Henry’s Fork

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.

Henry’s Fork, July 18th, 2023

With the warming weather and bright skies up and down the river, it is time for terrestrial insects to become an important food form for trout.. That means long, drag free drifts in front of well vegetated banks and dry structure such as snags, rocks and logs. Have hopper, ant, beetle, and even crane fly patterns on hand.

A productive trick follows: I swat any horsefly that lands on my self, then toss the splatted results into the water to drift downstream. On hearing or seeing a trout rise to take it, I have located another fish to try. Do the same with hoppers or any other resident terrestrial insect to locate a feeding fish. You might be accused by your fishing partners of low grade chumming, but the act is legal and informative!

Henry’s Fork at Bear Gulch

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

The big (positive) story continues to be how well water quality is holding up so far this summer, despite high outflow from Island Park Reservoir and warm weather. Water temperatures have stayed near average and well below last year’s readings at most locations. Daily maximum water temperatures exceeded 70 degrees yesterday at Buffalo River, Pinehaven, at St. Anthony and Parker. However, both duration and magnitude of high water temperatures are lower than at this time last year. For example, water temperature exceeded 70 degrees at Pinehaven yesterday for a little under 6 hours yesterday, compared with 7.5 hours on July 16 2022. Maximum water temperature yesterday was 71.8 degrees F at our sonde, compared with 72.9 degrees last July 16.

Turbidity is well below average at all locations except Flat Rock, where relatively high outflow from Henry’s Lake has kept turbidity a fraction of a turbidity unit above average.

Timing of aquatic insect hatches is still 5–6 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. At this point, I do not expect much change with respect to average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELL

Henry’s Fork, July 15th, 2023

Flav

The river below Ashton Dam is beginning to warm to the point that best fishing is early or late in the day. Big stonefly activity and the same for drake mayflies is about over. Afternoon caddis action remains , and PMD and flav spinner falls at these times of day will bring some top water action. Streamers can also be effective when direct sun light is off the water. Big bows and browns come out to forage at these times because of increased overhead cover.

After the July 10th increase in flow out of Island Park Dam, conditions have stabilized in the river below. Brown drake, flav, little PMD, and afternoon caddis activity still provide top water fishing with AM spinner falls and evening action. Expect recreational floaters to increase during day time hours.

Henry’s Fork, July 11th, 2023

Water quality and insect hatch Information from Dr. Rob Van Kirk’s Monday drainage water status report

Water quality us holding up well and is better than average for this time of year. Mean daily water temperatures have stayed at or below average and below last year’s values at all stations except Parker. Daily high water temperatures have exceeded 70 degrees over the past few days at Buffalo River, Pinehaven, St. Anthony, and Parker. Peak water temperatures are happening between 3:30 p.m. and 8:30 p.m. now. Dissolved oxygen concentrations are good at all locations.

Turbidity at Pinehaven increased briefly to 4.0 NTU on Friday as expected but has since dropped to less than 3.0 NTU, where it was prior to Friday’s outflow increase. For comparison, average turbidity values for the date are 6.1 NTU at the dam gates and 4.5 NTU at Pinehaven, Last year’s daily values on July 10 were 9.4 NTU and 7.9 NTU, respectively.

Timing of aquatic insect hatches has not changed much with respect to the 2014–2022 average over the past two weeks, as water temperatures have stayed close to or slightly below average. Hatch timing in the river reach between Island Park Dam and Pinehaven is 5–6 days later than average but at or a few days ahead of that in 2022.

Flow out of Island Park Dam was raised to about 1300 cfs yesterday, and since July 4th its progress graph looks like a step function as irrigation water demands kick in.The most significant insect happening is the building evening brown drake activity on the river below the US Highway 20 Bridge aside from AM spinner falls throughout the upper river. Especially from in Box Canyon down to Last Chance you will see the peak of recreational activity. Be patient and show the floating public that fly fishers are a “polite bunch.” ‘. The best way to escape the bulk of the plastic/rubber flotilla is during AM spinner falls and evening brown drake activity.

Henry’s Fork, July 7th, 2023

Big Osborn Spring at Lower Right

Fishing in the upper section of the Harriman State Park section will soon take a somewhat back seat during PMs to the lower section. March browns are mostly gone, Green Drake activity is tumbling, PMDs are getting smaller in size, but flavs remain for good PM fishing and AM spinner falls. But for brown drakes, the peak hatch peak is soon to come, and the best of their hatch will be accessed via Wood Road 16 (WR 16). For sure there will be some brown drakes throughout the entire Harriman reach (same with flavs), but there is one big reason why the best concentration is on the lower section. See the above photo? It was taken of a river section about a half mile above the WR 16 access.The US Highway bridge is at upper right. What is significant here is that water flowing from Last Chance on down to and past the bridge is warming because of water in Island Park Reservoir and inlets from Silver and other Park lakes are warming (all are shallow), and so is weather, helping to diminish dissolved oxygen in the river. However inflow from Big Osborn Spring adds a large amount of cool water to the river making better conditions for the brown drake population in the river below. Brown drakes emerge best during evenings but they are sure to peak soon. Fish are in the river “Twenty-Four Seven” and they will be gorged on these active insects just before their emergence peak. So consider being on this part of the river very soon. .

Water Quality Section from Dr. Rob Van Kirk’s Henry’s Fork Drainage report filed July 3rd.

Overall, water quality continues to be excellent throughout the watershed, with the exception that afternoon high temperatures are starting to exceed optimal at some locations. Mean water temperatures yesterday were very close to average throughout the watershed, but afternoon highs exceeded 70 degrees at Buffalo River, Pinehaven, St. Anthony, and Parker. Dissolved oxygen concentrations are good in all river reaches.

Timing of aquatic insect hatches has not changed much with respect to the 2014–2022 average over the past week except at Marysville, where it has moved from 9 days later than average to 8 days later. Hatch timing in the river reach between Island Park Dam and Pinehaven is 5–6 days later than average but at or a few days ahead of that in 2022. Hatch timing at Flat Rock, Ashton Dam, and St. Anthony is still 1–4 days behind average but 1–3 days ahead of last year.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELL

Henry’s Fork, June 30th, 2023

Golden stone flies are still around up and down the river. Some may flies have peaked (green drakes, gray drakes, March browns) and others are yet to peak ( brown drakes and flavs on the upper river). Afternoons will be the best times for finding the brown drakes and flavs.

The other happening that will begin its season is recreational floating. Going through Box Canyon and Big Springs to Mack’s Inn will have the biggest concentrations, but they will also be common through Harriman State Park to Riverside campground. Be patient with these folks as most of them have yet to form an etiquette. Gently explain to them that you realize they have a right to the river, but we appreciate you giving anglers present enough room to enjoy what they do. To be sure, you will learn a lot about human nature through these encounters.