Henry’s Fork, September 21st, 2021

Here are excerpts from Dr. Rob Van Kirk’s Henry’s Fork drainage status report of today.

Headlines:  

• Three-day precipitation totals ranged from 0.3 inch at Grassy Lake to 2.0 inches at Crab Creek, increasing the water year total to 82% of average.
• Natural flow has increased by around 130 cfs since last Friday, while diversion has dropped by about the same amount.
• At an average outflow of 236 cfs, Island Park Reservoir gained 415 ac-ft yesterday and is 42% full.

Details:

At 12 degrees F below average, yesterday’s mean temperature was the coldest since May 10. The daily minimum was the coldest since June 11. With complete data in hand, Saturday-Monday precipitation totals ranged from 0.3 inch at Grassy Lake to 2.0 inches at Crab Creek. Other stations receiving 1 inch or better were Grand Targhee, Pine Creek Pass, and Lewis Lake Divide. The watershed total was 0.86 inch for the three-day event. That was enough to move water-year total precipitation from 80% of average to 82% of average. If the forecast for dry weather holds for the next 10 days, total precipitation for the water year will end up 81% of average. That will rank 6^th lowest since 1989, the lowest since 2007, and around 2.5 inches less than in 2016. Dry conditions and above-average temperatures are expected for the next week.

Natural flow continued to increase yesterday in response to the weekend rain but is still only 73% of average for the date. For the water year, total natural flow is76% of average, which is about where it will end up once the water year is in the books next week. Meanwhile, diversion has decreased by around 9% since last Thursday, although a few canals increased diversion a little yesterday. Streamflow in the lower Henry’s Fork peaked yesterday afternoon at around 400 cfs above its irrigation-season target and is dropping this morning. The effect of yesterday’s outflow decrease from Island Park Reservoir has not yet reached the lower watershed. After things settle out in another day or two, I expect streamflow in the lower Henry’s Fork to drop back closer to but still above the target flow.

At an average outflow of 236 ac-ft, Island Park Reservoir gained 415 ac-ft yesterday and is 42% full, compared with 46% full on average and 17% full in 2016. Although fill rate will decrease a little as the effect of the weekend rain subsides, I expect the reservoir to end the water year at around 60,000 ac-ft (45% full), which would be right at the 1978-2020 average, despite a water year with natural flow at only 76% of average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX