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.
Headlines:
Temperatures returned to average yesterday, and water-year precipitation stayed at 82% of average.
Diversion continued to decrease yesterday, while natural flow stayed roughly constant.
At an outflow of 400 cfs, Island Park Reservoir gained around 40 ac-ft yesterday and remains 43% full, compared with 53% full on average and 20% full in 2016.
Details:
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.
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.
Headlines:
Yesterday was hot, dry, and smoky again, but relief from each of these things will arrive tonight and persist through the weekend.
Rain over the next five days could be heavy along the northern and eastern edges of the watershed.
Diversion increased yesterday while natural flow continued its slow recession; streamflow in the lower Henry’s Fork is at the target flow this morning.
At an average outflow of 724 cfs, Island Park Reservoir dropped by around 600 ac-ft yesterday and is 43% full, compared with 57% full on average and 23% full in 2016.
Details:
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.
Conditions much the same as in our last report. Terrestrial insect patterns seem most effective up and down the river with honey ants increasing in number and attracting trout accordingly along the upper. river.
It is time for terrestrial insect patterns almost anywhere you fish the river. As well as hopper patterns, include ant patterns, honey, red, and black in that fly box especially if you try the river in Island Park..
Here are excepts from Rob Van Kirk’s latest (today) analysis of the Henry’s Fork drainage water status.
Headlines:
Yesterday was dry but pleasantly cool.
Natural flow stayed at 75% of average yesterday but is dropping slowly in Fall River and Teton River.
Diversion continues to increase very slowly, including a couple of incremental increases in Crosscut Canal delivery since yesterday afternoon.
At an outflow of 740 cfs, Island Park Reservoir dropped by 594 ac-ft yesterday and is 46% full, compared with 62% on average and 28% in 2016.
Details:
Mean temperature yesterday was 4 degrees F below average, and no precipitation was recorded. The water-year total stands at 78% of average. Dry weather is expected to continue for the rest of the week, but forecasts continue to include a small chance of precipitation for next week.
Natural streamflow is decreasing very slowly on Fall River and Teton River but staying relatively constant in the upper Henry’s Fork, at least relative to a few flow drops at Ashton Dam over the past few days that confound the calculations. Natural flow was 75% of average again yesterday, about where it has been for the past few days. Natural flow is still a little higher than it was prior to the recent wetter weather that began two weeks ago today.
Diversion continues to increase very slowly, primarily on the Teton River. As a result, delivery through the Crosscut Canal was increased by around 15 cfs yesterday evening and another 10 cfs this morning. The supply-demand graph shows that my “600-cfs rule” continues to perform well. Natural flow minus diversion increased to around +600 cfs late last week (natural flow was 600 cfs greater than diversion), indicating little or no need for draft of Island Park Reservoir. Indeed, the reservoir actually stayed constant for a few days during that time. As we move farther away from last week’s wet weather, natural flow will continue to decrease, and diversion will continue to increase. However, stream flow in the lower Henry’s Fork remains well above the irrigation-season target today, which will absorb the increasing difference between natural flow and diversion without immediate need for additional outflow increases at Island Park Reservoir.
At an outflow of 740 cfs, Island Park Reservoir dropped by 594 ac-ft yesterday and is 46% full, compared with 62% full on average and 28% full in 2016. Together, the three reservoirs in the watershed are 65% full, compared with 74% on average and 55% full in 2016.
Outflow from the reservoir is being split between the dam gates and the power plant, resulting in lower turbidity than we saw last week, when all outflow was passing through the gates.
Whether you fish the upper or lower river, hopper patterns are now the ” bread and butter.” Spinner patterns come in next with PM caddis fly patterns following. So wherever you fish, consider that the higher the water temperatures, the tougher it is on fish to be caught, fought, then be released. Consider using heavier leaders, barbless hooks, keeping fish in the water for “hero” shots and keeping their heads pointed upstream until they appear to recover on their own. Considering the pleasure we gain from encountering them, we owe them as much gentleness as possible during these days of stressful conditions.
Here are excerpts from Ron Van Kirk’s July 27th analysis of the Henry’s Fork drainage water status. We offer these for those interested in the drainage water situation and its impact on fishing during this dry summer and for those planning a visit to any of the drainage’s major waters. Henry’s Fork fishing conditions have changed little since our Saturday, July 24th fishing report. Anywhere you fish the river, be sure to have terrestrial insect patterns in your fly box!
At 7 degrees F above average, yesterday was the warmest day in a week.
Forecasts are becoming more certain of substantial rain over the next week, with widespread totals of 1-2 inches.
Diversion has dropped to 77% of average, while natural flow is 68% of average, putting the difference between supply and demand right at average.
Outflow from Island Park Dam was reduced to 915 cfs yesterday. The reservoir is 51% full, compared with 73% on average and 42% in 2016.
