Henry’s Fork, May 13th, 2023

Flow is up with colder water throughout the river ( especially below Fall River and Warm River-Robinson Creek confluences) making for slower top water fishing even though BWO and Midge activity is good. Mother’s Day caddis are active in places. Streamer fishing can be productive when concentrated during low light periods and at out-of-direct-sunlight areas. Pattern choice is less important than understanding these conditions.

Stonefly nymphs are beginning to migrate and therefore becoming more available as a food form for trout. That’s another reason for slower top water fishing. Fish use less energy to feed along the river bed than rising to the surface. So as we move later in May, stonefly nymph patterns become more important in your fly box until adults appear.

Henry’s Fork, May 9th, 2023

Headlines from Dr. Rob Van Kirk’s SWE Report Filed Yesterday for the Henry’s Fork Drainage

• Temperatures and snow melt were lower yesterday than earlier in the week but still above average.
• Natural stream flow increased yesterday to average but is likely to level off today, depending on location and amount of rainfall.
• At an average outflow of 695 cfs, Island Park Reservoir gained 421 ac-ft yesterday and is 86% full, compared with 93% full on average.

Water quality

Turbidity continues to increase across the watershed in response to snow melt and increasing stream flow. Turbidity is particularly high right now in Fall River and other tributaries such as Robinson Creek. Water temperatures dropped back to average yesterday at all locations except Island Park Dam, where ice cover has kept temperature at 39 degrees F. Our water quality data show no signs of open water on the reservoir yet. Ice melted last year on May 6, the latest date of ice-off we have observed since installing our water quality instruments in 2014. This year’s ice-off date is likely to be 4-6 days later than that

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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, May 6th, 2023

Ground water flooding impacts access at the Vernon Bridge.

Headlines from Dr. Rob Van Kirk’s SWE Report Filed Yesterday for the Henry’s Fork Drainage

• The month of April was cooler and slightly drier than average.
• Watershed SWE peaked last Monday, at 119% of the average annual peak; 94% of this year’s peak SWE is still on the ground, compared with 87% on average.
• Natural flow is increasing in response to snowmelt; moderate rainfall is expected later in the week.
• Last week’s freshet exported 321 tons of sediment out of the Island Park to Pinehaven reach, compared with about 30 tons had the freshet not occurred.

Here is some interesting info from Dr. Van Kirk on the effects of the freshet performed for four days in late April.

Over the four days of the freshet operation, 321 tons of fine sediment were exported from the river reach between Island Park Dam and Pinehaven. Based on pre- and post-freshet numbers, we expect that about 30 tons would have been exported over those four days had the freshet not been done. For reference, the average annual net export of fine sediment from the reach over irrigation years 2016-2022 is 650 tons. So, the freshet moved half of the average annual sediment budget in three days. Freshet-related turbidity has dropped to near pre-freshet levels, but now increased stream flow from natural snow melt is increasing turbidity again, especially in the lower watershed.

Meanwhile, warmer water temperatures are accelerating aquatic insect development, although hatch timing remains later than average and even later than last year over the lower watershed. We expect rapidly warming conditions over the next few days, but the cooler forecast for late in the week could slow down progress toward average hatch timing ( His info on aquatic insect hatches helps explain the relatively slow recent fishing on the lower river).

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELLG

Henry’s Fork, May 2nd, 2023

Headlines from Dr. Rob Van Kirk’s SWE report filed yesterday for the Henry’s Fork drainage.

Headlines
• Temperatures and snow melt were well above average yesterday, but current SWE is 10% above the average annual peak and 27% above average for the date.
• Natural stream flow has increased 22% in the past two days and is 72% of average for the date.
• Precipitation over the next week is now forecast to be heavy, with much cooler temperatures.
• My forecast for April-September total natural stream flow is 105% of average for the watershed: 102% in upper Henry’s Fork, 103% in Fall River, and 106% in Teton River.

Henry’s Fork, April 25th, 2023

Headlines from Dr. Rob Van Kirk’s SWE Report Filed Yesterday for the Henry’s Fork Drainage

• Precipitation overachieved yesterday, pushing SWE to yet another seasonal peak, at 119.2% of average.
• Dry weather and a warming trend start today, which will greatly increase snow melt across the watershed.
• Island Park Reservoir will start today’s freshet delivery at 91% full and just a few inches below the rubber spillway collar.
• Outflow from Island Park will be increased to 2,000 cfs in four hourly increments this morning.

