Henry’s Fork

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

Henry’s Fork, January 3rd, 2023

Headlines from Dr. Rob Van Kirk’s SWE (snow-water equivalent) report for the Henry’s Fork Drainage filed yesterday are below.

Headlines:

• Mean temperature for the month of December was near average, but precipitation was 18% above average.
• Water-year precipitation to date is 113% of average, and SWE is 119% of average.
• Light to moderate precipitation is expected over the next week, with near-average precipitation forecast for the month of January.
• At an average outflow of 204 cfs, Island Park Reservoir gained 329 ac-ft/day over the last four days and is 71% full, 900 ac-ft ahead 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, December 20th, 2022

Here are details from Dr. Rob Van Kirk’s SWE report this AM for the Henry’s Fork drainage.

Note the waning he gives for weather conditions the next several days.

Mean temperature yesterday was 5 degrees F below average. Only very light precipitation was recorded, dropping the water-year total to 109% of average and snow water equivalent (SWE) to 117% of average. The latest precipitation forecasts are a little wetter than yesterday’s, now calling for up to 2 inches of water equivalent along the Teton crest from today through next Monday. Potentially heavy snow remains possible early next week.

In the meantime–for those who live or are considering traveling here over the next few days–an extremely strong cold front is expected to pass through the area tonight and early tomorrow morning, bringing 40+ mph winds and widespread blowing snow. Following the front, temperatures will drop well below zero on Thursday morning. If forecasts hold, the upcoming cold temperatures will be the lowest we have seen since December 2016. Travel is not advised and will not even be possible in some places tomorrow and Thursday.

Where stream gages are still reporting, natural flow is still in the 70-75% of average range, where it has been since September.

At an average outflow of 193 cfs, Island Park gained 309 ac-ft yesterday, staying well ahead of the fill rate needed to meet the reservoir volume target at ice-off, which occurs in late April on average. The reservoir is 68% full, only 376 ac-ft below average.

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, December 13th, 2022

Here is some great news from Rob Van Kirk’s Monday, December, 12th Henry’s Fork drainage SWE (snow-water equivalent) report.

Headlines:

• Precipitation over the past three days increased the water-year total to 124% of average and SWE to 145% of average.
• Since outflow from Island Park Reservoir was increased last Thursday, the reservoir has gained 344 ac-ft/day and is 64% full, compared with 65% full on average.

Details:

Precipitation over the past three days ranged from 0.04 inch at Alta to 1.3 inches at Grassy Lake and averaged 0.62 inch for the watershed. That increased water-year precipitation to date to 124% of average and snow water equivalent (SWE) to 145% of average for the date. SWE at all SnoTel stations is in the top one-third of water years since 1989. Generally wet, cold weather is forecast for the foreseeable future.

Since outflow from Island Park Dam was increased to around 195 cfs last Thursday, the reservoir has gained an average of 344 ac-ft/day. Around 75 ac-ft of that has been due to direct precipitation on the reservoir surface. The reservoir is 64% full and within 900 ac-ft 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, November 8th, 2022

Looks like skis or snowshoes are now standard equipment for fishing much of the Henry’s Fork

Headlines:  from Dr. Rob Van Kirk’s Henry’s Fork drainage snow-water analysis report of today.

• Precipitation averaged 0.77 inch across the watershed yesterday, favoring the Ashton/Fall River area.
• New SWE ranged from 0.5 inch to 1.5 inches; the watershed total is over two times average for the date.
•  At an outflow of 128 cfs, Island Park Reservoir gained 717 ac-ft yesterday and is 55% full, compared with 57% full on average.

Details:

Precipitation yesterday materialized as forecast. Station totals ranged from 0.33 inch at Rexburg to 1.4 inches at Black Bear SnoTel. The heaviest amounts fell in the Ashton/Fall River area. The watershed average was 0.77 inch, increasing the water-year total to 137% of average. All SnoTel stations received new snow water equivalent. Current SWE is the highest for the date in the 1989-2022 record at Lewis Lake Divide, Pine Creek Pass, and Island Park. Watershed total SWE is over two times the average for the date. An additional 0.25-1 inch of water equivalent is expected over the next two days, before cold, dry weather settles in later in the week.

Natural flow increased yesterday in response to a rain/snow mix early in the day and was 81% of average yesterday.

At an average outflow of 128 cfs, Island Park Reservoir gained 717 ac-ft yesterday, 200 of which was from direct precipitation on the reservoir surface. The reservoir is 55% full, compared with 57% 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