Henry’s Fork

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

Henry’s Fork, June 4th, 2022

Right now the most active big stone fly hatch with fish responding is in Cardiac Canyon. From the lower falls raft slide down through Bear Gulch and on to Warm River, fishing is great. Fishing around the Riverside area is also picking up. In a few days the stone flies in Box Canyon will become active, so this long-awaited event is progressing.

Henry’s Fork, May 31st, 2022

Those big bugs have been hunkering down for days along the river. With the improving weather, they will be out any where along the lower river and at Coffee Pot with those in Box Canyon not far behind.

Dr. Rob Van Kirk’s Henry’s Fork drainage snow-water situation as of today follows.

Headlines:  

• Precipitation over the weekend came in well above expectations, exceeding 3 inches along the northern edge of the watershed and moving the water-year total up to 87% of average.
• Natural stream flow doubled in response to a combination of high snowmelt and heavy rain but remains only 69% of average for the water year as a whole.
• To keep pace with rapid response to weather, outflow from Island Park Reservoir was increased to 1,200 cfs on Sunday and will be reduced to 980 cfs this morning as inflow recedes.
• The primary effect of the rain was to increase basin-wide reservoir storage by a percentage point; substantial improvement in drought will require an above-average snowpack next year.

Details:

Even by Memorial Day Weekend standards, the weekend’s storm was wetter than expected. Four-day precipitation totals ranged from 0.4 in Rexburg to 3.4 inches at Black Bear and White Elephant, both located on the Continental Divide at the northern edge of the watershed. Island Park received 2.6 inches, 1.5 of which occurred on Saturday alone. Saturday was the wettest day in the watershed since September 20, 2019. Subwatershed totals were 2.8 inches in upper Henry’s Fork, 2.15 inches in Fall River, 1.6 inches in Teton River, and 0.69 inch in the valleys. Water-year total precipitation increased from 82% of average on Friday morning to 87% of average this morning.

Mean temperature over the four-day period was 5 degrees F below average, despite warm temperatures on Friday. Accordingly, most precipitation fell as rain through Saturday evening but had turned to snow at most locations by Sunday night. For the four-day period as a whole, all but the highest SnoTel stations lost 1-2 inches of snow water equivalent (SWE). Grand Targhee and Black Bear gained enough new snow yesterday to make up for melt earlier in the weekend. At the watershed scale, current SWE is 51% of this year’s peak, compared with 42% on average. Despite a long, slow snowmelt season so far and above-average precipitation in April and May, current SWE is still only 87% of average for the date.

As expected, the combination of high snowmelt last Wednesday-Friday and very heavy rain on Saturday produced a dramatic increase in streamflow; natural flow doubled between Thursday and yesterday. As mentioned above, mid-elevation SnoTel stations lost 1-2 inches of SWE, and those same locations received 2-3 inches of rain. The net effect was to put an additional 3-5 inches of water onto the watershed over a four-day period, resulting in large increases in streamflow. I had expected streamflow to peak on Saturday, but rain was heavy enough to delay the peak in headwater areas until late on Sunday. All streams reached or even exceeded average for the date but are receding this morning. Even with the dramatic increase, watershed-total natural flow was only 96% of average yesterday and remains at 69% of average for the water year to date.

As expected, rain was heavy enough in valley areas Saturday and Sunday that diversion dropped a little yesterday to 87% of average. Cumulative moisture availability in the valley areas increased by 1 inch over the past few days but is still 5 inches below average. The difference between supply and demand increased substantially and is a little above average for the date for the first time this spring. Basin-wide stream flow exceeded diversion over the past few days, allowing upper Snake River Reservoir system storage to increase from 56% full to 57% full. We will need to wait another 5-6 days to see whether and how the physical reservoir gain translates into storage-account accrual. More on that below.

