Fishing Reports

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