Henry’s Fork

Henry’s Fork, September 5th, 2023

Now that cooling weather has arrived and fly fisher populations are dropping, some the best fishing days anywhere on the river are arriving. True, the autumn mayfly activity peak is not as great as that of late spring-early summer, but terrestrial insects will thrive along the river to feed trout until a killing frost. Streamer patterns will become most effective in interesting trout in the entire river, but as autumn marches on, brown trout in the lower river will become increasingly interested as they begin migrations to spawning areas.

Headlines from Dr. Rob Van Kirk’s Henry’s Fork Drainage Water Status Report of September 2nd

• The month of August was slightly cooler than average but 2nd wettest in the 1989-2023 record, with 215% of average precipitation for the month.
• Natural flow continued to decrease yesterday, while diversion increased again, dropping lower Henry’s Fork stream flow right to the irrigation-season target.
• At an average outflow of 507 cfs, Island Park Reservoir dropped by 109 ac-ft and is 62.6% full, compared with 49.4% full on average.
• Our Island Park Reservoir sampling yesterday confirmed well-mixed, turbid conditions on the West End, but so far that turbid water has not traveled to the dam. I still expect the possibility of increased turbidity this weekend, depending on whether all outflow is transferred to the power plant.

Details

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELL

208-652-3568 FAX

[email protected]

Henry’s Fork, September 2nd, 2023

With the great overhead cover coming during this weekend’s stormy weather, streamer patterns should be very effective, Box Canyon, above and below Coffee Pot, and the Tubs areas on the upper river should be good locations for trying these. On the lower river, with its great brown trout population, deep runs, undercut banks, submerged structure, soft water below islands, are among good locations for browns. Evenings will be the best time for trying streamers anywhere on the river. Also proper presentation trumps pattern selection.

If streamers are not your preference, BWOs will make an afternoon appearance when cloudy weather prevails around certain locations, but peak activity will come later this month..

Bring a raincoat anywhere you venture on the river this weekend!

Henry’s Fork, August 29th, 2023

If you see flying ants anywhere along the river, you have found the insect that trout will be keying on for weeks to come. Be sure to have patterns for them in that fly box. Also hopper-dropper combinations seem to work wellanywhere on the river.

Headlines from Dr. Rob Van Kirk’s Henry’s Fork Drainage Water Status Report Filed Monday, August 28th.

• Last week was on the warm and wet side, keeping water-year total precipitation 109% of average.
• Natural flow and diversion have stayed relatively constant over the past few days; irrigation demand and lower-watershed streamflow targets are currently being met with little to no reservoir draft.
• At an average outflow of 565 cfs, Island Park Reservoir gained 807 ac-ft since Friday due to rain and is 62.6% full, compared with 51.2% full on average.
• Water temperatures over the past few days have been well above average; turbidity remains well below average but could increase at Island Park Dam late in the week.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELL

208-652-3568 FAX

[email protected]

Henry’s Fork, August 22nd, 2023

Headlines from Dr. Rob Van Kirk’s Henry’s Fork Drainage Water Status Report Filed Yesterday (8/21)

• Yesterday was the wettest day in the watershed since December 1, 2022, increasing water-year total precipitation to 109% of average; we will end water year 2023 with at least 102% of average precipitation.
• Natural flow increased by 6% over the weekend, while diversion dropped by 5%.
• Outflow from Island Park Reservoir is being decreased by 200 cfs to around 670 cfs this morning. The reservoir is 62.3% full, compared with 54.5% on average.
• Water quality was excellent throughout the watershed yesterday.

• Rob Van Kirk, Ph.D.
• Senior Scientist
• Henry’s Fork Foundation
• P.O. Box 550
• Ashton, ID 83420
• 208-881-3407 CELL
• 208-652-3568 FAX
•  
• [email protected]

The current overcast and cooler weather is ideal for increasing the spinner falls, trico and speckled dun activity up and down the river. It may slow hopper activity a bit, but warmer, brighter days will return and bring back activity from these bugs. Flying ant populations are building, and for weeks to come trout will key on them.

Henry’s Fork, August 19th, 2023

Construction on U. S. Highway 20 will impact access to favored angling locations on the Henry’s Fork, waters around West Yellowstone, and access to Yellowstone Park’s west entrance. The highway will be reduced to a single lane with flaggers directing alternate travel during on-going work from Pinehaven to Island Park Lodge. Almost twenty miles of road is being paved with target completion being in September.

Early AM spinner falls and trico activity along with flying ant and terrestrial insect patterns later in the day provide the best times for success on the upper river. The same insect activity applies to the lower river where a relatively unsettled weather pattern for the upcoming week could improve fishing success without the access problems coming from road construction.

Henry’s Fork, August 15th, 2023

Water Quality Information from  Dr. Rob Van Kirk’s 8/14  HF drainage report

Water quality was good to excellent over the weekend, although afternoon high water temperatures have crept up a little over the past few days. However, afternoon highs have exceeded 70 degrees only at Buffalo River, St. Anthony and Parker, and then by lower margins than earlier in the summer. Mean daily water temperatures have been close to average at all locations. Dissolved oxygen concentrations have been good to excellent at all locations, benefiting from both cooler mean water temperatures (cooler water can hold more oxygen) and continued high photosynthesis from aquatic vegetation. Aquatic productivity is very high right now but not at the expense of high water temperatures and low dissolved oxygen.