Physical reservoir storage in the Henry’s Fork watershed benefited greatly from the weekend’s precipitation. Henry’s Lake gained over 2,000 ac-ft since Thursday, is 99.2% full, and will likely fill, given forecasts for continued wet, cool weather into next week. Grassy Lake gained 700 ac-ft over the past three days to reach 91% full. Grassy Lake could fill in the next 2-3 weeks if draft is not needed to meet irrigation demand before then. My April-1 simulation model gave only around a 5% probability that Grassy Lake would exceed 92% full before draft was needed to meet irrigation demand. The biggest factor in fill of Grassy Lake will be how long precipitation and physical natural flow in Fall River will meet irrigation demand there. Island Park Reservoir was within 0.5% of full pool on Friday and within that margin over the weekend, as outflow increases just kept pace with inflow. You can see that in the inflow/outflow graphs today. Outflow will be reduced this morning from 1,200 cfs to 980 cfs to keep the reservoir at full pool. Additional decreases will be made over the next few days as needed.

Although stream inflow responded to the weekend’s rain, over half of the additional inflow to the watershed’s three reservoirs came from direct precipitation on the respective reservoir surfaces. Net gain in reservoir storage since Friday was 3,441 ac-ft, of which 2,091 ac-ft came from direct precipitation. This gain will likely make the difference between filling or not at Henry’s Lake. If Grassy Lake fills, it will be because the rain increased natural flow and decreased irrigation demand enough to provide a longer fill season. In any case, the primary short-term effect of the weekend’s rain was to put the watershed’s three reservoirs within striking distance of full, something that looked very unlikely a month ago.

What does this mean for the drought? At the risk of seeming alarmist and overly pessimistic, I will answer this question at three time scales, based on the data at hand. In the long term (years), two months of above-average precipitation during the spring will not make any difference in the ongoing drought. Had we received this above-average precipitation during February and March, the effect on long-term conditions would have been a little better. Long-term drought conditions improve in our region with heavy snowpacks, not rain. The three-year average precipitation is the same today (11% below average) as it was in early April. 

In the medium term (6-12 months), cool wet weather during April and May has slightly improved soil moisture and offset some of the effects of dry conditions during February and March. For example, the one-year accumulated moisture deficit in agricultural areas of the watershed improved from 6 inches below average on March 1 to around 5 inches below average today. This is still a substantial deficit but is less bad than it would have been without recent precipitation.

In the short term (seasonal), recent cool weather has had the effect of doling out this year’s poor snowpack a little at a time. At both the watershed and basin scales, natural streamflow has been close to irrigation demand—a little ahead in the case of the Henry’s Fork and a little behind in the case of the upper Snake River basin. As a result, the upper Snake River reservoir system as a whole has been dropping slowly. This means that junior storage rights have not filled much this spring. The upper Snake River reservoir system achieved its peak of 59% full in mid-April, dropped to 56% full last week and has regained only around 51,000 ac-ft (1.2% of system capacity) over the past few days. This will probably result in a little accrual in storage accounts, primarily in the American Falls 1921 account.

On that note, the U.S. Bureau of Reclamation is today initiating delivery of upper Snake River reservoir storage for salmon flow augmentation in the lower Snake River. In most years, around 200,000 ac-ft of rented storage from the upper Snake River system is delivered for this purpose. This year, no rental water was available, so Reclamation’s powerhead storage in Palisades will be used for this purpose. That is around 53,000 ac-ft. This delivery of storage out of the upper Snake River system is equal to the physical gain the reservoir system made over the weekend.

Current weather forecasts call for continued relatively wet and cool conditions. If those forecasts materialize, we can expect to see continued slow snowmelt and relatively little additional physical fill in the basin-wide reservoir system. If this weather continues well into the summer—remember 1993, if you were here—natural flow will stay slightly higher than it otherwise would, and irrigation demand will stay slightly lower. The net effect will be to decrease reservoir draft, especially because junior storage users will gain little if any additional paper storage. This situation will benefit senior natural flow users, but the overall cost of a wet, cool summer would likely be decreased crop yields across the board. The best hydrologic outcome of a cool, wet summer (this is a big “IF”) would be better-than-expected reservoir carryover (bad instead of really bad), which would put the basin in a better position to recover from the drought next spring if snowpack is well above average next winter. 

