Henry’s Fork

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.

Henry’s Fork, May 16th, 2022

Giant stone fly nymphs are concentrating along banks and shallows of the lower river, and a few adults may be flying to announce beginning of the main event. Already numerous boats are on the water getting started to enjoy the famed adult stone fly event, and fish responding to ongoing may fly and caddis activity.

Here are a few things of note. The repaving of U.S.Highway 20 between Ashton and the Fall River bridge should be complete in a few days making for no flaggers on one lane sections and the long awaited seven mile smooth ride. Fall River is in run-off mode impacting fishing below Chester Dam. Mosquitos are out in forested areas

Dr.Rob Van Kirk’s Henry’s Fork drainage snow-water report of yesterday is below.

Headlines:  

• Yesterday was the warmest of the spring so far, and forecast showers did not materialize.
• Although cooler weather is forecast for the end of the week, it appears that our 6-week run of above-average precipitation has come to an end.
• Natural flow is increasing rapidly but is only 47% of average for the date at 71% of average for the water year.
• Island Park Reservoir gained over 400 ac-ft yesterday and is expected to fill next week.

Details:

Daily mean and daily maximum temperatures yesterday were both the warmest day since October 5. Showers did not materialize as forecast yesterday, but the Centennial Range received some measurable precipitation. The water-year total dropped a percentage point to 85% of average. Snowmelt was above average due to warm temperatures, and snow water equivalent at the three low-elevation stations is essentially zeroed out for the year. Relative to average, a fair amount of snow remains at the mid- and high-elevation stations in the northeastern corner of the watershed, where precipitation over the past few weeks has been heaviest.

Although cooler temperatures are still expected later in the week, forecasts have trended much drier over the past several days, and it looks like our 6-week run of above-average precipitation has ended. Only light precipitation in the northern-most areas of the watershed is expected over the next week, and long-range forecasts do not indicate any strong precipitation trend one way or the other.

Since April 6, the watershed has received 5.0 inches of precipitation, compared with 4.5 inches on average. Although this does not represent a large exceedance of average, the combination of moderate precipitation and very cold temperatures improved moisture availability in the agricultural regions by 2 inches. However, even with that improvement, one-year accumulated moisture availability is 4 inches below average. Three-year average precipitation improved slightly over the past six weeks but is still over 10% below average.

As anticipated, warm temperatures have increased natural flow watershed-wide but most notably in Fall River. However, natural flow is still exceptionally low—only 47% of average for the date and 71% of the 1978-2021 average for the water year to date. This is the lowest in the 1978-present record by a long shot—the next lowest year is 2002, at 79% of average. In the record of natural flow at Ashton (upper Henry’s Fork subwatershed) that goes back to 1930, accumulated natural flow for the water year to date has dropped to 4^th lowest, behind 1935, 1934, and 1931.

Also as anticipated, diversion continues to increase rapidly and is now 81% of average. Increase in stream flow in Teton River yesterday exceeded increase in diversion there, increasing flow in the Teton River from 25 cfs in each of the North and South forks on Sunday to about 45 cfs in each yesterday. I expect both stream flow and diversion to continue to increase through Friday, but natural flow increase will outweigh increase in diversion during that time. Fall River should reach its second and probably highest peak for the year late this week before cooler weather slows snowmelt again. The Teton River will probably reach a higher peak in two weeks or so. At the system-wide level, the reservoir system has been dropping for nearly a week now and is 58% full. The American Falls storage right has been in priority for only a few short periods over the past two weeks, meaning that paper accrual of storage system-wide has been minimal.

At an average outflow of 212 cfs, Island Park Reservoir gained 414 ac-ft yesterday and is 98% full. Stream inflow increased a little yesterday in response to snowmelt, but evaporation loss reduced net inflow by 26 cfs (6% of stream inflow). The reservoir will continue to gain around 350-400 ac-ft/day, which will put it at full pool early next week.

Henry’s Fork, May 10th, 2022

When not blown away or put down by wind, BWO March brown mayflies and early season caddis can offer good top water fishing afternoons on the lower river. sweeping big stone fly nymph patterns along the bottom is also effective.

Excepts from Dr. Rob Van Kirk’s Henry’s Fork drainage snow-water report of May 9th are below.

Headlines:  

• Mean temperature yesterday was 14 degrees F below average, and precipitation was heavy enough to move the water-year total up another notch to 85% of average.
• New snow accumulated at all SnoTel stations and was by far heaviest in the northern and eastern portions of the Island Park area.
• Natural flow dropped yesterday in response to colder weather but will increase in a day or two when yesterday’s new snow melts.  
• At an outflow of 276 cfs, Island Park Reservoir gained 449 ac-ft yesterday and is still on track to fill on May 20.

