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

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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

Henry’s Fork, March 7th, 2023

Headlines from Dr. Rob Van Kirk’s March 6th Henry’s Fork drainage SWE report

• Precipitation was light yesterday but still over-performed in the Ashton area.
• Heavy precipitation is expected Friday and possibly early next week.
• Current SWE is 108% of average for the date and 90% of the average annual peak.
• At an outflow of 233 cfs, Island Park Reservoir gained 218 ac-ft yesterday and is 83% full, compared with 79% 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, February 7th, 2023

Here is an important detail from Dr. Rob Van Kirk’s snow-water equivalent (SWE) report compiled yesterday for the Henry’s Fork watershed.

I will emphasize that average snow pack this winter—if we actually even achieve that—will not bring average stream flow. Stream base flows this winter are still in the range of 70-80% of average, and the springtime snow melt will be superimposed on that very low base—not on top of average stream flow. Average SWE this will winter will bring us natural stream flow in the range of 80-90% of average, which is much better than last year but still well below 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, January 31st, 2023

Headlines from Dr. Rob Van Kirk’s Snow-Water Equivalent Report Compiled Yesterday

• Yesterday’s watershed-averaged minimum and mean temperatures ended up just a fraction of a degree warmer than those on December 22, but most stations in the watershed set new low-temperature records for the date.
• Water-year total precipitation is 104% of average, and SWE is at 106% of average.
• If SWE stays at 106% of average the rest of the winter, expected April-September streamflow will be 90-95% of average.
• At an outflow of 266 cfs, Island Park Reservoir gained 104 ac-ft yesterday and is 77% full, compared with 75% full on average.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

Henry’s Fork, January 17th, 2023

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

• Thanks to a solid storm over the weekend, precipitation last week was above average, keeping the water-year total at 110% of average and increasing SWE to 115% of average.
• Generally dry conditions with below-average temperatures are expected for the next two weeks. New snow water equivalent (SWE) was 19% above average for the 8-day period, ranging from 1.4 inches at Phillips Bench (Teton Pass) and Grassy Lake to 3.0 inches at Grand Targhee. Current SWE is 115% of average: 104% in Teton River, 107% in Fall River, and 129% in upper Henry’s Fork. Upper-level atmospheric flow has shifted from southwest to northwest, finally putting an end to three weeks of continuous heavy precipitation in California. For us, the change means cooler-than-average temperatures and generally dry conditions for the next week or two. If that forecast proves to be correct, water-year precipitation here will drop relative to average over the next week to around 107% of average, and SWE will drop to 105% of average.
• At an outflow of 201 cfs, Island Park Reservoir is 75% full, compared with 73% full on average.
• Combined volume in the three reservoirs in the watershed caught up to average yesterday, at 79% full.

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, January 10th, 2023

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

• Yesterday was warm and wet, keeping water-year precipitation at 110% of average but increasing SWE a notch to 115% of average.
• Moderate precipitation is expected to continue on and off for the foreseeable future.
• Natural stream flow in the upper Henry’s Fork sub-watershed continues to hover in the range of 75-77% of average.
• At an average outflow of 203 cfs, Island Park Reservoir gained 390 ac-ft yesterday thanks to good precipitation and is 73% full, compared with 72% 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