Henry’s Fork

Henry’s Fork, July 23rd, 2022

Box Canyon, Warm River to Ashton, and Ashton Dam to Chester backwater sections are seeing a big increase in recreational floaters. The peak of this activity is usually from late morning to late afternoon. For the latter two locations, this time stretch coincides with the slowest fishing success. Minimize your interaction with these folks by fishing early AMs and during evenings when fishing success is much likely best during these warm days. Everywhere on the river, bank side terrestrial insects are taking over as being a most important food form for foraging trout. Carry fly patterns that simulate them and concentrate your presentations to banks above good holding water.

Here are headlines from Dr. Rob Van Kirk’s Henry’s Fork drainage water report compiled yesterday, July 22nd.

Headlines:  

• Dry weather with temperatures 2-4 degrees F above average is expected to continue for the foreseeable future, as drought redevelops along the northern edge of the watershed.
• Current USGS gage readings are around 80 cfs higher at Island Park and 300 cfs higher at Ashton than actual stream flow.
• At an outflow of 1423 cfs yesterday, Island Park Reservoir is 77% full, right at average for the date.
• No large changes in irrigation-system management are expected for another week or so.

Here are

Henry’s Fork, July 19th, 2022

Terrestrial insect populations are building along banks throughout the length of the river as the variety of aquatic insects decreases. Begin thinking patterns for such as ants, beetles and hoppers anywhere you fish the river, and target holding water along vegetated banks. Some turbidity is present in waters released from Island Park Reservoir and this condition continues on downstream through the Harriman State Park section. Best fishing is during AM spinner falls and remaining mayfly activity during late afternoons. This” best fishing times comment” applies to the entire river. Upcoming speckled dun and trico activity will “revitalize” this.

If you fish the river below Ashton Dam, consider that the warmer the water, the tougher it is for fish to revive after being caught and released.

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

Headlines:  

• Mean temperature yesterday was 3 degrees F above average, and water-year total precipitation at 90% of average.
• Natural flow has dropped a little to 80% of average over the past few days, while diversion increased a little yesterday to 98% of average.
• At an outflow of around 1450 cfs, Island Park Reservoir is dropping at around 1700 ac-ft per day and is 81% full.
• Current system management continues to keep stream flow in the lower watershed very stable, and no large changes are anticipated in the short term.

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

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Henry’s Fork, July 12th, 2022

Headlines from Dr. Rob Van Kirk’s latest water report for the Henry’s Fork follows.

Headlines:  

• Mean temperature yesterday was 2 degrees F above average; water-year precipitation stands at 90% of average.
• Natural flow dropped another 3% yesterday, while diversion increased to 99% of average. Water rights priorities are near the median for the date.
• Outflow from Island Park Reservoir was increased to 1460 cfs yesterday to accommodate increased diversion into the Crosscut Canal.
• Diversion and reservoir outflow will remain high for the next 2-3 weeks, as hot, dry weather continues.

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

Evenings ( try caddis and PMD and gray drake spinner patterns) and early AMs ( spinner patterns again) are the best times to fish the lower river where terrestrial insects are beginning to appear in good numbers. A bit early for these along the upper river except ant and beetle patterns should always be in that fly box Variable flow out of Island Park Reservoir impacts fishing from Box Canyon to Riverside. PMDs, Flavs and green drakes are diminishing a bit (AM and PM spinner falls bring action) except for PM brown drakes in lower Harriman offering best activity. PM caddis activity is reliable everywhere.

Henry’s Fork, July 9th, 2022

The river from Ashton Dam downstream is beginning to warm as our “dog days” are here. This means best fishing will be early AMs and evenings, Major aquatic insects are diminishing, but terrestrial insects will soon be plentiful. Wading and boat traffic on the Last Chance-Harriman section is high, but fishing is good as the usual aquatic insect hatches continue at good levels.

Plenty of water ( currently about 170 cfs) is flowing through the river at the Flat Ranch Preserve. This flow started late because of water managing efforts in this drought year. That means fewer fish were swept downstream out of Henry’s Lake than usual. Nevertheless, the river here is worth a try.

