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Henry’s Fork

Jimmy's All Seasons Angler / Henry’s Fork (Page 16)

Henry’s Fork, July 24th, 2021

Bear Gulch

It is time to try terrestrial patterns on nearly the whole river above St. Anthony. Catching and releasing will be toughest on fish from Ashton Dam downstream. From Ashton Reservoir to Warm River as well as the Cardiac Canyon section provides the best fishing on the lower river. You will find the Cardiac Canyon section to be the least crowded section because hard sided boats are not legal ( rafts and flotation devices only) there and walk-in fishing is pretty much the name of the game. Flow out of Island Park dam is down to near historic levels and top water fishing below has picked up. Bead head nymphs are producing in Box Canyon and any pattern the size of a hopper or golden stone fly will also bring action there, but be tolerant of recreationists.

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Henry’s Fork, July 17th, 2021

Flow out of Island Park Dam is dropping. Look for top water fishing to improve. Do not overlook presenting terrestrial insect patterns from Last Chance on downstream, but such as may fly spinner patterns and caddis fly life cycle patterns will be effective. The lower river is warming enough to be good for daytime swimming!

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Henry’s Fork, July 13th, 2021

Flow out of island Park Reservoir was upped to about 1700 cfs a week ago with decreasing discolor in the water. Normal flow for this date is around 1100 cfs. The current flow has placed somewhat of a damper on the top water fishing (there is plenty of food at depth, so no need for fish to expend energy to go to the top for it) in the Last Chance to Pinehaven section, but the mayflies are still active. Nymph patterns work best in Box Canyon. Flow is up in the lower river, but especially below Ashton Dam it has warmed to the point that early AM and evenings are the best times for fishing success. Because of our unusually hot, dry summer expect that flows will stay high as long as irrigation demand is present and the system can deliver water.

We place much of the Henry’s Fork Foundation’s Rob Van Kirk’s July 13th Henry’s Fork watershed information on our report. This because because Rob’s work offers much that explains reasons for the current water situation and what to expect with respect to fishing conditions on major streams within. See below.

Headlines:  

  • Yesterday was the warmest day of the summer so far, and water-year precipitation to date dropped to 75% of average.
  • Natural streamflow and diversion continue to stay relatively constant over the watershed, although diversion has been changing one way or the other at several canals on a daily basis.
  • At an outflow of 1472 cfs, Island Park Reservoir is 65% full, compared with 85% on average and 61% in 2016.
  • Turbidity in the Henry’s Fork between Island Park and Ashton has improved substantially in the last few days. See water quality section at the bottom of the report.

Details:

At 7 degrees F above average, yesterday’s mean temperature edged out June 15 as the warmest so far this summer. Mean temperature yesterday was the warmest since August 17 of last year, and the maximum was the warmest since last September 5. No precipitation was recorded, dropping the water year total to 75% of average. Afternoon high temperatures will drop a degree or two over the next few days, due in part to smoke cover. A few showers are possible over Island Park and the Yellowstone Plateau.

Natural flow continues to drop very slowly and is staying in the neighborhood of 57% of average for the watershed: 75% of average at Island Park, 89% of average for the upper Henry’s Fork subwatershed as a whole, 50% of average in Fall River, and 25% of average in Teton River. Total natural flow for the water year to date is 78% of average. Diversion has also stayed relatively constant at around 96% of average over the watershed. However, individual canals have made changes on the order of 50-150 cfs one way or the other over the past few days. Total diversion for the irrigation year to date is 99% of average. Crosscut Canal delivery to the Teton River has been fairly constant at around 375 cfs for the past three days. Streamflow in the lower Henry’s Fork has been stable for the past 48 hours within 75 cfs of target flows.

At an outflow of 1472 cfs yesterday, Island Park Reservoir dropped by 1965 ac-ft yesterday and is 65% full, compared with 85% full on average and 61% full in 2016. Total storage in the watershed’s three reservoirs is dropping at 2200 ac-ft/day. The three-reservoir system currently at 73% of capacity, compared with 83% on average and 70% in 2016.

Water quality:

Turbidity has decreased substantially in the Henry’s Fork between Island Park and Ashton. Turbidity at the dam gates has decreased from around 13 turbidity units (FNU) in mid-June when the gates were first opened substantially for the first time this year to around 5 FNU now. On the power-plant side of the dam, turbidity dropped from a high of 5 FNU two weeks ago to around 2.5 FNU today. Turbidity on the power-plant side of the dam was below average yesterday for the first time in a month.

