Henry’s Fork

Henry’s Fork, February 8th, 2022

During nicer days midge activity is dense enough to provide some good top water fishing up and down the river, but getting to the river can be a problem in many places.

Dr. Rob Van Kirk’s report for today, February 8th, on snow and water conditions in the upper Henry’s Fork drainage are given below.

Headlines:  

• Water-year precipitation stayed at 96% of average yesterday, but SWE dropped to 80% of average.
• Island Park Reservoir gained 187 ac-ft yesterday, a little higher than it was 7-10 days ago but well below the rates of 250-350 ac-ft/day we saw during periods of heavy precipitation in December.
• Natural flow in the upper Henry’s Fork sub-watershed remains around 75% of average.
• Dry weather will continue until at least next Monday.

Details:

Mean temperature yesterday was 2 degrees F below average. No precipitation was recorded, leaving the water-year total at 96% of average. Snow water equivalent (SWE) dropped to 80% of average. Forecasts are hinting at precipitation next Monday, but we’ve seen such forecasts fall apart week after week over the past month.

Most stream gages in the watershed remain affected by ice, but the Island Park and Ashton gages on the Henry’s Fork show that natural flow in the upper Henry’s Fork watershed is around 75% of average, where it has been since late last summer.

Island Park Reservoir gained 187 ac-ft yesterday, higher than what we saw 7-10 days ago but nowhere near the rates of 250-350 ac-ft/day observed back in December during periods of precipitation. Since December 1, outflow has average 218 cfs, and fill has averaged 247 ac-ft/day. The reservoir is 81% 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

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

Henry’s Fork, January 11th, 2022

Excerpts From Dr. Rob Van Kirk’s Henry’s Fork drainage water report for today are below.

Headlines:  

• Dry conditions have set in for around 10 days, during which water-year precipitation and SWE will fall with respect to average and reservoir fill rates will drop.
• Water-year precipitation is 116% of average, and SWE is 104% of average this morning.
• At a mean outflow of 218 cfs, Island Park Reservoir gained 227 ac-ft yesterday, compared with 265 ac-ft/day needed over the remainder of the winter to reach the April-1 target.

Details:

Mean temperature yesterday was 5 degrees F below average. Water-year precipitation dropped to 116% of average and SWE to 104% of average. Dry conditions are expected to last into the beginning of next week, by which time SWE will be 5-7% below average again.

At a mean outflow of 218 cfs, Island Park Reservoir gained 227 ac-ft yesterday and is 77% full, compared with 72% full on average. Since December 1, the reservoir has filled at 295 ac-ft/day, ahead of the 265 ac-ft/day needed over the remainder of the winter to reach the April-1 target 126,000 ac-ft (93% full). If precipitation in Island Park stays above-average for the rest of the winter, as is expected, the reservoir will reach the April-1 target at an average outflow of 216 cfs, within measurement error of current outflow. If reservoir fill rates deviate too much one way or the other from forecasts, outflow adjustments will need to be made later in the winter to keep the reservoir on track to meet the target.

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]

HFF blog

Henry’s Fork, January 8th, 2022

Dr. Rob Van Kirk’s Henry’s Fork drainage snow water equivalent (SWE) report of January 7th is below. It’s data suggests that we are off to a good start in accumulating snowfall in the drainage.

DATA NOTE: The Grassy Lake and Black Bear SnoTel stations have been reporting incomplete data for the past few days. I have estimated these using available data, but I anticipate adjustments once complete data are reported again. Those adjustments may affect precipitation totals and temperatures.

 Headlines:  

• Precipitation yesterday was the heaviest since December 23, increasing the water year total up to 119% of average and SWE up to 111% of average.
• Reservoir fill rates increased again yesterday due to the heavy precipitation, especially at Henry’s Lake and Grassy Lake.
• At an outflow of 222 cfs, Island Park Reservoir gained 309 ac-ft yesterday, compared with 261 ac-ft/day needed over the remainder of the winter to reach the April-1 target.
• Warm, wet weather is expected to continue today before a lengthy dry spell sets in.

