Many fly-fishers are interested in how winter’s progress will impact fishing quality in the Henry’s Fork and its drainage during the upcoming season. Dr. Rob Van Kirk, Henry’s Fork Foundation Senior Scientist, compiles and offers a water supply report sequentially throughout the year for the river and drainage. We at Jimmy’s believe that placing on our web site Rob’s presentation and analysis of data impacting Henry’s Fork water supply provides to a wider range of interested fly fishers more information with which to judge quality in its upcoming season. We therefore will provide Rob’s updates here during the winter season and into the irrigation season. The second of these is below.
Henry’s Fork Water Supply, Jan. 19, 2021
- Continued dry weather has dropped water-year precipitation and SWE below 80% of average.
- Over the past four days, Island Park Reservoir has gained 144 ac-ft/day; a fill rate of only 89 ac-ft per day is needed to reach the targeted capacity by April 1.
- The reservoir is 84% full, compared with my projected value of 82% full and the long-term average of 73% full.
Weather over the past 10 days has been dry, with seasonable temperatures. Mean temperature last week was 1 degree F above average, and precipitation was around two-thirds of average for a mid-January week. Water-year precipitation to date is 79% of average this morning, and snow water equivalent (SWE) is 78% of average. Both indices have hovered around 80% of average for the past month. Snow accumulation will need to be 120% of average for the remainder of the winter just to reach average by early April.
The overall weather pattern over the western U.S. is expected to shift toward wetter conditions on Thursday. Areas of the drought-stricken Southwest will pick up the largest amounts of moisture. Over the next week, the Henry’s Fork watershed is expected to receive around 0.25 inch of precipitation in the valleys and around 1 inch in the Teton Range. Precipitation will favor the southern part of the watershed. Medium- and long-range outlooks continue to trend toward colder, wetter conditions for the entire western U.S., including our region.
Streamflow continues to be affected by cycles of river-ice formation and breakup. Natural streamflow yesterday was 92% of average in both upper Henry’s Fork and Teton River. The streamflow gage on Fall River at Chester has not reported accurate data since ice first formed in early December, but stream gages farther upstream show streamflow to be near average.
Island Park Reservoir fill continues to exceed both my predictions and the rate needed to reach the targeted content of 120,000 ac-ft by April 1. At an average outflow of 379 cfs, the reservoir has gained 144 ac-ft/day over the past four days, of which 28 ac-ft/day was due to direct precipitation on the reservoir surface. A fill rate of only 89 ac-ft/day is needed to reach 120,000 ac-ft by April 1. The reservoir is currently 84% full, compared with 73% full on average. By current USGS gage data, which is subject to change based on the next rating-curve adjustment, mean reservoir outflow since December 1 is 350 cfs, compared with 348 cfs on average. The Drought Management Planning Committee’s winter target outflow of 330 cfs is within streamflow measurement uncertainty around the nominal 350 cfs.
- Weekly climate summary table
- Weekly SWE summary table
- Watershed SWE
- Island Park Reservoir inflow/outflow: 15-minute data
- Island Park Reservoir volume: 15-minute data
- Island Park Reservoir volume: predicted vs. observed
Rob Van Kirk, Ph.D.
P.O. Box 550
Ashton, ID 83420