Turbidity over the past month has been caused primarily by delivery of fine sediment from the reservoir bottom, as we have seen little evidence of algae or other phytoplankton production in the reservoir over that time period. Most likely, the reason for the recent decrease in sediment delivery is because sediment deposited behind the dam gates from last September’s wind-driven “sloshing event” has now been moved out of the area behind the gates due to a few weeks of high outflow through the gates. Had water-supply conditions allowed delivery of a high spring freshet flow—as occurred in 2018, 2019, and 2020, most of that sediment would have been moved out during April or May. Until irrigation demand necessitated reservoir outflow in excess of power plant capacity back in mid-June, the gates had remained closed since the beginning of August of last year, allowing sediment to accumulate there for over 10 months.

Turbidity at Pinehaven has dropped from around 8 FNU to 4 FNU and is now right at average after spending around 8 weeks above average. Similarly, turbidity at Marysville has dropped over the past several days from 5-6 FNU back down to its long-term average of 2.5 FNU. At these locations, the drop in turbidity is due in part to decreased sediment delivery at the dam and in part to increased aquatic vegetation growth in the river between Island Park and Pinehaven, which traps fine sediment as it moves through the reach.

As a result of above-average air temperatures and sunny weather, water temperatures have been above average across the watershed since the end of the cool, rainy spell we had at the end of May. However, temperatures have generally remained in the optimal range for trout—roughly 55-67 degrees F at all of our sonde locations except Pinehaven, St. Anthony, and Parker-Salem (Red Road bridge). Water temperatures have regularly exceeded 70 degrees at Pinehaven and St. Anthony during late afternoon and evening—from 1 p.m. to midnight on most days. Maximum temperatures have reached as high as 73 degrees at Pinehaven and 76 degrees at St. Anthony. Water temperature at Parker-Salem is cooling below 70 degrees only for a few hours in the morning, and we recorded maximum temperatures exceeding 80 degrees for about 10 days at the end of June and beginning of July. Although no fishing restrictions are in place as a result of high water temperatures, please be aware of these stressfully high water temperatures and fish responsibly.

Fortunately, dissolved oxygen levels remain good throughout the watershed, due to a combination of factors that include photosynthesis by aquatic vegetation and turbulent flow at the Island Park Dam gates, through canyon reaches, and over lava flows and irrigation dams in the lower watershed.    

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

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Henry’s Fork, July 10th, 2021

Here are excerpts form Rob Van Kirk’s latest Henry’s Fork drainage water report.

Headlines:  

  • Last week brought slightly cooler temperatures and daily afternoon and evening showers, favoring the Centennial Range and Island Park area.
  • Accounting for stream gage shifts, natural flow has stabilized at 49% of average, while diversion has increased slowly to 96% of average.
  • At an average outflow of 1364 cfs over the past week, Island Park Reservoir has dropped at a rate of 1,837 ac-ft/day and is 75% full, compared with 90% on average and 71% in 2016.

Details:

Compared with conditions over most of the month of June, the past week brought somewhat cooler and wetter weather. Mean temperature was 5 degrees F above average, and the warmest day of the period—July 3—was still cooler than the warmest day we experienced in the middle of June. Afternoon and evening showers fell every day over the past week, totaling 0.19 inch for the watershed. By comparison, average weekly precipitation during early July is 0.27 inch. Showers were heaviest over the Centennial Range and Island Park, the areas that need it the most. Total precipitation for the water year to date is 76% of average. A few mountain showers aside, hot, dry weather is forecast for the foreseeable future.

The rate of natural streamflow recession has flattened out greatly over the past week. After accounting for apparent stream gage shift at Ashton, natural flow is 49% of average for the watershed: 73% of average at Island Park, 80% of average for the upper Henry’s Fork subwatershed as a whole, 44% of average in Fall River, and 24% of average in Teton River. Watershed-total natural flow since the beginning of the water year is 78% of average. Meanwhile, diversion has dropped slowly over the past few days and was 96% of average yesterday.

At the daily scale, streamflow has stabilized in the lower watershed and has even increased a little in the lower Henry’s Fork. However, short-term fluctuations have occurred over the past few days due to a trip at the Ashton power plant and changes in flow at the Marysville power plant on Fall River. The latter changes were made to accommodate a recreational streamflow requirement in the plant’s bypass reach (Marysville Canal diversion to Kirkham Bridge) that applies over the July 4 weekend.

Aquatic vegetation is growing rapidly in most reaches of the Henry’s Fork, and apparent flows at Island Park, Ashton, and St. Anthony are somewhat higher than actual flows at those locations.