Outflow will be increased to 2,000 cfs in four hourly increments this morning and held at 2,000 cfs until at least Thursday and most likely until Friday morning. Post-freshet outflow is tentatively planned to be set at 650 cfs, but the exact figure will be determined by inflow rate. Current inflow is around 500 cfs, but I expect that to jump quickly over the next few days to 800 cfs and likely to exceed 1,000 cfs by the weekend. This will impact flow downstream to Ashton Reservoir and has purpose of helping to flush silt from the river bed.

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, April 18th, 20-23

Headlines From Dr. Rob Van Kirk’s SWE report for the Henry’s Fork drainage filed yesterday.

• A little precipitation yesterday kept the water-year total at 115% of average.
• Snowmelt was very modest yesterday, keeping SWE around 2% below its annual peak.
• At an outflow of 271 cfs over the past week, Island Park Reservoir is 89% full and filling very slowly.
• A freshet operation designed to transport fine sediment out of the Island Park to Riverside reach will occur next Tuesday-Friday. See details on the Henry’s Fork Foundation website.
• We have posted a lot of information about the ecological benefits of the freshet, as well as information on other aspects of the freshet on the HFF web site and will continue to add to that information as the freshet approaches.

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, April 15th, 2023

Although snow currently dominates the banks of the river and limits access, here is a soon-to-be-performed action that will impact flow and therefore fishing from Island Park Dam downstream at least to Ashton Reservoir. Normal flow out of Island Park Dam is around 200 cfs this time of the year.

The Drought Management Planning Committee (DMPC) approved a spring freshet for April 25-27. Freshets are most effective at sediment scour and thereby aquatic insect habitat improvement when flows increase rapidly, to high outflows, and last for 48-72 hours. Flows are expected to be bumped to 2,000 cfs on the 25th and remain there until morning of the 27th. This was not easy to accomplish and has been most of a year in the making. For more information on this action, visit the Henry’s Fork Foundation web site.

FAQ: What is a Spring Freshet?

Updated: 2 hours ago

Freshet:

A high rate of streamflow for a short amount of time. Typically a natural process in rivers caused by heavy rain or rapid snowmelt.

The Two Freshet Types on the Henry’s Fork

On the Henry’s Fork, a freshet can be:

1. Natural: from heavy rain or rapid snowmelt into a full or nearly full Island Park Reservoir, requiring a rapid increase in outflow from the dam
2. Managed: a planned freshet, negotiated with water managers and decision makers, to achieve (or even improve on) the benefits a natural freshet brings

T

Henry’s Fork, April 11th, 2023

The river below Ashton Dam is providing fishing with midge and BWO activity making for top water action during PMs. Flow is a bit below normal for the time of year. For browns and post spawning rainbows, try streamers around locations out of direct sunlight and during low light periods.

Headlines from Dr. Rob Van Kirk’s Henry’s Fork Drainage Snow-Water Equivalent Report filed Yesterday, April 10th

• Natural stream flow has increased 10% since Thursday due to snow melt but is still only 59% of average.
• At an outflow of 266 cfs, Island Park Reservoir is 88% full and filling very slowly.

At 9 degrees F above average, yesterday was the warmest day since last October 21.Based on updated data, SWE appears to have peaked for the year last Thursday at 117% of the average peak.Natural .

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, April 4th, 2023

Headlines From Dr. Rob Van Kirk’s SWE Report as of April 3rd for the Henry’s Fork Drainage

• Yesterday was cold but relatively dry, leaving water-year precipitation at 116% of average for the date and SWE at 116% of the average annual peak.
• Natural stream flow is 63% of average for the date and around 75% of average for the water year so far.
• At an outflow of 280 cfs, Island Park Reservoir is 87% full, compared with 84% full on 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, March 28th, 2023

Here are Dr. Rob Van Kirk’s Snow-Water Equivalent (SWE) report headlines for the Henry’s Fork drainage today, March 28th.

Headlines

• Mean temperature yesterday was 13 degrees F below average, extending what has been a very cold winter into spring. Precipitation was light yesterday, but after updating data, the water-year total is 112% of average, a little higher than I reported yesterday. Snow water equivalent (SWE) stayed at 119% of average for the date and 109% of the average seasonal peak.

There is no end to cold, wet weather in sight. The latest 7-day quantitative forecast is now calling for widespread precipitation exceeding 0.75 inch over even the driest parts of the watershed, with up to 3 inches of water equivalent at the highest elevations. Below-average temperatures are expected to continue well into the month of April, with high confidence in that outlook.

At an outflow of 250 cfs, Island Park Reservoir is 86% full, compared with 82% full on average. The three reservoirs in the Henry’s Fork watershed are 88% full, compared with 86% full on average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-881-3407 CELL