With the stormy weekend and first of next week predicted, brown trout will be active on the lower river, so break out those streamer patterns. Other than that BWO activity will be plentiful anywhere on the river. For the river in Island Park, the best news is that Highway 20 repaving between the Sheep Falls Road and Riverside Campground is complete, so no more flaggers and long lines of vehicles to slow down traffic.
Aquatic weeds are breaking up everywhere giving better chances for landing larger fish especially on using finer tippets when presenting BWO, mahogany dun or midge life cycle patterns. Wherever you try the river, streamer patterns are a must. Brown trout are moving in the lower river, and locations in the upper river such as Box Canyon, Cardiac Canyon holes and runs, and The Tubs now host large foraging trout.
Below are excerpts from Dr. Rob Van Kirk’s summary of water conditions up to this morning in the Henry’s Fork drainage.
REPORT NOTE: I will be presenting a brief summary of water year 2021 at the Henry’s Fork Watershed Council meeting tomorrow, following a presentation on upper Snake River reservoir operations by the US Bureau of Reclamation. The meeting starts at 9:00 a.m., and you can join via Zoom at https://us02web.zoom.us/j/81474786741
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Mean temperature last week was 7 degrees F below average, and precipitation totaled over 1 inch for the watershed. All stations received precipitation over the week, ranging from 0.02 inch at Ashton to 2.6 inches at Lewis Lake Divide. Total watershed precipitation for the water year to date is 2.10 inches, and with more on the way, the first month of water year 2022 will very likely receive above-average precipitation. That happened in only four months during water year 2021. Precipitation this evening is now forecast to be lighter than anticipated a few days ago, with most areas receiving 0.1-0.2 inch. However, very heavy precipitation is expected next weekend and into the beginning of next week. Totals over the next week are expected to range from 0.75 inch at the western edge of the watershed to as much as 2.5 inches along the Teton crest. Temperatures will generally remain near to slightly above average over that time period, but snow will continue to accumulate at elevations above 7,500 feet.
Natural flow has dropped a little from the effects of precipitation early last week and is 75% of average across the watershed and within a few percentage points of that in each of the three subwatersheds. Accumulated natural flow for the water year to date is 74% of average. Diversion is still on a generally downward trajectory with two weeks remaining in the administrative irrigation year.
At a mean outflow of 110 cfs, Island Park Reservoir gained an average of 592 ac-ft/day last week and is 51% full, compared with 50% full on average. The upper Snake River reservoir system is at 14% of average and filling slowly.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-652-3567 OFFICE
208-881-3407 CELL
208-652-3568 FAX
The challenge now will be getting to the river to fish especially in the Island Park area! October storms like this has happened before, and after it passes a lot of good fishing will remain. BWO and midge activty will offer top water fishing and presenting streamer patterns will become increasingly effective. Another “plus” coming from this weather is that it will help clear aquatic vegetation ffom All waters.
Here are excerpts from Dr. Rob Van Kirk’s update for today on water conditions in the Henry’s Fork drainage. There is some welcome info here to begin the water storage year.
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Precipitation yesterday exceeded expectations, with even more falling after midnight and continuing this morning. Through midnight, water-equivalent totals ranged from 0.02 inch at Ashton to 1.4 inches at Lewis Lake Divide, with a watershed average of 0.55 inch. Mean temperature yesterday was 32.8 degrees F, 6 degrees below average and the coldest since April 20. Precipitation fell as snow at all elevations, and all SnoTel stations are reporting snow on the ground this morning, ranging from 0.2 inch to 1.7 inches of water equivalent. Snow is expected to continue on and off through Thursday, with Friday morning’s low temperatures well down into the teens in most locations. Dry weather with a slow warming trend is expected over the weekend, which will melt most snow at the low and mid-elevations. Above-average temperatures are expected next week.
Natural flow dropped a little yesterday to 75% of average due to colder weather. Meanwhile, diversion dropped by nearly 100 cfs yesterday, so supply now exceeds demand by over 1100 cfs, the highest it has been since early June.