Henry’s Fork, July 5th, 2022

We have reports of recreational floating traffic on the river; Box Canyon and the entire lower river between above Chester seem to be favored locations. Consider that these folks do not understand angling etiquette. So be patient. They have just as much right to enjoy the river as do anglers. Best time to fish around them is early and late in the day.

All traditional hatches are in full swing along the river: Green drakes, flavs, PMDs, PM caddis abound with a few golden stones. Brown drakes are appearing in evenings on lower the river at Harriman East making Wood Road 16 an up and coming favored location. AM and PM spinner falls are good times to be on the river whether it be Last Chance-Harriman State Park waters or the lower river. These times of day minimize recreational traffic. Also dry-dropper combos seem to work everywhere. Try a dry blond humpy over a bead head nymph of your choice

Headlines from Dr. Rob Van Kirk’s latest Henry’s Fork drainage water conditions report follow:

Headlines:  

• The first 4 days of July were slightly on the warm side of average, and a few thundershowers kept water-year precipitation at 91% of average.
• Natural flow has dropped by 1-2% per day over the past four days, while diversion has stayed relatively constant.
• Since an outflow increase on Saturday, Island Park Reservoir outflow has averaged 991 cfs, and the reservoir has drafted at 930 ac-ft/day. Additional incremental increases in outflow are expected.
• Water quality: Water temperatures have been optimal except at St. Anthony, while turbidity is still above average at all locations upstream of Ashton Reservoir. 

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

Henry’s Fork, June 21st, 2022

Yesterday was a tough one along the upper river. Thunderstorms with rain and hail, air temperatures staying in the mid-forties in Degrees. F,, and hypothermic conditions for anyone getting wet. Today, the first partial day of summer, is different with good weather, air temperatures warming to seasonal levels, and aquatic insect hatches ( PMDs, some drakes, a few golden stones, March browns, and flavs, plus reliable PM caddis) becoming more active. Flow out of Henry’s Lake has been raised to around 200 cfs, which will flush some fish into the river below to benefit fishing in the Flat Ranch section. For a while wet flies ( streamers, bugger types, Henry’s Lake patterns) will be best bets here. Yesterday’s foul weather also hit the lower river but better weather now prevails to benefit hatches. Dry- dropper methods seem to work well anywhere on the river. Angler numbers are most numerous on the river from Ashton to Chester.and some social situations, mostly from boating anglers, are happening. Please be considerate.

Dr. Rob Van Kirk’s latest report on snow-water conditions in the Henry’s Fork drainage follows.

• Another cold, wet day moved water-year precipitation up a percentage point to 92% of average and added a little new snow at high elevations.
• Rain and cooler weather has kept both diversion and natural flow constant over the past few days.
• Island Park Reservoir stayed constant yesterday at 99.1% full, and outflow will be decreased to 600 cfs today.
• Drier, warmer weather is forecast for the rest of the week, albeit not quite as dry or warm as predicted a few days ago.

Details:

Mean temperature yesterday was 12 degrees F below average, and moderate precipitation was widespread across the watershed. Precipitation favored the Teton subwatershed, where Grand Targhee picked up 0.7 inch. Amounts in the Fall River and upper Henry’s Fork subwatersheds were fairly uniform at 0.1-0.3 inch. Valley areas received as much as 0.2 inch. Watershed average precipitation since the beginning of the water year moved back up to 92% of average. White Elephant and Lewis Lake Divide lead in that department, each at 98% of average. Because of cold temperatures, yesterday’s precipitation fell as snow at the highest elevations, keeping snow water equivalent (SWE) at 11% of the season’s peak. Over 13 inches of SWE remains yet to melt at Grand Targhee.

Much warmer and drier conditions are expected for the remainder of the week, but forecast temperatures are now a little lower than expected a few days ago. The current forecast shows temperatures staying near average for this time of year. In addition, monsoonal moisture is expected to creep up into our area from the south, bringing a chance of showers back into the forecast by Friday. The monsoonal (seasonal southerly moisture feed from the Gulf of Mexico) pattern always favors the southern end of the watershed, which has received less precipitation than the northern end over the past month. Water-year precipitation in the Teton subwatershed is 88% of average, compared with 95% of average in each of Fall River and upper Henry’s Fork. The monsoonal pattern also brings higher temperatures, especially at night, and forecasts at the 1-3 week time scale hint at above-average temperatures and near-average precipitation.