In part as a result of Friday’s outflow decrease and in part because of improving water quality in the reservoir, turbidity at Island Park Dam has decreased by 1-1.5 Nephelometric Turbidity Units (NTU) since then and was below average over the weekend. Turbidity at Pinehaven is near the lowest we have recorded this time of year.

All in all, streamflow, water quality, and aquatic ecosystem productivity are about as good as they can get throughout the whole watershed right now. With another round of cooler, wetter weather expected next week, I anticipate conditions to remain excellent for another two weeks.

• Rob Van Kirk, Ph.D.
• Senior Scientist
• Henry’s Fork Foundation
• P.O. Box 550
• Ashton, ID 83420

Patterns for Grass Hoppers are Effective During Summer Days

• Same as our last report: Best fishing success is during AM spinner falls and trico emergences. Then again as evening approaches through using long drag-free drifts of hopper or flying ant patterns each accompanied by a small dropper of your favorite nymph pattern.

Henry’s Fork, August 12th, 2023

Headlines from Dr. Rob Van Kirk’s Henry’s Fork drainage water status report filed yesterday.

Yesterday’s mean temp was right at average for this time of year, and water-year precipitation stayed at 108 % of normal.

Lower watershed stream flow is still well above irrigation season targets, so outflow from Island Park Reservoir was reduced to 575 cfs this morning. As natural flow continues to drop, an increase in out flow may happen next week.

Island Park Reservoir is 63.7 % full compared to 60.6 % on average.

Turbidity at Island Park Dam continues to drop , and is at average as of today.

Dr. Rob Van Kirk

Senior Scientist

Henry’s Fork Foundation

There is no significant change in fly fishing conditions since our last report. Mornings and evenings remain the best times for fishing success up and down the river.

Henry’s Fork, August 5th, 2023

Although this last week has been a reprieve from the hot, dry weather, such will soon return. This means that up and down the river, mid day is the slowest time with respect to fishing action. Overhead cover diminishes with increased sunlight. Water temperatures, already high because of upstream shallow reservoirs, increase further during daytime. The result is reduced dissolved oxygen making fish less active. Try your luck during AM spinner falls, and as cooler air prevails around sundown, try drag free floats with hopper-dropper combinations adjacent to banks an at the heads of deeper water. Also do not overlook drifting a mouse pattern along these sections during the same time of day. You will not encounter many fish by doing so, but one of those you do could be the fish of the year.

Henry’s Fork, August 2nd, 2023

Flow out of Island Park Dam has been reduced to the usual amount for this time of the season. Up and down the river AM spinner falls and terrestrial insects are abundant. Hopper and ant patterns and hopper-dropper rigs work anywhere during PMs. Long, drag free floats are are required with any of these. The above photo should be enough to tell you that in many locations, terrestrial insect populations are not only abundant but the major food form for trout this time of year. If you are experienced in long, drag free floats, consider using a slightly stronger tippet in order to play that trout quickly giving it a better chance to survive the warmer summertime waters..

One other thought; particularly in the Big Springs- Macks Inn, Box Canyon, Harriman State Park and Warm River to Ashton sections, recreational boaters abound. This means early AM spinner fall periods and late afternoon terrestrial insect activities are best conditions for minimizing interruptions.

Henry’s Fork, July 22nd, 2023

The Tubs

Water Quality Information from Dr. Rob Van Kirk’s Henry’s Fork Drainage Report Filed Yesterday.

Water quality continues to exceed expectations throughout the watershed, especially given hot dry weather and very high reservoir outflow. Water temperatures yesterday were a little cooler than on Wednesday due to cloud cover and close to average. Afternoon high water temperatures exceeded 70 degrees at our usual warm spots—Buffalo River, Pinehaven, St. Anthony, and Parker—but the duration and magnitude of temperatures greater than 70 degrees was lower than on Wednesday. Water temperatures stayed within the optimal range of 53–68 degrees all day at all of our other sonde locations. Dissolved oxygen continues to stay good to excellent—and above the state cold-water standard of 6 mg/L—at all locations.

Turbidity is still well below average at all locations between Island Park Dam and St. Anthony and among the lowest readings we have observed during mid-July since first installing our water-quality network in 2014. This is despite outflow through Island Park Dam gates higher than we have seen this time of year since 2013. The common denominators among the years with lowest turbidity—2017, 2018, 2019, and 2023—are near- to above-average snowpack and high Island Park Reservoir levels.

Timing of aquatic insect hatches is still around 5 days later than average in the river reach between Island Park Dam and Pinehaven, 8 days later in the Warm River to Ashton reach, and 4 days later than average between Ashton Dam and St. Anthony.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELL

There are a few comments on why we continue to place Dr. Rob’s observations on the fishing report. We do so because many of these give reasons for fly fishing success. They also help in deciding where and when to try your luck on drainage waters.

Consider that although aquatic insect hatches remain a bit late in much of the drainage, terrestrial insect populations are building and are becoming quite important as a trout food along the entire river and its tributaries.