Details:

Yesterday was the warmest and wettest day since December 23. Temperature was 7 degrees above average, and precipitation averaged 0.54 inch across the watershed. The Teton and Fall River subwatersheds were favored. Water-year precipitation is now 19% above average for the watershed and above average at all 12 stations. New snow water equivalent (SWE) accumulations ranged from 0.2 inches at Crab Creek to 2.3 inches at Grand Targhee. This increased the watershed-total SWE to 11% above average. SWE is above average at all SnoTel stations except Grand Targhee, which is at 98% of average.

Warm, windy, and wet conditions are expected today before an extended dry period sets in. Temperatures in inversion-prone valley areas will be average to below average, while elevations above inversions will see above-average temperatures next week.

In response to precipitation and warmer temperatures, total gain in the watershed’s three reservoirs increased from 220 ac-ft on Monday to 482 ac-ft yesterday. The increases were most notable at Grassy Lake and Henry’s Lake. Grassy Lake gained only 11 ac-ft on Monday but 33 ac-ft yesterday and is 67% full, compared with 76% full on average. Henry’s Lake actually lost some volume last weekend but gained 140 ac-ft yesterday. Henry’s Lake is 88% full, compared with 89% full on average.

At an outflow of 222 cfs, Island Park Reservoir gained 309 ac-ft yesterday. Average fill rate since December 1 is 307 ac-ft/day, compared with 261 ac-ft/day needed the rest of the winter to meet the April-1 target. The reservoir is 77% full, compared with 71% full on average. Together, the three reservoirs in the Henry’s Fork watershed are 81% full, compared with 78% on average. The upper Snake River reservoir system as a whole is 36% full, compared with 56% full on average.

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

Henry’s Fork, December 14th, 2021

Anywhere you try the Henry’s Fork open to fishing, winter conditions will be your companion. Happy Holidays!

Excerpts from Dr. Rob Van Kirk’s Henry’s Fork watershed report released yesterday follow.

Headlines:  

• Yesterday was warm again but the wettest since November 19, moving water-year precipitation up to 94% of average.
• New SWE accumulation was heavy yesterday, especially in the upper Henry’s Fork, increasing watershed-averaged SWE to 68% of average.
• Natural flow increased to 83% of average in response to warmer temperatures and low-elevation rain.
• Island Park Reservoir gained 625 ac-ft yesterday, over half of which was due to direct precipitation, and is 71% full, compared with 67% full on average.

Details:

Mean temperature yesterday was 9 degrees above average, and moderate to heavy precipitation fell across the watershed. Low elevations received a mix of rain and wet snow, while all SnoTel stations accumulated new snow water equivalent (SWE). Precipitation totals ranged from 0.02 inch at Alta to 1.3 inches at White Elephant. As has been the case recently, the upper Henry’s Fork and Fall River subwatersheds received the most precipitation. The watershed average was 0.43 inches, the highest one-day total since November 19. Water-year total precipitation increased from 91% of average yesterday to 94% of average this morning. New SWE totals ranged from 0.2 inch at Pine Creek Pass to 1.3 inches at White Elephant and averaged 0.7 inch over the nine SnoTel stations. This improved SWE from 62% of average yesterday to 68% of average today. Temperatures will drop back to average tonight and stay near to slightly below average for the next week or two. Above-average precipitation is expected to continue for the foreseeable future.

Watershed-total natural streamflow increased in response to yesterday’s warm temperatures and low-elevation rain. Natural flow was 17% below average yesterday. Cumulative natural flow for the water year to date is still 23% below average.

