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
Warm River to Ashton makes for the best fishing on the lower river where streamers and terrestrial insect patterns, with or without a nymph dropper, work well. With relatively limited walk-in locations fishing through boating is the best way to enjoy this section. Last Chance-Harriman reach features early morning tricos. Later fish respond best to terrestrial insect patterns. Small BWOs are beginning to show up and down the river. Try streamer patterns of choice in Box Canyon. Lots of aquatic vegetation throughout the river.
No big change since our last report. Terrestrial insect patterns, especially those for ants and hoppers are most effective just about anywhere you try the river. Other than that early in the day trico activity brings trout to the top.
Not much change from our last report. AM trico, and may fly spinner patterns early, then switch to terrestrial insect and caddis patterns later in the day.
Here are excerpts from Rob Van Kirk’s Henry;s Fork Drainage status report published today.
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Temperatures returned to average yesterday, as both the daily maximum and daily average were the highest since last Tuesday. A little residual precipitation that fell after midnight on Monday morning was recorded, leaving water-year total precipitation at 82% of average. Since August 1, the one-year cumulative moisture availability in the agricultural regions of the watershed has improved by 4 inches, although it is still 7 inches below average. The improvement made up for a large decrease in this metric during the month of June, which was the hottest and driest June in the 1989-2021 record. Three-year average watershed-wide precipitation improved by around 1 inch but is 2 inches below average. Dry conditions and near- to below-average temperatures are forecast for the next week or two.
Diversion continued to drop yesterday, while natural flow stayed roughly constant for the watershed as a whole, dropping a little on Fall River and the Henry’s Fork and increasing a little on the Teton River. Diversion is 66% of average for the date and 94% of average cumulatively for the irrigation year so far. Natural flow is 79% of average for the date and 77% cumulatively for the water year. Streamflow in the lower Henry’s Fork dropped by around 200 cfs yesterday but has been stable so far today at around 160 cfs above the target flow. I expect diversion to remain fairly close to its current level for a few more days, while natural flow will recede now that the peak generated by Saturday’s rain has made its way through the lower Teton River.
At an outflow of around 400 cfs, Island Park Reservoir gained 38 ac-ft yesterday and is 43% full, compared with 53% full on average and 20% full in 2016. The USGS gage at Island Park remains accurate based on the rating adjustment on August 12, as no large changes in aquatic vegetation growth are apparent. We will measure outflow again on Friday to quantify any gage shift one way or the other that has occurred since last Friday. Barring substantial change in diversion, no increase in outflow from Island Park Reservoir will be needed in the next few days to accommodate streamflow recession in the lower watershed.
Water temperatures have fallen to their lowest values in over two months and were within the optimal range for trout yesterday at all locations downstream of Island Park Dam. Water temperatures upstream of the reservoir have been a little colder than optimal over the past few days—this isn’t a stressful situation for trout but just means that they won’t grow as fast as they would in warmer water. Turbidity at Island Park Dam has stabilized since last Friday’s flow transfer to the power plant and is slightly above average at the dam and at Pinehaven.
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
Not much change from our August 14th posted fishing report. Look for AM Trico activity to increase and the same for trout interest in honey ants and other terrestrial insects. Excerpts from Rob Van Kirk’s Henry’s Fork drainage status posted today follow.
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Mean temperature yesterday was 7 degrees F above average despite continued heavy smoke cover. The water-year precipitation total to date stayed at 78% of average. Much cooler, wetter weather is forecast to arrive tonight and persist at least through the weekend. Expected precipitation amounts continue to increase with each forecast cycle. The current 5-day forecast calls for around 0.25 inch at lower elevations on the Snake River Plain to 0.5-0.75 inch in Island Park to 2 inches at the highest elevations in the Teton Range. These amounts are slightly lower than what we received 2-3 weeks ago but will be concentrated into a shorter time period. Although precipitation is expected to end on Sunday, temperatures are forecast to remain near average, if not below, through next week.
Natural streamflow continues to recede slowly and was 68% of average yesterday for the watershed and within two percentage points of that in each of the three subwatersheds. Diversion increased a little yesterday, and streamflow in the lower Henry’s Fork is at its target this morning after staying just a little above it for several days. Delivery to the Teton River through the Crosscut Canal has been stable at 210 cfs since Sunday afternoon. Upcoming precipitation is expected to be heavy enough to increase natural flow by around 100 cfs again, and I expect diversion to decrease a little as well, depending on precipitation amounts in the valleys.
At an outflow of 724 cfs, Island Park Reservoir dropped by 596 ac-ft yesterday. The reservoir is 43% full, compared with 57% on average and 23% in 2016. Together, the three reservoirs in the Henry’s Fork watershed are 62% full, compared with 70% on average and 52% in 2016. Reservoir draft rate will slow tomorrow through the end of the week due both to direct precipitation on the reservoir surface and to increased stream inflow. After some higher values last Thursday and Friday, peak water temperatures have decreased watershed-wide by 3-4 degrees since then, due primarily to increased wildfire smoke. Turbidity continues to increase at Island Park Dam due both to increased algae growth in the reservoir and continued reservoir draft. Data from past years shows that sediment transport out of the reservoir accelerates when the reservoir drops below about 60,000 ac-ft, which happened on Saturday. The upcoming rain will slow algae growth but is also likely to generate more suspended sediment from shoreline erosion. In general, I expect that turbidity in reservoir.
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