Over the past week, outflow from Island Park Reservoir averaged an approximate 1364 cfs, somewhat lower than indicated by the real-time gage. The reservoir is dropping at a rate of a little over 1800 ac-ft/day and is 75% full, compared with 90% full on average and 71% full in 2016. Storage in the watershed’s three reservoirs is 85% of capacity, compared with 93% on average and 83% in 2016.

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]

The river below Ashton Dam has warmed to the point that early AMs and evenings provide the best fishing success. The flow into the Flat Ranch preserve has been increased to around 80 cfs which could bring some Henry’s Lake fish into it. The normal procession of may fly emergences is ongoing from Last Chance to Pinehaven and some terrestrila insect are beginning to show and will become maajor food forms for trout as we advance into summer.. .

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Henry’s Fork, July 3rd, 2021

The river below Ashton Dam is warming to the degree that early AMs and evenings will soon offer the best fishing. Try caddis life cycle and nymph patterns presented deep.With the giant stone flies appearing in good numbers on the South Fork, boat anglers are beginning to decrease along the Warm River to Ashton section. The famed mayfly emergence continues on the Last Chance-Harriman-Riverside section with emphasis shifting to flav and brown drake life cycle patters being more effective. Many fly fishers remain, boating and wading, to enjoy resulting activity.,

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Henry’s Fork, June 26th, 2021

Flow out of Island Park Reservoir was raised to 1230 cfs on June 19th. This is 50% above mean flow (800 cfs) for this time of year, and it has put a “:crimp” in the dry fly fishing downstream to Pinehaven. The green drakes, PMDs, caddis and other aquatic insect are still there and so are the fish. But with increased flow, less effort is required by fish through staying near the bottom and picking off food items freed up by the increased flow. This suggests that presenting nymphs and wet flies may be a good option as is proving to be the case in Box Canyon.

Plenty of fly fishers remain on the lower river whether boating or wading. A few drakes and golden stones and PMDs remain there. With some flavs and caddis they provide top water action. The best way to minimize crowding is to fish during evening hours. Be aware, however, that the river below Ashton Dam will begin to warm as we get into July. This will means early AM and evenings will be the best times for fishing success.

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Henry’s Fork, June 22nd, 2021

Flow out of Island Park Dam increased from about 800 cfs a few days ago to 1200 cfs yesterday. All insect hatches are ongoing. Trout (and whitefish) are responding resulting in good top water fishing. As is common this time of the season plenty of fly fishers are trying their skills from Box Canyon to Pine Haven and recreationists are present . So be patient and courteous. Fewer visitors are on the lower river and fishing whether through wading or boating remains good with flavs, PMDs, caddis and a remnant of golden stones and green drakes present. A significant appearance of grey drakes has not happened.

Henry’s Fork, June 22nd, 2021

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Henry’s Fork, June 19th, 2021

The famed hatches are ongoing on the Last Chance-Harriman section of the river. PMD,early flav, and green drake life cycle patterns are best with rusty spinner and caddis life cycle patterns also effective In Box Canyon where some golden stone flies remain. Crowds of fly fishers from all corners of the earth are present and so are etiquette varieties. If you intend to fish the Henry’s Fork, crowds on the lower river have thinned considerably above and below Ashton Dam and the hatches ongoing from the upper river are present and will be until the river warms up (from Ashton Dam to Chester). We have no word on the presence of a significant gray drake hatch here. Because of Henry’s Lake water management actions this spring, flow out of the lake has been meager ( about 20 cfs). This results in fewer fish escaping into the stream below. So fishing in the Flat Ranch section will just be OK.

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Henry’s Fork, June 15th, 2021

It seems almost a tradition that the green drake emergence begins this week on the Harriman State Park section of the Henry’s Fork. Just a glance at the encampments along Harrriman East (See picture) tells you that folks are lining up to enjoy this world class event. Some years the green drakes appear in larger numbers than others, so we will just have to see what happens this season. Mid morning is usually the best time to begin on the river, and consider that green drake life cycle patterns are not the only ones that will be effective. For example: try drifting dry stone fly imitations to rising fish. The initial green drake appearance overlaps the end of the stone fly event, and the fish being present 24-7 are conditioned to take these imitations because they get a bigger reward for the effort of a rise. Cloudy and mild days will produce the densest emergences, but it looks like the foreseeable future days will be bright,and warm.

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Henry’s Fork, June 12th, 2021

Now that the big stone fly event is fading into the rear view mirror, every fly fishing action is gearing up for the green rake event. It is ongoing on the lower river, but when Harrriman State Park opens to fishing, emphasis will shift to the upper river. Already campers/fly fishers are setting up quarters from which to fish. So expect the usual crowds along the river.

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