USGS measured outflow from Island Park Reservoir at 213 cfs yesterday afternoon. Updated outflow data are around 7% lower than I have been reporting for the past month or so. The reservoir gained 625 ac-ft yesterday, over half of which was from direct precipitation on the reservoir surface. Since December 1, reservoir outflow has averaged 205 cfs, and the reservoir has gained 325 ac-ft/day. An average fill rate of 270 ac-ft/day is needed to reach the April-1 reservoir target of 93% full. The reservoir is currently 71% full, compared with 67% full on average.

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]

HFF blog

Henry’s Fork, November 23rd, 2021

Snow shoes or X-country skis will help if you intend to fish the river in Island Park! Nymph patterns, large and small, and streamer patterns will be best candidates for action here and in the lower river as well.

There is near global interest in how 2021-2022 winter snowfall/water conditions will impact next season’s fishing conditions in the Henry’s Fork drainage.

Through the upcoming winter we therefore will post the Henry’s Fork Foundation’s Dr. Rob Van Kirk’s weekly summary of these conditions. Information from Dr. Van Kirk’s summary prepared on November 22nd follows.

Headlines:  

• Yesterday was warm and dry, and with the exception of a little snow today and tomorrow, warm and dry conditions will continue for the next week.
• Natural flow dropped yesterday in response to a cold morning and is 78% of average.
• At a mean outflow of 119 cfs, Island Park Reservoir gained 481 ac-ft yesterday and is 66% full, compared with 61% full on average.

Details:

Despite a cold start to the day, mean temperature yesterday was 3 degrees F above average. No precipitation was recorded. Except for some light snow today and tomorrow, generally warm and dry conditions are forecast for the next week. Precipitation is tentatively forecast to return in the middle of next week.

Natural flow decreased a little yesterday as a result of cold overnight low temperatures. River icing was apparent in the Fall River hydrograph yesterday. Watershed-wide natural flow was 78% of average yesterday and within a few percentage points of that in each of the three subwatersheds. Cumulative natural flow for the water year so far is 77% of average.

At an average outflow of 119 cfs, Island Park Reservoir gained 481 ac-ft yesterday and is 66% full, compared with 61% full on average. At the current outflow, reservoir gain due to stream inflow has been in the range of 475-500 ac-ft/day. Direct precipitation on the reservoir surface has added an additional 1000 ac-ft since the start of the water year, equivalent to an additional 10 cfs of inflow, on average.

: 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, November 9th, 2021

Streamers and nymph patterns are the ” Name of the Game” up and down the river now.

All of us are interested in what upcoming winter snowfall and water situations will be for the Henry’s Fork drainage. Such information can provide likely water conditions during the next angling season. Thus we will continue reporting what information Dr Rob Van Kirk assembles each week on the drainage water conditions. Information Rob offers from his Monday,November 8th report follows.

Headlines:  

• Yesterday’s mean temperature was the coldest since March 31, and some precipitation lingering from the weekend was recorded very early yesterday morning.
• Natural flow has dropped a little over the past day or two to 73% of average for the date.
• At a mean outflow of 120 cfs, Island Park Reservoir gained 473 ac-ft yesterday and is 60% full, compared with 57% full on average.
• Above-average temperatures and near- to above-average precipitation are expected to continue.

Details:

Yesterday’s mean temperature was the lowest since March 31, although it wasn’t even all that cold, at 3 degrees F below average. A little precipitation leftover from Sunday was recorded very early yesterday morning at a few locations in the northern and eastern areas of the watershed. Snow accumulation so far this fall remains below average, despite above-average precipitation. At least for now, the combination of warm temperatures and above-average precipitation is favorable to increase soil moisture prior to the inevitable arrival of widespread freezing temperatures and snow accumulation. In general, temperatures are expected to remain at or above average, and near- to above-average precipitation is also expected to continue.

Watershed-total natural flow has dropped a little over the past day or two and is 73% of average for the date and 75% of average for the water year so far.  

At an average outflow of 120 cfs, Island Park Reservoir gained only 473 ac-ft yesterday on a day without precipitation. Over the weekend, a substantial fraction of reservoir fill was due to direct precipitation on the reservoir. Precipitation forecast over the next week will add around 250 ac-ft over and above that due to stream inflow. The reservoir is 60% full, compared with 57% full on average.