In the meantime, enjoy average streamflow for a few days.

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 28th, 2022

Those big bugs taht are out are hunkering down. Give them a warm day or two, and their numbers will increase, they will fly, then drop to the water, and the fish will respond maybe way up the river.

Henry’s Fork, May 24th, 2022

It looks like a late year for the giant stone fly event beginning in good numbers. Let’s see what the warming weather of the next few days brings. Looking at what Dr. Rob Van Kirk observes in the Henry’s Fork drainage with respect to effect of cold weather on the drainage, it is no wonder that insect activity is behind for the date. See below

His snow-water analysis for Henry’s Fork drainage of yesterday follows.

Headlines:  

• Another cool, dry day dropped water-year total precipitation to 83% of average but kept snow melt relatively low.
• Natural flow continued to drop yesterday and was 45% of average for the date and 70% of average cumulatively for the water year.
• Diversion held steady at 93% of average.
• At an average outflow of 389 cfs, Island Park Reservoir gained 77 ac-ft and is 99.6% full.

Details:

Mean temperature yesterday was 6 degrees F below average, continuing what has been a very long run of below-average temperatures. Precipitation dropped to 83% of average for the water year so far. Snow melt was a little below average for the date, and 65% of this year’s peak snow water equivalent (SWE) remains yet to be melted, compared with 53% on average. Current SWE at the mid-elevation stations is about average for the date. Cold weather continues to keep aquatic insect hatches delayed by about 5-6 days in the lower watershed. Crop development also continues to lag average due to cooler temperatures.

A couple of warmer days are in store tomorrow and Thursday before cooler weather and precipitation return on Friday and persist through the Memorial Day weekend. Temperatures will be around 5 degrees below average. Precipitation forecasts have not been very consistent, due to a pretty strong gradient between a relatively dry forecast to our southwest and a very wet forecast to our northeast. The latest 7-day quantitative forecast calls for around 0.1 inch along the western edge of the watershed to around 1 inch along the Montana border. Areas just to our northeast, in Yellowstone National Park and southern Montana, could receive up to 2 inches. So, slight changes in moisture feed could mean large differences in actual precipitation amounts. At this point, rain in the valleys would be beneficial, but current forecasts suggest a continuation of the recent pattern of showers in the mountains but little if any in the valleys.

Natural stream flow continued to drop yesterday in response to cooler weather. Natural flow is currently 45% of average for the date and 70% of average for the water year so far. These numbers are truly astounding. In the 1978-2022 record, natural flow for the water year to date in the next driest year to this one (2002) is 79% of average, illustrating just how far this year is away from anything close to “normal.” Diversion stayed constant at 92% of average, dropping the difference between supply and demand down to only 600 cfs above the indicator of need for reservoir draft to meet irrigation demand. With warmer weather on the way—as well as some precipitation—natural flow will meet demand by a small amount at least into next week, although stream flow will be far less than half of average for this time of year throughout the watershed.

At an average outflow of 389 cfs yesterday, Island Park Reservoir gained a very small amount of water and is 99.6% full. Current outflow is just at the right level to keep the reservoir within a few hundred ac-ft of full for the next several days. Outflow may need to be increased a hair at the end of the week to accommodate increased inflow in the form of rain, should forecast amounts creep upward. On a full reservoir, 1 inch of rain adds 625 ac-ft, which is less than the difference between full pool and current reservoir content. Right now, forecasts call for around 0.5 inch in Island Park later in the week.   

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, May 21st, 2022

A few big stone flies reported out, and plenty of boats are on the lower river. Any day now the big bugs will be out in force, and so will those who are now waiting for them whether they be trout or fly fishers.