outlooks issued yesterday call for above-average temperatures and above-average precipitation on all time horizons. Upcoming precipitation is expected to be heavy enough that removal of drought designation for some parts of the upper Snake River basin is expected.

It is always possible that predictions of above-average precipitation will prove to be wrong. Last fall, long-range outlooks consistently predicted above-average precipitation for our region, but November and February were the only two fall and winter months with above-average precipitation. We ended the winter with well below average snowpack and accumulated precipitation. Last fall’s outlooks were based on a La Nina ocean-temperature regime, which usually produces above-average precipitation in the Pacific Northwest. However, last year’s La Nina was relatively weak and resulted in good precipitation in the lower 48 states only in Washington. This year’s La Nina is much stronger, and its effects have reached farther south and east so far this fall and are expected to do so for the next three months.

Streamflow. Current natural streamflow in the watershed primarily reflects the accumulated effects of the current drought, which began in July 2020. Watershed-wide natural flow is currently 22% below average (78% of average) and within a few percentage points of that in each of the three subwatersheds. Accumulated natural flow for the first two months of the water year is 23% below average, despite above-average precipitation over that time period. Natural flow will remain about where it is relative to average until next spring, when this winter’s snowpack begins to melt.

Consistent with streams throughout the watershed, natural stream inflow between Henry’s Lake and Island Park Reservoir was 22% below average for the month of November and predicted to stay pretty close to that through the winter. Mean outflow from Island Park Reservoir for the month of November was around 120 cfs, subject as always to gage shift updates. This estimate is based on three streamflow measurements we have made at Island Park since the last USGS rating adjustment on November 1. The last field measurement we made was 123 cfs on November 18, with a measurement error of 3.5%. The reservoir gained 15,120 ac-ft during the month and is currently 68% full, compared with 64% full on average. This difference is worth an additional 27 cfs of winter flow, all other factors (inflow, reservoir management targets, etc.) being equal. About 3% of the reservoir gain during the month of November was the result of direct precipitation on the reservoir surface.

Island Park Reservoir Management. The Henry’s Fork Drought Management Planning Committee met yesterday to set a winter management strategy for Island Park Reservoir. I’m attaching the analysis prepared for that meeting to save substantial space here. Low reservoir outflow this fall allowed substantial fill without negative impact on the fishery. Long story short: data show no negative effects of low flows during the fall, when water temperatures are relatively warm and aquatic vegetation is abundant enough to provide cover and maintain water depth. However, juvenile trout survival downstream of Island Park is strongly dependent on streamflow during December-February, when water temperatures are cold and aquatic vegetation has moved out of the stream channel. However, outflow so far this fall has been below the minimum required to operate the hydroelectric plant, so while no negative effects on the fishery are expected, low flows have negatively affected Fall River Electric’s operations.

To allow the power plant to operate during the winter, provide better flow for the fishery, and attain an April-1 reservoir target of 126,000 ac-ft (93% full), the Committee yesterday agreed to increase flow to the power-plant minimum of 200 cfs. That increase was made yesterday afternoon. The USGS gage currently shows 184 cfs, which is certainly well below the actual outflow due to gage shift. Based on our estimate of gage shift, this corresponds to an actual outflow of around 215 cfs. Regardless of the details, the power plant is now running at minimum capacity, which corresponds to an outflow of roughly 200 cfs. The 126,000 ac-ft target is based on the maximum reservoir volume that can be reached while ice is on the reservoir without damage to the spillway infrastructure. Ice-off usually occurs sometime in April although can occur as late is the first week of May.



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