Henry’s Fork

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.

Henry’s Fork, April 12th, 2022

We are in a period of “BWO” weather. If you can stand the accompanying wind and precipitation, afternoon top water fly fishing can be worth a visit along the lower river. . Dress warmly and wade carefully! Banks are clearing making access an improving situation. Presenting streamer and stone fly nymph patterns will also bring action during this period of relatively good overhead cover.

Excerpts from Dr. Rob Van Kirk Henry’s Fork drainage SWE report of this morning are below. It contains some long awaited good news.

Headlines:  

• Yesterday was the wettest since January 6, increasing water-year precipitation to 80% of average and SWE to 67% of average.
• Natural flow dropped yesterday in response to cold temperatures.
• Due in part to direct precipitation, Island Park Reservoir gained 444 ac-ft yesterday and is 93% full, compared with 86% on average.
• Cold, unsettled weather is expected to continue for the rest of the week.

Details:

As mentioned yesterday, we could have used our current weather back in February. Mean temperature yesterday was 9 degrees F below average and roughly equal to average at the end of February. Precipitation overachieved for a change and was the highest one-day total since January 6, the last day of the only extended wet period we had all winter. Precipitation totals ranged from 0.11 inch at Rexburg to 0.8 inch at White Elephant and averaged 0.38 inch over the watershed. This was enough to move the water-year total from 78% of average to 80% of average.

Cold temperatures put a temporary end to snowmelt, and all SnoTel stations except Island Park gained snow water equivalent (SWE) yesterday. New SWE ranged from 0.3 inch at Crab Creek to 1.3 inches at Phillips Bench and averaged 0.6 inch over the watershed. This moved SWE up two percentage points to 67% of average. It also moved SWE back ahead of 2001 and 2015, the two years with lowest SWE in the 1989-2022 record. SWE on the ground this morning is the highest for the season so far at all high-elevation stations and most of the mid-elevation stations. Date of peak SWE for the whole watershed is still March 22, but tomorrow’s SWE will most likely be the new peak for the season. Average peak date is April 12.

Very cold and unsettled weather will continue for another few days. Although temperatures will warm, precipitation remains in the forecast off and on into the beginning of next week. The 7-day quantitative forecast calls for 0.25 inch along the western edge of the watershed, 0.5-0.75 inch in the Island Park area and up to 1.75 inches along the Wyoming border. That will be enough to keep the water-year total up above 80% of average and is likely to push SWE up to 70% of average. Snowmelt will remain minimal at least through Friday.

After a small, short-duration bump, natural streamflow dropped yesterday in response to cold temperatures over the past two days. Natural flow is 71% of average: 76% in upper Henry’s Fork, 68% in Fall River, and 62% in Teton River. I expect natural flow to stay pretty constant for the next few days.

Due in part to direct precipitation on the reservoir surface, Island Park Reservoir gained 444 ac-ft yesterday to each 93% full, compared with 86% full on average. Outflow will likely be increased again to prevent reservoir ice from encroaching on spillway infrastructure as the reservoir continues to fill.  

Henry’s Fork, March 15th, 2022

Excerpts from Dr. Rob Van Kirk’s Henry’s Fork drainage water report of this morning are below.

 Headlines:  

• Water-year precipitation stayed at 83% of average yesterday, while SWE dropped to 69% of average.
• Stream gages are starting to come back on line after a very long period of ice, showing natural flow in the range of 75-80% of average.
• Temperatures and precipitation are expected to stay near climatic averages for the next week.

Details:

Mean temperature yesterday was 2 degrees F below average, extending a streak of nine consecutive days of temperatures at or below average. With snowpack as low as it is, cold weather is critical to keeping that snow on the ground as far into the spring as possible. After a dry day, snow water equivalent (SWE) dropped back to 69% of average yesterday, while water-year precipitation stayed at 83% of average.

Current SWE is still ahead of that for the date in 2001 and 2010. Subsequent spring and summer weather differed substantially between those two years, resulting in two very different outcomes for the water year as a whole. April-September precipitation in 2001 was 21% below average, while that in 2010 was 19% above average. That difference resulted in 2001 ending up with the lowest natural streamflow in the 1978-2021 record, at 73% of average, compared with 75% of average in 2016 and 76% of average last year. By contrast, natural flow in 2010 ended up at 87% of average, ranking above the 33^rd percentile of water years since 1978. So, a wide range of outcomes is still possible, although anything close to average water supply is not one of them.

Light precipitation is expected this afternoon through tomorrow and again on Saturday night and Sunday. Total precipitation amounts over the next week are expected to range from around 0.1 inch along the western edge of the watershed to around 1 inch along the Wyoming border. If those totals are realized, precipitation this week will be about the same as it was last week and close to average for mid-March. Temperatures are expected to stay near average. After that, above-average temperatures are in the forecast beginning next Tuesday, although long-range outlooks do not show any strong indication that either temperature or precipitation will deviate much from average.

