Henry’s Fork

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.

Henry’s Fork, June 27th, 2023

Upper Coffee Pot Area

Water Quality and Insect Hatch Information From Dr. Rob Van Kirk’s Henry’s Fork Watershed Report of Today

Overall, water quality continues to be excellent throughout the watershed. Turbidity continues to decrease across the watershed. Water temperatures yesterday were near to slightly below average throughout the watershed. Temperatures were within the optimal range for rainbow trout most of the day in most river reaches. Dissolved oxygen concentrations are optimal.

Timing of aquatic insect hatches at Flat Rock, Ashton Dam and St. Anthony is 1–4 days later than the 2014–2022 average but 1–4 days earlier than last year. 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 lags average by the largest margin at Marysville, where it is 9 days later than average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELL

Dr. Rob Van Kirk’s information above explains why fishing is better on some parts of the river than others. For example a good population of big stoneflies remain in Box Canyon, but on cool, cloudy days streamer fishing produces best. Golden stoneflies are coming out up and down the river., Gray drakes and flavs are showing up during PMs on the river below Ashton Dam. The fabled green drake hatch is yet to be significant in the Harriman State Park reach and is declining on the lower river.

The lower Coffee Pot area will one of the better locations to find relative solitude on the river. The Henry’s Fork giant stonefly hatch ends there, and golden stone fly, PMD and caddis activities can bring on good fishing.

Henry’s Fork, June 24th, 2023

Green Drakes are out on the lower river with best concentrations below Ashton Dam to above Chester backwaters. Although they inhabit many section of the river, this is the best hatch in terms of numbers. They may have peaked here, but there are enough to make a visit worthwhile. Days with higher relative humidity, little wind, and even overcast will bring out the best concentrations. But as with the feeding on stoneflies a few weeks earlier, they will become filled and head to the depths to digest. A strategy is to give them time to digest, then come back later with life form patterns, and you will have some action.

Henry’s Fork, June 20th, 2023

The Tubs

Here is some good info from Dr. Rob Van Kirk’s Water Status report filed yesterday for the Henry’s Fork watershed.

Water quality

Overall, water quality continues to be excellent throughout the watershed. Turbidity has increased a little at most locations over the past day or two due to rain but remains much lower than it had been during the peak of snow melt a few weeks ago. Water temperatures are in or slightly below the optimal range for rainbow trout watershed-wide. Dissolved oxygen concentrations are also optimal. Timing of aquatic insect hatches at Flat Rock, Pinehaven, Ashton and St. Anthony is staying around 1–4 days later than average but 1–3 days earlier than last year. Hatch timing at Marysville and Island Park Dam is 7–8 days later than average and about 1–3 days later than last year. I do not expect hatch timing to change much with respect to average for the foreseeable future.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELL

Consider that Dr. Rob is talking about the peak of insect hatches. This means that a few insect are emerging now, but achieving peak concentrations are delayed.

The above photo shows The Tubs area above Mack’s Inn. Try pitching streamer patterns into these deep holes. You will find some relative solitude before recreational floating gets going.

Henry’s Fork, June 17th, 2023

Today there are more folks on the upper river than in the rest of Fremont County. Look at the attached photo to see anticipation camps on the river below Osborn Bridge.Today is also the last of the Henry’s Fork Foundation Henry’s Fork Days event sited at Last Chance. Harriman State Park opened to fly fishing a few days ago, and green drakes should begin hatching in numbers soon. If you are looking for them, they are most active on cool days having higher air relative humidity. Seems like we have had a lot of those days lately! They are getting going on the lower river along with golden stones and caddis and the number of anglers are less than on the river above.

Henry’s Fork, June 10th, 2023

The giant stonefly hatch is heading up the river but at a pace slower than last year. If you own a soft- sided boat ( rubber raft, etc.) a good trip considering active fish and lack of crowds, put in at the lower Mesa Falls access and float down to the Stone Bridge access just below the Warm River confluence. It is a beautiful trip, just beware of little Surprise Falls about a third of the way down the trip. Afternoon caddis activity is going on good up and down the river.

Headlines From Dr. Rob Van Kirk’s Water data report filed June 6th for the Henry’s Fork Drainage.

• As expected, snowmelt was lower yesterday because of cooler temperatures and loss of remaining mid-elevation snow. Snow remains only at the highest elevations.
• Natural streamflow was roughly constant yesterday at 110% of average, as Fall River and Teton River respond to the last gasps of high-elevation snowmelt.
• Heavy precipitation in the forecast could temporarily increase streamflow again next week.
• At an average outflow of 843 cfs, Island Park Reservoir gained 281 ac-ft yesterday and is 99% full. Outflow will continue to be reduced gradually as inflow decreases.