G

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

Henry’s Fork, November 6th, 2021

Just about anywhere on the lower river brown trout are on the move. Presenting big streamer and woolly bugger patterns are the best way to encounter them.

Henry’s Fork, November 2nd, 2021

Presenting streamer patterns in deeper waters throughout the river is becoming very effective.

Excerpts from Dr. Rob Van Kirk’s Monday analysis of water conditions in the Henry’s fork drainage are below

Headlines:  

• The first month of water year 2022 was a little on the warm side but wet, with 140% of average precipitation.    
• Natural flow has dropped steadily since last Wednesday’s rain-driven peak and is 80% of average.
• At a mean outflow of 116 cfs, Island Park Reservoir has gained 535 ac-ft/day over the past three days and is 57% full, compared with 54% full on average.

Details:

Mean temperature for the month of October was 1 degree F above average, due primarily to consistently warm daily minimum temperatures over the last 12 days. The warm overnight lows have accompanied generally high humidity (for around here, anyway) and persistent precipitation, at least in some locations. For the month as a whole, precipitation was 140% of average. By subwatershed, monthly totals relative to average were 149% in Fall River headwaters, 144% in upper Henry’s, 136% in the Teton headwaters, and 120% in the valleys. The only station with below-average precipitation during the month was Ashton, coming in at 93% of average. Warm temperatures prevented much snow accumulation, despite above-average precipitation. The only two stations with appreciable snow water equivalent on the ground right now are the two highest in elevation: Grand Targhee at 81% of average and Black Bear at 67% of average.

Over the first month of the new water year, the three-year average watershed precipitation increased by almost 1 inch but remains 1 inch below average. One-year accumulated moisture availability in the agricultural areas increased 2.5 inches but remains 4.5 inches below average. New outlooks for the month of November issued yesterday give our area better-than even odds of continued warm temperatures, above-average precipitation, and slow improvement in drought conditions. In the short term, light to moderate precipitation is expected tonight/tomorrow morning and again Thursday night/Friday morning. Forecast confidence is low after that.

Watershed-total natural flow has dropped steadily since Wednesday’s rain-drive peak and was 80% of average yesterday. Accumulated natural flow for the water year so far is 77% of average, compared with 76% of average for the entirety of water year 2021 and only 69% of average for April-September. Diversion stayed fairly constant near the long-term average over the weekend but will decrease today as canals shut down after the end of administrative irrigation season 2021. Based on data available to date, diversion was 92% of average for irrigation year 2021. That figure may change a little once Water District 1 adds data from pumps that are not measured in real time and adjusts data for rating-curve shifts. The process of approving all diversion and water-rights accounting data for the previous irrigation year is usually completed in early March, but at the watershed scale, the approved data are usually not much different than data available at this point.

We measured outflow from Island Park Reservoir at 122 cfs on Friday, right on the trajectory determined by measurements over the previous two weeks. Mean outflow since Friday was 116 cfs, at which the reservoir gained 535 ac-ft/ day. The reservoir is 57% full, compared with 54% full on average. Total storage in three reservoirs in the Henry’s Fork watershed is 68% of capacity, around 630 ac-ft above average for the date.

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

Henry’s Fork, October 30th, 2021

On the lower river, Box Canyon, Cardiac Canyon holes and runs deep, and the Tubs, presenting streamer patterns is the name of the game. If you favor top water fishing, BWO life cycle patterns provide the best action.

More information from Dr, Rob Van Kirk on Henry’s Fork drainage water status follows.

Headlines:  

• Precipitation yesterday was moderate and widespread again yesterday, averaging 0.4 inches across the watershed.    
• Natural flow has increased by 330 cfs (17%) since Friday and is 89% of average.
• At a mean outflow of 124 cfs, Island Park Reservoir gained 781 ac-ft yesterday, 110 of which was due to direct precipitation on the reservoir surface. The reservoir is 55% full, compared with 53% full on average.