Details:

Mean temperature yesterday was 14 degrees F below average, setting a new low for the date in the 1989-2022 record. Precipitation ranged from 0 in valley areas to nearly one inch of water equivalent in areas of the upper Henry’s Fork watershed and was enough to move the water year total up one more percentage point to 85% of average. For the two-day storm, mean temperature was 11 degrees below average, and precipitation totaled 0.42 inch over the watershed. Valley areas received only a few hundredths, while White Elephant and Black Bear SnoTel each received over 1 inch of new snow water equivalent (SWE). Precipitation heavily favored the Island Park area, where driving yesterday was very difficult and people reported 20 inches of new snow in places. Over the whole watershed, net change in SWE for the two-period was a gain of 0.5 inch, compared with an average change for the date of 0.7 inch of melt.

Very light precipitation is possible Thursday and Friday, before what appears to be an extended period of dry weather sets in. Temperatures remain below average into the weekend but will creep back up to average or even above early next week. So, it appears that our five-week run of cool, wet weather will end soon.

Natural stream flow dropped yesterday in response to cold weather, but I expect it to rise again in a few days as yesterday’s new snow melts. With temperatures back up to average by the weekend, I expect stream flow to increase substantially early next week, especially on Fall River. Watershed-total natural flow is 73% of average for the water year so far, the lowest in the 1978-2022 record by quite a margin. Natural flow system-wide has increased enough relative to diversion that the American Falls Reservoir storage right is accruing water again after a week or so without any accrual. Physical storage in the upper Snake River system has been increasing very slightly over the past few days but is still no higher than its peak a few weeks ago at 59% full.

At an average outflow of 276 cfs, Island Park Reservoir gained 449 ac-ft of physical water yesterday t0 reach 97% full. At this rate, the reservoir will fill around May 20. However, some of yesterday’s gain came from direct precipitation on the reservoir surface, and with only very light precipitation forecast for the next week, fill rate will likely be a little lower than yesterday’s. On the other hand, stream inflow will get a small boost as fresh snow from the past few days melts.

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 2nd, 2022

Good BWO weather continues. The same continues for successful streamer presentation and for responses to big stone fly nymphs as these bugs begin moving along the bottom towards banks.

Dr. Rob Van Kirk’s latest water report for the Henry’s Fork drainage follows.

Headlines:  

• Heavy precipitation across the watershed last night and this morning increased water-year precipitation to 84% of average.
• At an outflow of 425 cfs, Island Park Reservoir gained 520 ac-ft yesterday, of which 180 ac-ft was from direct precipitation on the reservoir surface.
• Recent cold weather has delayed snowmelt, but peak SWE this year was 8.3 inches below average, equivalent to 620,000 ac-ft (4.5 times the capacity of Island Park Reservoir) of water that has not fallen on the watershed this year and will not contribute to stream flow or reservoir fill.
• Water-year natural flow to date is 75% of average and the lowest in the 1978-2022 record. The best-case outcome for the water year is 77% of average stream flow, 6^th driest on record.
• The best water-management outcome possible is that spring and summer precipitation is high enough that reservoir draft is lower than expected, and the reservoir system ends the season better than expected.

Details:

A strong and unusually wet cold front brought heavy precipitation to the entire watershed last night and this morning. Precipitation prior to midnight was enough to move the water year total up a percentage point to 84% of average. Much more precipitation fell after midnight. Warmer temperatures over the past few days increased natural stream flow, but diversion increased just as much. Current natural flow is only about 1,500 cfs greater than diversion right now. After another reduction in outflow, Island Park Reservoir gained 520 ac-ft yesterday, 180 of which was from direct precipitation on the reservoir surface. Outflow will be reduced again tomorrow to around 275 cfs to fill the remaining 7,838 ac-ft as soon as possible, hopefully in 10 days or so.  

Will additional snow gained over the past month increase water supply? The short answer is: only very slightly. Here’s why. Current snow water equivalent (SWE) is 80% of average for the date. That may sound good, but there are three reasons why it is not. First, 80% of average is 20% below average and still only in the bottom one-third of years since 1989. Second, on average, 15% of the snowpack has already melted at this point, so the current value is being compared to one that is already lower than the peak. Third, this year’s peak of 20.1 inches of SWE is 8.3 inches below the 1989-2021 average. That 8.3 inches did not fall on the watershed this winter and will not fall over the remainder of the spring, no matter how slowly our remaining snowpack melts.

About 70% of the Henry’s Fork watershed, or 1.49 million acres, accumulates snowpack. Over that area, the 8.3 inches of missing SWE (relative to average) is a little over 1 million ac-ft of volume in SWE. Around 60-65% of that, when melted, ends up as stream flow during the subsequent spring and summer. The remainder replenishes soils and aquifers to support future stream flow or is lost to evapotranspiration. So, that 8.3 inches of missing SWE is worth about 620,000 ac-ft of missing stream flow that will not flow down the river this spring and summer. For perspective, 620,000 ac-ft is 4.5 times the volume of Island Park Reservoir, 2.6 times the volume of the three reservoirs in the watershed combined, and 70% of total annual irrigation diversion in the watershed. Across the whole upper Snake River basin, the analogous deficit means that there is not enough water to fill the reservoir system beyond its current level of 59% full and meet ongoing irrigation demand.