Stream gages are starting to come back on line after a very long period during which ice has prevented real-time flow observations. The St. Anthony gage on the Henry’s Fork and Ashton gage on Fall River are the first two to return to real-time transmission. Based on data available as of yesterday, natural streamflow is in the range of 75-80% of average in both the upper Henry’s Fork and Fall River.

Island Park Reservoir gained 232 ac-ft yesterday and is 87% full, compared with 80% full on average. Reservoir content will reach 120,000-123,000 ac-ft by April 1.

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, March 12, 2022

Nicer but unsettled weather is predicted for the coming week. With increasing BWO activity on the lower river, some decent top water fishing should take place but river still has limited approaches. Streamer and stonefly nymph patterns remain effective when presented in deeper runs and holes where you can get to them..

Excerpts from Dr. Rob Van Kirk’s Henry’s Fork drainage Snow Water Equivalent (SWE) report of yesterday, March 11th, are below. The data give a grim picture of current SWE in the area.

Headlines:  

• More low temperature records were set yesterday, which started out with the coldest minimum in over five years.
• Water-year precipitation dropped to 82% of average, while SWE stayed at 68% of average.
• Reservoir fill rates were even lower yesterday than on Wednesday. Island Park is 86% full, compared with 79% full on average.
• After today, temperature and precipitation are expected to be near climatic normal for the next week or two.

Details:

Yesterday morning was the coldest in the watershed since December 17, 2016. In the 12-station 1989-2021 record I use for the whole watershed, yesterday’s minimum, mean, and maximum temperatures all set new records for the date. The low of -16.5 degrees F broke the old record of -3.2 degrees set in 2006. The daily maximum of 17.0 degrees broke the old record of 21.1 degrees, set in 2009. The daily mean was a whopping 27 degrees below average for the date and the second coldest day of the winter, behind January 1. That beat the old March-10 record by 9 degrees. Even at stations with very long periods of record, yesterday’s low temperature broke records for the date by wide margins. Idaho Falls set a new record low of -7 degrees, breaking the old record of 4 degrees, set in 1969.  

No precipitation was recorded yesterday, dropping the water-year total to 82% of average. Snow water equivalent (SWE) stayed at 68% of average. After another day of below-average temperatures, forecasts and long-range outlooks all call for temperature and precipitation near climatological averages. In the short term, light precipitation is expected on Sunday, with slightly heavier precipitation on Tuesday and Wednesday. The latest 7-day precipitation forecast looks a little better than it did yesterday, with 0.25-0.5 inch expected over most of the lower elevations of the watershed and up to 1.25 inches along the Teton crest. If realized (big if), that would be roughly average for this time of year.

As you might imagine, stream gages throughout the watershed remain affected by ice. The Island Park and Ashton gages are the only two that have provided reliable data all winter, and both show natural flow in the upper Henry’s Fork at around 75% of average.

Reservoir fill rates were even lower yesterday than they were on Wednesday. Island Park Reservoir is 86% full, compared with 79% on average, and the three reservoirs in the watershed are 87% full, compared with 84% full on average. The upper Snake River reservoir system is 50% full, compared with 74% 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, March 8th, 2022

Access to the lower river from the Ora bridge downstream to Chester is opening up. Increasing BWO activity during afternoons and midge hatches are providing some top water fishing that will increase as we move through this month. Life cycle patterns of these insects will produce. Streamer and giant stone fly nymphs will attract post-spawning rainbows almost anywhere in the river.

Fun Farm Bridge at Dusk

Excerpts from Dr. Rob Van Kirk’s Henry’s Fork drainage water status report of this morning follow.

Headlines:  

• Light snow late yesterday kept water-year precipitation at 82% of average and SWE at 67% of average.
• Island Park Reservoir continues to fill slowly but steadily at a rate of around 200-210 ac-ft/day.
• Light to moderate snow is forecast watershed-wide today.

Details:

Mean temperature yesterday was 12 degrees F below average and will stay that cold for the next several days. Snowfall arrived yesterday evening as forecast, but only a few hundredths of an inch were recorded through midnight, mostly in the valleys. Widespread snow is expected today, accompanied by strong winds and very cold temperatures this afternoon and evening. Unfortunately, water-equivalent totals now look to be lower than anticipated yesterday, a consistent pattern over the past two months. The heaviest snowfall appears to be headed to our south again, as happened a few days ago. More precipitation is expected Sunday and into next week, but this far out, forecast precipitation could easily disappear.