Details:

Moderate precipitation continued yesterday at all locations, averaging 0.4 inch across the watershed. Precipitation was uniformly distributed across the watershed, and even valley areas received 0.2-0.3 inch. This pushed the water-year total to date to 3.63 inches, compared with the long-term average of 2.75 inches for the whole month of October. Temperatures dropped yesterday to 2 degrees F below average, the coolest in 11 days. As a result, most of the SnoTel stations gained new snow water equivalent (SWE) yesterday. White Elephant, Black Bear, and Grand Targhee each have over 1 inch of SWE on the ground. Small chances of precipitation continue on and off over the next week, with temperatures staying near average.

Watershed-total natural flow has increased by 330 cfs (17%) since Friday, with most of that increase occurring Monday and yesterday. Although streamflow in headwater areas peaked yesterday, some of that is still making its way to the lower watershed this morning. Natural flow yesterday was 89% of average for the date, and cumulative natural flow for the water year to date has increased a percentage point to 75% of average. Diversion incidentally increased by around 30 cfs yesterday due to higher river stage and was 93% of average. Total diversion for the irrigation year to date—which ends on Sunday—is 92% of average.  

At an average outflow of 124 cfs, Island Park Reservoir gained 781 ac-ft yesterday. Around 110 ac-ft of that gain was due to direct precipitation on the reservoir surface. The reservoir is 55% full, compared with 53% full on average. Over the next day or two, fill rate will gradually drop back to around 550 ac-ft/day, where it was prior to the recent precipitation event.

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]

Henry’s Fork, October 26th, 2021

As is our usual practice, excerpts from Dr. Rob Van Kirk’s Monday AM analysis of the Henry’s Fork drainage water status are given below.

REPORT NOTE: USBR data sites are down for maintenance this morning, so diversion, streamflow and reservoir data are current only through Saturday. I’ve filled in missing data for USBR weather stations with data from the nearest non-USBR stations.

Headlines:  

• Precipitation during the much-anticipated atmospheric river event has so far been highly variable, ranging from a few hundredths in the valleys to over 1 inch in the Centennial Range.    
• Diversion and natural flow were around 75% of average until rain increased natural flow in Fall River on Saturday.
• At a mean outflow of 110 cfs, Island Park Reservoir gained 549 ac-ft/day for the week ending Saturday and is 54% full, compared with 52% on average.

Details:

Mean temperature last week was 3 degrees F above average. At 8 degrees F above average, Friday was the warmest day since October 6. Precipitation for the week-—most of which has fallen in the past two days—was highly variable across the watershed, ranging from a few hundredths in the valley locations to 1.3 inches at White Elephant. The Centennial Range and Yellowstone Plateau areas have received by far the highest totals thus far during the much-anticipated atmospheric river event that is bringing heavy precipitation to most areas of the West. Lewis Lake Divide, Black Bear, White Elephant and Crab Creek all received over 0.6 inch of water equivalent last week. Because of warm temperatures, new snow accumulation has been limited to the highest elevations, where Black Bear and Grand Targee have 1.7 inches and 2.5 inches, respectively, of snow water equivalent on the ground–around 80% of average for the date. Precipitation is expected to continue today and tomorrow, with somewhat cooler temperatures arriving this afternoon.

Watershed-total natural flow and diversion stayed around 75% of average until rain on Friday night and Saturday morning increased streamflow in Fall River by 40 cfs or so. Streamflow in the Teton River is also increasing a bit this morning.

We measured outflow from Island Park Reservoir at 114 cfs on Friday. For the week ending Saturday, mean outflow was 110 cfs, and the reservoir gained an average of 549 ac-ft/day. Projecting that gain through the end of yesterday, the reservoir is 54% full, compared with 52% full on average.   

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