Can a cool, wet spring and summer increase water supply? The short answer: again, very little. First, long-range forecasts call for a hot, dry summer. But, even if those are wrong, spring and summer rain cannot make up for missing snowpack, because most rain is taken up by plants and soils and does not contribute to stream flow the way winter snowpack does. So far this water year, natural streamflow is only 75% of average (25% below average) and the lowest in the 1978-2022 record. Considering poor base flow (the fall and winter “base” on which this year’s snowmelt is added) and a still very poor snowpack, even well above-average precipitation from here on out will yield natural flow over the remainder of the water year that is only around 78% of average. That would be much higher than last year’s May-September stream flow and an outcome that is very unlikely. But, even if this best-case scenario occurs, total water-year natural flow will end up at 77% of average and the sixth driest in the 1978-2022 record. It is just not possible to make up for 9 months of very low base flow (reflective of the long-term state of soils and aquifers) and a very low snowpack with spring and summer rain. Only several years of high snowpack will do that.

Can a cool, wet spring and summer increase end-of-summer reservoir levels? This one gets a definite “yes,” if precipitation is heavy enough. The previous two answers explain why overall water supply (natural streamflow) is not going to improve much over the next five months and also why the reservoir system will not fill much more than it already has. However, if rain is heavy enough and temperatures are cool enough, irrigation demand will be low because direct precipitation will make up a greater fraction of crop needs than average. In the Henry’s Fork watershed, precipitation makes up around 10-15% of irrigated crop needs in the watershed. A wet summer could move that up to 15-25%. That alone would reduce the need for diversion by 40,000-80,000 ac-ft. That’s 17-34% of the total volume of the Henry’s Fork watershed’s three reservoirs. In addition, continued rain will keep streamflow a little higher than it would otherwise be, in line with the 78% of average (vs the current 75% of average) referred to above. This would not necessarily limit reservoir draft but would keep natural flow water rights priorities a little higher, allowing irrigators to divert more water with their natural flow rights, on top of already limited storage rights they have. Thus, the best-case scenario would reduce the need for reservoir draft by 40,000-80,000 ac-ft. Compared with expected conditions, this would make the difference between Henry’s Lake, Island Park, and Grassy Lake combined ending up at 64% full instead of 39% full as expected. For reference, average carryover is 62% full. 

If this happened across the whole upper Snake River system, then reservoir carryover will be higher than it would have been without the spring and summer rain. This scenario adds very little to the water supply, but it reduces the need for reservoir draft. This has happened in the past, albeit not in a year with this poor of a water supply and low system-wide reservoir carryover. Years with heavy spring/summer rain that greatly limited reservoir draft were 2010, 2011 and 2014. Even last year, rain limited reservoir draft due to decreased demand. However, the cost in 2014 and 2021 was in poor hay and grain yields and quality for many producers.Bottom line: the best outcome possible this year is lower-than expected reservoir draft that will put end-of-season reservoir storage near average here in the Henry’s Fork watershed. Carryover in the upper Snake River system as a whole will still below average but better than the ~10% carryover expected at this point. For agricultural producers, the “best case” water situation will likely come at the expense of crop yields and quality this year but provide a greater chance of reservoir fill and higher storage allocations next year. For anglers, this “best case” will keep outflow from Island Park Reservoir lower than expected this year, leading to better water quality this summer and higher winter flows next year. The cost will be very low stream flows between Island Park Dam and Chester Dam for much of the summer, due to decreased need for high outflows from Island Park Reservoir but ongoing low

Henry’s Fork, April 26th, 2002

A Message from the Henry’s Fork Foundation.

Hi all,

Fremont County is beginning work on N 3100E at Vernon today (4/26) and the road will be closed coming from the north until early afternoon. The boat ramp area can still be accessed on that road from the south side during that time.

Also, on Wednesday (4/27), crews will be conducting rock crushing work. Anticipate possible delays as heavy equipment moves in and out of the gravel pit and parking area.

I know this is lining up to be a very busy season. We will continue to stay in contact with the county with the primary goal of avoiding loss of access. Yellow Sally is working hard to park shuttled vehicles in a way that makes the most of limited available parking at Vernon. We will continue to ask non-guided anglers and river users to do the same. 

Thank you for all you do to lead by example when it comes to how this river deserves to be treated.

Jamie

— Jamie Laatsch 
Communications and Outreach Director
Henry’s Fork Foundation

Good top water fishing can be had on the lower river during wind-free PMs as small stone flies,midges, BWO and March brown mayflies are active. Streamers are effective especially during unsettled weather and anywhere good overhead cover is present. Presenting large stone fly nymph patterns will become increasingly effective as we move into May.