At an outflow of around 125 cfs, Island Park Reservoir gained 662 ac-ft yesterday, nearly 100 ac-ft of which was due to direct precipitation on the reservoir surface. The reservoir is 48% full and only 118 ac-ft below average for the date. However, the upper Snake River reservoir system as a whole continues to draft and is only 9% full.
As a final note this morning on streamflow at both Ashton and Island Park, which are critical to natural-flow calculations, I suspect that the current gaged flow at Ashton is lower than actual flow and that my estimate of Island Park flow based on our measurement last Friday is a little high. We will measure flow at both locations this Friday to get better estimates. Changes in the stream gage rating curves are large relative to actual flow in the river right now, as aquatic vegetation is decaying quickly and moving out of the stream channel.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-652-3567 OFFICE
208-881-3407 CELL
208-652-3568 FAX
It’s time to present your favorite streamer patterns anywhere on the lower river. The same can be said for The Tubs area above Mack’s Inn, Box Canyon, and any deep holes you can find in Cardiac Canyon.
BWOs and early in the day trico in small sizes (#18-22) and mahogany duns are making for good near-the-top or top water fishing almost anywhere on the river. While presenting hopper patterns will soon begin to diminish in being effective as cold weather settles in, consider that presenting streamer patterns will become increasingly effective in Box Canyon, the Tubs and anywhere on the lower river where brown trout are present.
Here are excerpts from Dr. Rob Van Kirk’s Henry’s Fork drainage status report of today.
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At 12 degrees F below average, yesterday’s mean temperature was the coldest since May 10. The daily minimum was the coldest since June 11. With complete data in hand, Saturday-Monday precipitation totals ranged from 0.3 inch at Grassy Lake to 2.0 inches at Crab Creek. Other stations receiving 1 inch or better were Grand Targhee, Pine Creek Pass, and Lewis Lake Divide. The watershed total was 0.86 inch for the three-day event. That was enough to move water-year total precipitation from 80% of average to 82% of average. If the forecast for dry weather holds for the next 10 days, total precipitation for the water year will end up 81% of average. That will rank 6th lowest since 1989, the lowest since 2007, and around 2.5 inches less than in 2016. Dry conditions and above-average temperatures are expected for the next week.
Natural flow continued to increase yesterday in response to the weekend rain but is still only 73% of average for the date. For the water year, total natural flow is76% of average, which is about where it will end up once the water year is in the books next week. Meanwhile, diversion has decreased by around 9% since last Thursday, although a few canals increased diversion a little yesterday. Streamflow in the lower Henry’s Fork peaked yesterday afternoon at around 400 cfs above its irrigation-season target and is dropping this morning. The effect of yesterday’s outflow decrease from Island Park Reservoir has not yet reached the lower watershed. After things settle out in another day or two, I expect streamflow in the lower Henry’s Fork to drop back closer to but still above the target flow.
At an average outflow of 236 ac-ft, Island Park Reservoir gained 415 ac-ft yesterday and is 42% full, compared with 46% full on average and 17% full in 2016. Although fill rate will decrease a little as the effect of the weekend rain subsides, I expect the reservoir to end the water year at around 60,000 ac-ft (45% full), which would be right at the 1978-2020 average, despite a water year with natural flow at only 76% of average.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-652-3567 OFFICE
208-881-3407 CELL
208-652-3568 FAX
No significant change since our 9/14/21 fishing report. Upcoming unsettled weather should improve fishing on the river from Ashton Dam to Sr. Anthony whether through mayfly activity or streamer effectiveness.
The late summer mayfly activity peak is on going up and down the river. BWOs, mahogany duns, and tricos lead with respect to quantity and are accompanied by diminishing PMDs and speckled duns. Add a good number of terrestrial insects remaining streamside and a great time to be dry fly fishing here continues.
