Henry’s Fork, August 24th, 2021
Not much change from our last report. AM trico, and may fly spinner patterns early, then switch to terrestrial insect and caddis patterns later in the day.
Here are excerpts from Rob Van Kirk’s Henry;s Fork Drainage status report published today.
Headlines:
- Temperatures returned to average yesterday, and water-year precipitation stayed at 82% of average.
- Diversion continued to decrease yesterday, while natural flow stayed roughly constant.
- At an outflow of 400 cfs, Island Park Reservoir gained around 40 ac-ft yesterday and remains 43% full, compared with 53% full on average and 20% full in 2016.
Details:
Temperatures returned to average yesterday, as both the daily maximum and daily average were the highest since last Tuesday. A little residual precipitation that fell after midnight on Monday morning was recorded, leaving water-year total precipitation at 82% of average. Since August 1, the one-year cumulative moisture availability in the agricultural regions of the watershed has improved by 4 inches, although it is still 7 inches below average. The improvement made up for a large decrease in this metric during the month of June, which was the hottest and driest June in the 1989-2021 record. Three-year average watershed-wide precipitation improved by around 1 inch but is 2 inches below average. Dry conditions and near- to below-average temperatures are forecast for the next week or two.
Diversion continued to drop yesterday, while natural flow stayed roughly constant for the watershed as a whole, dropping a little on Fall River and the Henry’s Fork and increasing a little on the Teton River. Diversion is 66% of average for the date and 94% of average cumulatively for the irrigation year so far. Natural flow is 79% of average for the date and 77% cumulatively for the water year. Streamflow in the lower Henry’s Fork dropped by around 200 cfs yesterday but has been stable so far today at around 160 cfs above the target flow. I expect diversion to remain fairly close to its current level for a few more days, while natural flow will recede now that the peak generated by Saturday’s rain has made its way through the lower Teton River.
At an outflow of around 400 cfs, Island Park Reservoir gained 38 ac-ft yesterday and is 43% full, compared with 53% full on average and 20% full in 2016. The USGS gage at Island Park remains accurate based on the rating adjustment on August 12, as no large changes in aquatic vegetation growth are apparent. We will measure outflow again on Friday to quantify any gage shift one way or the other that has occurred since last Friday. Barring substantial change in diversion, no increase in outflow from Island Park Reservoir will be needed in the next few days to accommodate streamflow recession in the lower watershed.
Water temperatures have fallen to their lowest values in over two months and were within the optimal range for trout yesterday at all locations downstream of Island Park Dam. Water temperatures upstream of the reservoir have been a little colder than optimal over the past few days—this isn’t a stressful situation for trout but just means that they won’t grow as fast as they would in warmer water. Turbidity at Island Park Dam has stabilized since last Friday’s flow transfer to the power plant and is slightly above average at the dam and at Pinehaven.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-652-3567 OFFICE
208-881-3407 CELL
208-652-3568 FAX