As far as drought recovery goes, all indicators point to strong short-term improvement and incremental long-term improvement. The one-year accumulated moisture availability index has increased by nearly 5 inches over the past month and is now less than 2 inches below average after spending most of the winter and early spring down around 6 inches below average. That index bottomed out at 11 inches below average last July. Three-year average precipitation has improved slightly over the past month, moving from 12.5% below average to 10% below average.

Although this spring has certainly been wetter and colder than average, it is nowhere near the coldest and wettest, even in recent decades. Mean temperature since April 1 is 3 degrees F below average, but that ranks only in the coldest 15% of years since 1989. The springs of 2010, 1998, and 2011 were all colder than this year’s. The spring of 2011 averaged 1.4 degrees colder than what we have experienced this year. Similarly, precipitation since April 1 this year is 22% above average, but that ranks only around the 70th percentile of years since 1989. The same three years mentioned above—2010, 2011, and 1998—were all wetter than this spring and some by a large margin. April 1 – June 20 precipitation in 2011 totaled 14.9 inches, compared with 10.6 inches this year.

Natural streamflow has remained fairly constant for the past three days, as rain has offset decreased snowmelt due to colder temperatures. Natural flow yesterday was 95% of average: 87% in upper Henry’s Fork, 96% in Fall River, and 101% in Teton River. Diversion has also stayed fairly constant and was 91% of average. Natural flow is 3300 cfs greater than demand, which is a little above average for the date, something that was extremely unlikely based on conditions in early April. Recall that last year at this time, Island Park Reservoir had been drafting for 10 days and was already less than 94% full. I expect diversion to increase back up to average by the end of the week, while natural flow will drop relatively slowly. Warmer temperatures will increase melt of remaining high-elevation snow, which should produce one more peak in Teton River streamflow. Fall River and upper Henry’s Fork will drop relatively slowly. I expect the positive effects of delayed snowmelt and recent rain on streamflow to persist into next week, delaying need for reservoir draft until then.

At an outflow of 800 cfs, Island Park Reservoir was constant yesterday at 99.1% full. Now that heavy precipitation appears to be over for the foreseeable future, outflow will be reduced today to 600 cfs to slowly bring the reservoir back to full pool until draft is needed to meet within-watershed irrigation demand. As mentioned above, that is not likely until at least next week, so expect relatively low and stable flow downstream of Island Park Dam during that time period. For reference, the long-term average outflow from Island Park Reservoir right now is 1,000 cfs. Lower outflow right now is due to an unusual combination of low natural inflow due to long-term drought, and no need for reservoir draft to meet irrigation demand due to a good short-term water supply.

The upper Snake River reservoir system continues to gain physical water and is 63% full, compared with its previous peak of 59% full back in April. The reservoir system has gained 296,000 ac-ft over the past few weeks after dropping slowly for much of April and May. Net gain since the mid-April peak is around 165,000 ac-ft. Outflow from Palisades Reservoir is around 9210 cfs, which is 62% of average for the date.

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

Henry’s Fork, June 18th, 2021

Aquatic insect activity seems to be in a delayed mode throughout the river. Looks like warmer weather is needed to speed things up throughout the river..

Henry’s Fork, June 14th, 2022

Recent heavy rains will impact the dry fly fishing for a few more days,. The river is running higher than normal throughout and fish have enjoyed life forms washed into it from the impact of high water on banks and stream bottoms. The Harriman State Park reach opens tomorrow, but expect a slowdown in fly fishing there for a while.

Dr. Rob Van Kirk water status report compiled yesterday for the Henry’s Fork drainage follows. It contains information on how recent rains will impact fishing throughout the drainage for a while.

Headlines:  

• Record-setting rain and flooding just to our north and east in Montana and Yellowstone National Park made national headlines yesterday, and these conditions extended down into Island Park as well.
• Natural streamflow appears to have peaked at most locations in the watershed late yesterday and was 120% of average for the date.
• Outflow from Island Park Reservoir will be increased to 1100 cfs this morning to keep pace with increasing inflow.
• Flooding during a drought? Short- and medium-term drought indicators continue to improve, while long-term low water supply conditions continue.

