Henry’s Fork

Henry’s Fork, January 19th, 2020

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

Headlines:

• 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.

Details:

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.

Graphics:

• 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

Station guide and disclaimer

Glossary of Terms

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

Henry’s Fork, January 5th, 2021

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 first of these is below.

Henry’s Fork water supply, Jan 05 2021

Headlines:

• Another warm, wet day increased water-year precipitation to 82% of average and SWE to 85% of average.
• At an outflow of 329 cfs, Island Park Reservoir gained 443 ac-ft yesterday, one-third of which was from direct precipitation on the reservoir surface.
• The reservoir is 82% full and filling at well over twice the rate needed to reach the April-1 target.

Details:

Mean temperature yesterday was 10 degrees F above average, and precipitation was recorded at all stations except Alta. As predicted, precipitation heavily favored the upper Henry’s Fork for a change, where amounts through midnight ranged from 0.3 inch at Island Park to 0.7 inch at White Elephant and Black Bear. Ashton was again the winner in the valleys with 0.16 inch. Water-year total precipitation increased to 82% of average, and snow water equivalent (SWE) increased to 85% of average. Precipitation is continuing this morning, so storm totals will end up being quite a bit higher than reported through midnight. Only light precipitation is forecast for the next seven days, and the long-term outlooks call for generally dry conditions next week.

At an outflow of 329 cfs, Island Park Reservoir gained 443 ac-ft yesterday, one-third of which was from direct precipitation on the reservoir surface. The reservoir is 82% full, compared with 70% full on average. Fill rate has averaged 229 ac-ft/day since December 1, and a fill rate of only 103 ac-ft/day is needed for the rest of the winter to meet the April-1 target. Reservoir content is 1,880 ac-ft ahead of my November 19 prediction, which assumed an outflow of 330 cfs. As mentioned previously, higher-than-expected fill rate is due to a combination of slightly higher inflow and slightly lower outflow, at least according to our streamflow measurements earlier in December and to the power plant operator. According to USGS data, updated in mid-December, outflow since December 1 has averaged 344 cfs, compared with 341 cfs on average.

Graphics:

• Watershed SWE
• Island Park Reservoir inflow/outflow: 15-minute data
• Island Park Reservoir volume: 15-minute data
• Island Park Reservoir volume: predicted vs. observed

Station guide and disclaimer

Glossary of Terms

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

[email protected]

Rob’s blog

Henry’s Fork, October 27th, 2020

See the Main Stem Snake river fishing report for some timely information.

Henry’s Fork, October 20th, 2020

The lower river offers BWO and midge activity with responding trout for the top water enthusiast and streamer fishing for the brown trout enthusiast.  Now and during the upcoming weekend the upper river offers winter-like weather with BWO and midge activity and responding trout. Streamer and nymph presentation in deep Box Canyon holes and runs may entice a big rainbow to hit, but dress for winter especially with reliably leak proof waders.

Henry’s Fork, October17th, 2020

Downstream from Island Park Reservoir flow is lower than normal at every gauge location.  BWOs are abundant everywhere. Look for sheltered areas along the river for best activity from them and responding fish.  Do not forget to place streamer patterns in that fly box, because brown trout are beginning spawning migrations and are in an aggressive mood. Low light conditions will give the best conditions for encountering them.

Henry’s Fork, October13th, 2020

With respect to BWO and mahogany dun activity and steamer presentation on the lower river, what applies to the South Fork applies here. Wind can diminish the mayfly activity, but unsettled conditions do not hurt streamer fishing. Flow throughout is a bit below average for the time of year.

Henry’ Fork, October 10th, 2020

With the river so low below Island Park Dam (135 cfs today), it is time to pitch streamers into the big Box Canyon holes where fish will concentrate.  Look for weeds braking up along the Last Chance-Harriman reach and fish responding to BWOs and what is left of early AM tricos. Hoppers along this part of the river are soon to be things of the past. The lower river now offers better fishing with BWO, mahogany duns, a fading hopper population and best of all browns responding to streamers. Look for streamer fishing to improve with the unsettled weather coming up in the next few days.

Henry’s Fork, October 3rd, 2020

Look for fish responding to BWO, mahogany duns, morning tricos, afternoon caddis, ants, beetles and hoppers anywhere you try the river from Last Chance to below St. Anthony. Crowds are gone just about everywhere, but especially from the Last Chance-Harriman section. A killing frost could happen any day, especially on the upper river and wipe out hoppers and slow ants and beetles. But BWOs and caddis will remain active for  quite a while.  Streamer fishing will pick up on the lower river as browns begin to migrate.

Henry’s Fork, September 26th, 2020

Brown trout are beginning their runs to spawning areas. The best time to encounter them is in the evening or at first light in the AM. Use streamer and woolly bugger type patterns to encounter them. On windy days look for diminished aquatic insect activity.  When the wind dies and if overcast and cool weather remains the BWO, mahogany dun and midge activity will return.

Henry’s Fork, September 19th, 2020

Flow out of Henry’s Lake has been cut in half. This means fish in the Flat Ranch section of the river will be moving downstream into the Henry’s Fork. Until we have a killing frost, terrestrial insect patterns will be most effective along the Last Chance-Harriman section of the river and also along the lower river.  BWO life cycle patterns remain effective everywhere as will patterns for PM caddis activity.