Henry’s Fork, June 7th, 2022
The big stoneflies are making their run followed by that of the golden stones up the river. These soon will be relinquished on the lower river to green drakes. Also, anywhere along the river afternoon caddis activity attracts fish where stone fly activity has peaked.
Dr. Rob Van Kirk’s report on water status in the Henry’s Fork drainage follows.
Headlines:
- Yesterday was cool yet again, and precipitation was locally heavy in the Island Park area.
- Although soil moisture has improved in valley areas, the three-year average precipitation is still 10% below average and not improving.
- Natural streamflow continued to increase yesterday but is still only 73% of average for the date and 69% of average for the water year.
- Island Park Reservoir outflow will be increased at noon today to 725 cfs to match increased inflow.
Details:
Guess what? Another day of below average temperatures is keeping the spring of 2022 among the coldest in the past three decades. Mean temperature since April 1 is in the bottom 10th percentile of all years since 1989, at over 3 degrees F below average. Meanwhile, precipitation overachieved again yesterday, especially in the Island Park area, where Black Bear and White Elephant each received 0.8 inch. Water-year total precipitation stayed at 88% of average. Despite cool temperatures relative to average, temperatures are warm enough to continue to melt mid-elevation snow. Only a few inches of water equivalent remains at the mid-elevation stations, leaving appreciable snow only at the highest elevations.
In terms of drought indicators, the one-year accumulated moisture availability (precipitation minus evapotranspiration) has improved from a little over 6 inches below average to less than 4 inches below average just in the past 10 days. So, while still pretty far below average, a 2.5-inch net gain is substantial and represents a short-term improvement in the drought. In the longer term, the three-year average precipitation is still around 11% below average, where it was in early March, indicating persistence of long-term drought.
Natural flow continued to increase yesterday in response to rain and mid-elevation snowmelt but is still only 73% of average: 70% in upper Henry’s Fork, 81% in Fall River, and 68% in Teton River. The Fall River number is highest because it has the most drainage area in the mid-elevations at which snowmelt has been highest over the past 7-10 days. Streamflow in Teton River is yet to experience its largest peak for the season, which will happen when high-elevation snow melts. Warmer temperatures are forecast today through Saturday, which will melt some of that snow, but cooler weather early next week will slow melt a little. Overall, it looks like our slow, extended, low-magnitude snowmelt will continue. This will result in lower streamflow peaks than normal but extend the period of moderate flow farther into the summer.
Island Park Reservoir gained more volume than expected yesterday due to unexpectedly heavy rain. Outflow will be increased from around 635 cfs to 725 cfs today to keep pace with increased inflow. Outflow will continue to be adjusted in small increments to keep the reservoir within a fraction of a percent of full pool until draft is needed to meet within-watershed irrigation demand. With heavier rain than forecast yesterday and more expected Sunday and Monday, the need for reservoir draft will be pushed out at least until sometime next week. We will beat the odds for draft need today, as my predictive model based on April-1 conditions gave us a 50% chance of needing reservoir draft by June 6. Last year, draft was first needed to meet irrigation demand on June 8, and we will definitely beat that this year. Mean date of first draft is June 23.
Meanwhile, Grassy Lake has filled, and Henry’s Lake is less than 100 ac-ft short of full. Both of these outcomes have also beat the odds based on conditions on April 1.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-881-3407 CELL
208-652-3568 FAX