Details:

By now, most of you have seen regional and national news coverage of record-setting rain and a rain-on-snow event causing extreme flooding and damage in Yellowstone National Park and adjacent areas. Some streams set new all-time record highs. Island Park sat on the southern edge of the heaviest precipitation and has also experienced flooding, especially from streams that drain the Centennial and Henry’s Lake mountains.

Two-day precipitation totals for our watershed ranged from 0.67 inch at Alta to 3.3 inches at White Elephant. For those of you who are not familiar with the SnoTel stations, White Elephant sits on the eastern side of Mt. Sawtelle in Island Park and is a good indicator of conditions along the Continental Divide. The watershed average was 1.57 inches, which brought the water-year total up to 91% of average. Total for the first 13 days of June is 2.5 inches, which is the average total for the whole month of June. More precipitation is in the forecast, and this June’s monthly precipitation is likely to end up in the top 25^th percentile of all years since 1989.

Mean temperature over the past two days was 5 degrees F below average, which kept melt of remaining snow below average. Some locations even received a little new snow this morning. Remaining snow water equivalent (SWE) is 19% of this year’s peak and 67% of average for the date. Warmer temperatures later this week will increase high-elevation snowmelt and maintain relatively high streamflow in Teton River well into next week.

Natural streamflow peaked in headwater areas late yesterday, and that peak is now making its way down through the lower watershed. Streamflow at most locations yesterday was above average or even well above average for the date. Total natural flow yesterday was 120% of average: 89% in upper Henry’s Fork, 131% in Fall River, and 136% in Teton River. The reason for the low figure in the upper Henry’s Fork is because spring-fed base flow is still very low and reflective of long-term drought. Discharge from the large springs at Big Springs, Buffalo River, Warm River and other locations responds to changes in precipitation and snowmelt on the scale of years to decades. We will need to receive above-average precipitation—and particularly snow—for several years to see those base flows increase from near-record lows back up to average.

However, Fall River and Teton River both respond rapidly to rain and snowmelt, and both increased substantially due to both factors over the past two days. As mentioned several times over the past week, the Memorial Day rain appears to have saturated soils to the point where more recent rains have produced strong and rapid streamflow response. I expect Fall River to drop over the next few days, while Teton River will increase again later this week due to melt of high-elevation snow. Diversion decreased by 13% yesterday, and I see no indication that reservoir draft will be needed any time soon to meet irrigation demand.

For some context on current streamflow, a reader suggested I look at some all-time high flows in the watershed. Current watershed-wide natural flow is around 7,000 cfs, which ranks in the bottom quartile (25^th percentile) of the annual peak in water years since 1978. The highest natural flow in that record was 17,423 cfs in 2011. That was a year in which heavy rains during the spring and summer fell on top of an above-average snowpack. As mentioned above, this year’s heavy spring rains fell on a very poor snowpack and low base flows.

Current natural flow at Island Park is 1,330 cfs, which ranks 20^th lowest out of the 51 years since 1972. The maximum natural flow in that record was 3,155 cfs, in 1984. In the lower watershed, current flow in the Henry’s Fork at Rexburg is around 5,500 cfs. That ranks 15^th lowest among maximum annual flows in that same set of 51 water years. Streamflow at Rexburg will continue to increase for another day or two as water makes its way to the lower watershed. The highest daily flow recorded at Rexburg was 79,000 cfs during the Teton Dam flood. Otherwise, the highest annual maximum was 15,100 cfs, in 2011. So, current flows, while near or above average for the date, are generally in the bottom one-third of annual peak flows

Island Park Reservoir fill rate slowed yesterday after an increase in outflow to 950 cfs, but outflow is still lower than inflow. Outflow from Henry’s Lake was increased yesterday to around 110 cfs to keep pace with very high inflow. That will increase inflow to Island Park Reservoir later today. Outflow from Island Park is being increased this morning to 1,100 cfs, which should get ahead of inflow by tomorrow and put the reservoir on track to stay at full pool for the next week. Average outflow on the Harriman Ranch opener is 1,000 cfs. With any luck, outflow will not change for at least several days, but precipitation keeps beating expectations, and more is forecast for Sunday and Monday. The upper Snake River reservoir system continued to gain volume yesterday and is 61% full, its highest level of the year so far. The American Falls storage right is currently accruing paper water, which has happened on only a very few days since early April.