At an average outflow of 214 cfs, Island Park Reservoir gained 201 ac-ft yesterday and is 85% full, compared with 79% full on average. Yesterday I reported that the mean winter (December-February) outflow from Island Park Reservoir was 218 cfs, compared with 360 cfs on average. Winter flow in the Buffalo River was 200 cfs, compared with an average of 214 cfs, so total flow through Box Canyon—the most important variable determining survival of juvenile trout—was 418 cfs, 73% of average. However, natural stream flow was much lower than average this winter regardless of the effect of storage in Island Park and Henry’s Lake. In absence of the two reservoirs, stream flow through Box Canyon this winter would have been only 516 cfs. Thus, the actual stream flow through Box Canyon this winter was 81% of the river’s natural flow, not a bad outcome following last year’s very dry irrigation season.

:

Senior Scientist

Rob Van Kirk, Ph.D.

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, February, 19th, 2022

Dr. Rob Van Kirk’s water report of February 18th for the the Henry’s Fork drainage is below.

Headlines:  

• Water-year total precipitation stayed at 90% of average yesterday, while SWE dropped to 75% of average.
• Island Park is 83% full, compared with 77% full on average, and the upper Snake River Reservoir system is 45% full, compared with 64% full on average.
• The new seasonal outlook issued yesterday calls for drought to persist through the spring.

Details:

Mean temperature yesterday was 3 degrees F below average. Light precipitation was recorded at scattered locations, averaging 0.04 inch over the watershed. The water-year total stayed at 90% of average, but snow water equivalent (SWE) dropped to 75% of average. Current SWE is fifth lowest for the date in the 1989-2021 record, ahead of 1991, 1994, 2001, and 2010. April-1 SWE in those other four years ranged from 59% of average in 2001 to 85% of average in 1991.

All signs point to a similar outcome this year. After light to moderate snow on Sunday and Monday, very cold but dry conditions are expected next week and into the first week of March. The new seasonal outlook for March-May issued yesterday calls for drought conditions to persist. Our best-case scenario this year is a snowpack about like last year’s, in the 80-85% of average range. However, when combined with near record-low stream base flows this winter, water supply will definitely be lower than it was last year.  

Reservoirs continue to fill slowly but steadily. Island Park Reservoir is 83% full, compared with 77% full on average. Henry’s Lake is 90% full and very close to average. The upper Snake River reservoir system is 45% full, compared with 64% full on average. Even with above-average precipitation over the remainder of the winter, snowpack will not be sufficient to fill the upper Snake system.

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, February 25th, 2022

Excerpts from Dr. Rob Van Kirk’s February 24th Henry’s Fork water supply report are below.

Headlines:  

• A very dry week dropped water-year precipitation to 92% of average and SWE to 77% of average.
• Since Wednesday’s reduction in outflow, Island Park Reservoir fill rate has averaged 233 ac-ft/day, up from 150-190 ac-ft/day for most of the last five weeks.
• Weather forecasts have been highly variable from day to day over the past week but are certain of widespread snowfall tonight and tomorrow and chances of precipitation over the next week.

Details:

Mean temperature last week was 2 degrees F above average, and yesterday’s watershed-average maximum temperature of 39 degrees was the warmest since December 4. Last week was as dry as any in the current dry spell, now in its sixth week. Precipitation was recorded at only four of the 12 stations, amounting to only a few hundredths of one inch when averaged across the watershed. This dropped water-year precipitation from 96% of average last week to 92% of average this morning. In the bigger picture, the three-year running average precipitation, which is a good measure of overall drought conditions at a scale relevant for ecological process and long-term water supply, dropped from just a fraction of an inch below average in early January to 2.5 inches below average over the past five weeks.

The figures for snow water equivalent (SWE) are even worse. Net accumulation of new SWE over the week was 0, dropping SWE from 81% of average to 77% of average over the week. If we have average SWE accumulation between now and early April, this year’s snowpack will end up around 20% below average. At best, we could barely hit average if the remainder of the winter is extremely wet. In the 1989-2021 record, only two years—2011 and 1989—were wet enough between February 14 and April 8 to push this year’s snowpack up to average. That’s only two observed scenarios in the past 33 years, giving us roughly a 6% chance of getting to average in the modern climatic regime.

Weather forecasts and climate outlooks at all scales have not only performed poorly over the past month but have become highly variable from day to day over the past week. Forecasts for this week went from wet a week ago to dry by the end of last week to wet again this morning. Widespread precipitation is expected tonight and tomorrow, followed by moderate chances over the remainder of the week. More widespread precipitation is expected again next weekend. The current 7-day quantitative precipitation forecast is the best it has looked in over a month, calling for widespread amounts of 0.25 inch in the valleys to over 1 inch along the Wyoming border. However, confidence is low past Wednesday of this week. Nonetheless, we will see at least some change from persistent high pressure.  

Since Wednesday’s 20-cfs reduction in outflow from Island Park Reservoir, fill rate has averaged 233 ac-ft/day, up from 150-190 ac-ft/day over most of the past five weeks. The reservoir is 82% full, compared with 76% on average. If precipitation over the next week materializes as predicted, gain will be enhanced by about 50 ac-ft/day due to direct precipitation. Outflow has averaged 217 cfs since December 1.

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