Below are excerpts from Dr. Rob Van Kirk’s analysis for today of Henry’s Fork drainage water status.
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Mean temperature yesterday was 1 degree F above average, and no precipitation was recorded. Updated precipitation data from SnoTel stations confirmed that water-year precipitation to date is 81% of average. With only 17 days remaining in the water year and no heavy precipitation in the forecast, precipitation for the water year will end up in the neighborhood of 80% of average. That will put 2021 somewhere in the range of 4th-7th driest since 1989. For reference, precipitation in 2016 was 88% of average. Temperatures over the remainder of the week are expected to stay around where they were yesterday. Precipitation is possible on Sunday and Monday, although amounts look light.
Diversion decreased by around 25 cfs yesterday, all of which occurred on the lower Henry’s Fork. Meanwhile, natural flow stayed roughly constant, although that in Fall River dropped by around 5 cfs from a small bump over the weekend resulting from rain on Friday night. The net result was an increase in stream flow of around 20 cfs on the lower Henry’s Fork. Otherwise, conditions in the lower watershed have been very stable for the past week.
All measures of water quality remain very good in all river reaches. Turbidity at Island Park Dam continues to decrease on both sides of the dam.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-652-3567 OFFICE
208-881-3407 CELL
208-652-3568 FAX
The fall mayfly peak is going on on the upper river. AM tricos followed by later BWOs and mahogany duns bringing action. . Until a killing frost happens look for terrestrial insect patterns to be effective. Either streamers or two nymph rigs are working in Box Canyon. All these suggestions apply to the Warm River-Ashton reach of the lower river.
BWOs are beginning to show during afternoons on the lower river. These are small (#18-20), but fish are becoming interested . It’s a good sign that the river is cooling and indicates that streamer fishing will become more effective as we advance toward the fall season ,
Excerpts from Rob Van Kirk’s Henry’s Fork drainage status as of today follow.
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As forecast, weather over the long weekend was warm, dry and smoky. Mean temperature over the three-day weekend was 2 degrees F above average. Yesterday’s mean and maximum temperatures were the warmest since August 17, the day before the four-day mid-August rain event. Dry weather since then has lowered water-year precipitation from 82% of average to 81% of average. Warm temperatures will continue for the next several days before dropping to average at the end of the week. Precipitation chances increase on Friday, but timing of precipitation for the weekend and early next week is uncertain. Regardless of timing, amounts look light at this point—a few hundredths of one inch in the valleys to around one-third inch in the mountains.
After increasing for a week or so, diversion has dropped 140 cfs since Thursday and was 80% of average yesterday. Total diversion for the irrigation year so far has dropped to 92% of average. Natural flow has stayed roughly constant since receding from the mid-August rain and was 71% of average yesterday: 69% in upper Henry’s, 68% in Fall River, and 79% in Teton River.
Based on Thursday’s conditions, when streamflow in the lower Henry’s Fork had dropped back to its irrigation-season target, outflow from Island Park Reservoir was increased by around 90 cfs on Friday morning. However, daily fluctuations of 100-200 cfs at Ashton Dam were larger in magnitude than either the flow increase or daily changes in diversion, making it difficult to determine actual conditions in the lower Henry’s Fork. As a consequence, streamflow in the lower Henry’s Fork yesterday ended up around 230 cfs above its irrigation-season target, even without any increase in natural flow.
At an average outflow of 490 cfs, Island Park Reservoir has lost around 200 ac-ft/day since the outflow increase on Friday morning. Actual outflow is around 95 cfs higher than indicated on the real-time gage. The reservoir is currently 42% full, compared with 42% full on average and 15% full in 2016.
All measures of water quality remain very good in all river reaches. Turbidity at Island Park Dam increased on the west side when additional flow was released from the gates on Friday but remains near average on the power-plant side. Turbidity at Pinehaven slightly above average for the date but about as low as it has been all summer.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-652-3567 OFFICE
208-881-3407 CELL
208-652-3568 FAX