The lake is 90% of capacity and the blue-green algae bloom is extensive. All this suggests warm water and lack of dissolved oxygen that slows fishing. An extensive, hard cold snap would diminish the algae bloom and improve fishing.
Not much of a change since our last report. Flow throughout is gradually dropping to seasonal normal. Mutant golden stones are active but diminishing. We need some cool, cloudy weather to get the autumn BWOs and mahogany duns going in significant numbers.
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.
Flow throughout the river is about normal for this date. . Early AM mutant golden stone activity continues to provide best action, but being on the river at first light begins the best time to enjoy fish responding to it.
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.
Mountain Ash Creek
The park lifted its hoot owl closures on all streams, but these waters offer sub par fishing even though water temperatures have cooled (except for in the Firehole River). Low flows mean reduced cover that concentrates fish in deeper water and causing stress. Best fishing in the park is found in higher altitude lakes such as Grebe and Cascade Lakes. Beula and Hering Lakes in Fall River Basin and Riddle Lake also offer good fishing. As summer winds down crowding such as is present on northeast corner streams ( Lamar River, Slough and Soda Butte Creeks) will diminish. Migrating browns and rainbows are beginning to migrate out of Hebgen Lake into the Madison River.
Horseshoe Lake
IDF&G has placed a salvage regulation on Condie and Winder Reservoirs. Let’s hope that more such regulations are not upcoming. Some of the fastest still water fishing can be had at Horseshoe Lake, Sand Creek Ponds and Paul’s Reservoir. Horseshoe features rainbow and some grayling, Sand Creek features rainbows and Paul’s features cutthroat. In general the fish are not large, but they provide action. Try speckled dun life cycle, small leech and damselfly nymph patterns.
In the last week flow out of Palisades Dam has dropped from around 14000 cfs to just under 9000 cfs, ( now 10000 cfs at Heise, 5320 cfs at Lorenzo). These flow rates are at historic amounts for the river at this date and make walk-in fishing safer and open up more locations for doing so. Mutant golden stones are active early in the AM, then their activity ceases by mid day. Add fishing such as parachute adams or purple haze ( #16) blind to the list of currently effective patterns for the river
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