Henry’s Fork, June 11th, 2021

Fishing on the entire river can be described as “spotty”. Whether BWO, golden stone, grey drake, March brown,or PMD activities with fish responding, it’s all matter of being at the right place at the right time. Afternoon caddis activity goes on up & down the river, but where fish are taking them is up to discovery. So far it been a strange year when it comes to consistent fish activity.

Henry’s Fork, June 7th, 2022

The big stoneflies are making their run followed by that of the golden stones up the river. These soon will be relinquished on the lower river to green drakes. Also, anywhere along the river afternoon caddis activity attracts fish where stone fly activity has peaked.

Dr. Rob Van Kirk’s report on water status in the Henry’s Fork drainage follows.

Headlines:  

• Yesterday was cool yet again, and precipitation was locally heavy in the Island Park area.
• Although soil moisture has improved in valley areas, the three-year average precipitation is still 10% below average and not improving.
• Natural streamflow continued to increase yesterday but is still only 73% of average for the date and 69% of average for the water year.
• Island Park Reservoir outflow will be increased at noon today to 725 cfs to match increased inflow.

Details:

Guess what? Another day of below average temperatures is keeping the spring of 2022 among the coldest in the past three decades. Mean temperature since April 1 is in the bottom 10^th percentile of all years since 1989, at over 3 degrees F below average. Meanwhile, precipitation overachieved again yesterday, especially in the Island Park area, where Black Bear and White Elephant each received 0.8 inch. Water-year total precipitation stayed at 88% of average. Despite cool temperatures relative to average, temperatures are warm enough to continue to melt mid-elevation snow. Only a few inches of water equivalent remains at the mid-elevation stations, leaving appreciable snow only at the highest elevations.

In terms of drought indicators, the one-year accumulated moisture availability (precipitation minus evapotranspiration) has improved from a little over 6 inches below average to less than 4 inches below average just in the past 10 days. So, while still pretty far below average, a 2.5-inch net gain is substantial and represents a short-term improvement in the drought. In the longer term, the three-year average precipitation is still around 11% below average, where it was in early March, indicating persistence of long-term drought.

Natural flow continued to increase yesterday in response to rain and mid-elevation snowmelt but is still only 73% of average: 70% in upper Henry’s Fork, 81% in Fall River, and 68% in Teton River. The Fall River number is highest because it has the most drainage area in the mid-elevations at which snowmelt has been highest over the past 7-10 days. Streamflow in Teton River is yet to experience its largest peak for the season, which will happen when high-elevation snow melts. Warmer temperatures are forecast today through Saturday, which will melt some of that snow, but cooler weather early next week will slow melt a little. Overall, it looks like our slow, extended, low-magnitude snowmelt will continue. This will result in lower streamflow peaks than normal but extend the period of moderate flow farther into the summer.

Island Park Reservoir gained more volume than expected yesterday due to unexpectedly heavy rain. Outflow will be increased from around 635 cfs to 725 cfs today to keep pace with increased inflow. Outflow will continue to be adjusted in small increments to keep the reservoir within a fraction of a percent of full pool until draft is needed to meet within-watershed irrigation demand. With heavier rain than forecast yesterday and more expected Sunday and Monday, the need for reservoir draft will be pushed out at least until sometime next week. We will beat the odds for draft need today, as my predictive model based on April-1 conditions gave us a 50% chance of needing reservoir draft by June 6. Last year, draft was first needed to meet irrigation demand on June 8, and we will definitely beat that this year. Mean date of first draft is June 23.

Meanwhile, Grassy Lake has filled, and Henry’s Lake is less than 100 ac-ft short of full. Both of these outcomes have also beat the odds based on conditions on April 1. 

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