Details:
Mean temperature yesterday was 7 degrees F above average and the warmest since last Monday. The daily maximum was the warmest since Sunday the 18th. Water-year precipitation to date is still 75% of average. Forecasts are becoming more confident in widespread, wetting rain beginning tomorrow and continuing for at least a week. The current seven-day forecast calls for widespread rainfall totals of at least 1 inch, with up to 3 inches possible on the Yellowstone Plateau. Temperatures will gradually cool to average—something we have not seen since late May. Outlooks at the time frame of 7-14 days give greater-than-even odds of above-average precipitation, something we have also not seen since late May. Overnight low temperatures will increase with the increase in moisture, but daytime highs will cool substantially, reaching 8-10 degrees F BELOW average by early next week.
If the forecast rainfall totals materialize, streamflow will increase substantially by this time next week, especially in Fall River and Teton River. Precipitation of 1-3 inches in headwater areas could increase natural flow by 50% or so, at least briefly bringing natural flow up to average. That much rain would also directly add 500-1000 ac-ft to the surface of Island Park Reservoir, on top of any increases in stream inflow. In addition, an extended period of rain in valley areas would decrease irrigation demand, although that may come at the cost of damage to cut hay and to ripening grain. The duration of precipitation is a critical determinant of the latter—if valley rain is concentrated in a narrow window of a day or two followed by a return to dry weather, grain will dry without sprouting on the stalk. An extended period of rain, such as we experienced in August of 2014, could result in widespread loss of grain crops. The best scenario for both water supply and crop quality is 1-2 days of rain in the valleys, with daily showers in the mountains over an extended period. Right now, that scenario is essentially what is forecast.
For now, natural flow remains very stable at 68% of average: 76% in upper Henry’s Fork, 68% in Fall River, and 57% in Teton River. Diversion continued to drop yesterday and is 77% of average: 81% on Henry’s Fork, 64% on Fall River, and 79% on Teton River. The difference between irrigation demand and natural flow has dropped to only 100 cfs, about average for this time of year.
As expected, natural flow water-rights priority has continued to drop and was at 7/10/1889 yesterday, which reduced available natural flow on several canals in the watershed, prompting further reductions in diversion. However, the upcoming rain will increase natural flow enough that some of the natural-flow rights cut over the past week or so will at least briefly come back into priority next week, allowing more diversion without storage charges to those water users.
As I mentioned yesterday, water management over the next few weeks will be determined by a complex combination of water-rights availability, hay prices (very high right now), and effects of rain on streamflow, crop development and harvest. After two months of very stable albeit hot and dry conditions, August is promising to start off with a little more excitement for us water-data geeks. That said, no long-term changes to drought conditions are expected. Keep in mind that while the heavy rain we received at the end of May temporarily helped fill reservoirs and lowered irrigation demand, it had essentially no effect on drought conditions, which are worse now than they were in May.
Continued decreases in diversion have allowed continued cuts in Crosscut Canal delivery to the Teton River and resulted in increased streamflow in the lower Henry’s Fork. As a result, outflow from Island Park Reservoir was decreased to 915 cfs yesterday, and that decrease is just making its way to the lower watershed this morning. For the 47 days of Island Park Reservoir draft so far this season, streamflow in the Henry’s Fork downstream of all diversions has averaged 342 cfs, compared with the target flow of 350 cfs. The coefficient of variation in daily flow is 30%, meaning that the typical daily departure from the average is around 100 cfs. Outflow from Grassy Lake will be cut to zero this week, further reducing reservoir draft. If the rain materializes as forecast, it is possible that we may even have a few days next week when total reservoir storage in Grassy Lake, Henry’s Lake, and Island Park combined stays constant.
Island Park Reservoir is 51% full, compared with 73% full on average and 42% full in 2016. Total storage in the three reservoirs in the Henry’s Fork watershed is 68% of capacity, compared with 81% full on average and 64% in 2016. The three reservoirs currently contain over 9,000 ac-ft more than on this date in 2016.
It is time to try terrestrial patterns on nearly the whole river above St. Anthony. Catching and releasing will be toughest on fish from Ashton Dam downstream. From Ashton Reservoir to Warm River as well as the Cardiac Canyon section provides the best fishing on the lower river. You will find the Cardiac Canyon section to be the least crowded section because hard sided boats are not legal ( rafts and flotation devices only) there and walk-in fishing is pretty much the name of the game. Flow out of Island Park dam is down to near historic levels and top water fishing below has picked up. Bead head nymphs are producing in Box Canyon and any pattern the size of a hopper or golden stone fly will also bring action there, but be tolerant of recreationists.
Flow out of Island Park Dam is dropping. Look for top water fishing to improve. Do not overlook presenting terrestrial insect patterns from Last Chance on downstream, but such as may fly spinner patterns and caddis fly life cycle patterns will be effective. The lower river is warming enough to be good for daytime swimming!
