Henry’s Fork

Henry’s Fork, May 24h, 2021

Below are some excerpts from Rob Vankirk’s latest report on the water situation in the Henry’s Fork drainage. If you plan on visiting the river in the near future to fish, what is given below could help in planning. One thing is certain; with a three-day weekend coming up and the stone fly event in full swing, you will have plenty of company.

Latest forecasts are calling for one more round of decent precipitation this evening through tomorrow night. Totals will range from 0.25 in the valleys to 0.75 inch in the northeastern corner of the watershed. Temperatures remain below average through Friday or Saturday, rising to average by Sunday. Warm, dry conditions are expected to return next week, putting an end to our 10-day break.

Given the higher-than-expected rainfall totals, streamflow response to the rain was higher than I expected. Natural flow reached its highest peak for the season to date yesterday at around 7,000 cfs. Although 2,000 cfs greater than the late-April peak, 7,000 cfs is still only 90% of average for the date. Total accumulated natural flow for the water year so far is 84% of average. Natural flow has been receding since early yesterday morning, and I expect recession to continue until late tonight, when it should increase again in response to rain this evening. However, lower rainfall totals forecast for the next 24-36 hours will probably keep total natural flow a little lower than it was yesterday. Diversion decreased another 6% yesterday and was 86% of average.

Island Park Reservoir and Henry’s Lake both reached full pool on Sunday, and Grassy Lake gained enough over the weekend that it will most likely fill, especially if rain favors the Fall River headwaters again today and tomorrow. Outflow from Island Park Reservoir was increased yesterday from 210 cfs to 925 cfs to match inflow, which reached a maximum of around 875 cfs on Sunday. Much of that inflow was from direct precipitation on the reservoir surface, but natural reach gain between Henry’s Lake and Island Park did increase in response to rain from around 450 cfs on Thursday to 822 cfs yesterday. Although this number will change a little with addition of more data over the next day or two, it will be close to the snowmelt-driven peak of 790 cfs back on April 29. Stream inflow to Island Park Reservoir is around 750 cfs this morning, and I expect that to drop throughout the day. Rain this evening and tomorrow will probably not produce much more streamflow response but will add another 300 ac-ft of inflow on top of stream inflow. Once rain ends tomorrow night, inflow will drop fairly rapidly. Outflow from Island Park Reservoir will be adjusted as needed to keep the reservoir full until draft is needed to meet downstream demand.

The biggest, and most critical question is: when will that happen? Based on April-1 conditions, I predicted that need for reservoir draft would begin on June 25. After the driest April in the last 33 years and a very dry start to May, it looked more like draft would be needed as soon as the Teton River reached its snowmelt-driven peak for the spring shortly after June 1. At this point, I expect natural flow to be sufficient to meet irrigation demand until at least June 10, if not even June 15. This won’t be as favorable as June 25 (or last year’s July 4), but it’s much better than June 1.

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

Henry’s Fork, May 22nd, 2021

Cold, rainy weather kept the lower river giant stone fly activity down yesterday.  Nevertheless boats and rafts were numerous everywhere above Chester on the lower river. Yesterday the upper river experienced wind and snow making fishing difficult and travel downright dangerous. In fact, US Highway 20 was closed past Island Park, so getting into Montana by that highway and State Highway 87 was not possible. The same could happen to fishing and travel tomorrow according to weather predictions. As we warm up after tomorrow, look for the stone fly activity to move up the river, likely into Box Canyon where crowding is sure to happen.  Even with the stone fly activity it is a good idea to have caddis life cycle patterns in that fly box because at times fish key on them during stone fly activity. Also it might be prudent to check highway conditions before venturing to the upper river because of bad weather predicted for tomorrow.

Henry’s Fork, May 18th, 2021

The giant stone flies are out big time from Ashton Dam to Chester. Fish have been on them for a few days, so expect the best activity to be moving up the river. Also expect plenty of company from boats to walk-in wade anglers throughout the lower river especially on weekends. Right now the best place to avoid crowds on the lower river would be to hike down Bear Gulch and fish upstream.  Hard sided boats are not allowed for accessing the river from the trail below Lower Mesa Falls, but rafts can be eased down the trail and launched.  Thus an occasional raft will go by to take out at Stone Bridge.  Few anglers take time to walk down the mile or so to access the river at Bear Gulch.  With relatively low water currently in the river, use caution if you are taking a raft down the trail below Lower Mesa Falls. Be aware that small but dangerous Surprise Falls can easily flip a raft and that rocky areas just below the surface are abundant and there is no easy access out of the river down to Bear Gulch.

A good strategy for fishing the lower river is to wait several days after the peak of the hatch passes a given lower river location.  This allows fish time to digest captured big stone flies, then return to that location where fish will be looking for a repeat. Through doing such most anglers will be upstream in places such a Box Canyon and the Coffee Pot area.

Henry’s Fork, May 4th, 2021

Courtesy Dave Pace

This weekend a lone adult giant stone fly was spotted by a team of drift boat fly-fishers along the river between Stone Bridge and Ashton. Certainly an “early bird”, it best suggests that it is time to offer giant stone fly nymph patterns when fishing the river from Warm River to Chester. For now the other patterns we have suggested in recent fishing reports ( BWO, caddis, and midge life cycle patterns, streamers, and small nymphs of choice) will work fine until later this month when the bulk of the big stone fly activity begins.

Henry’s Fork, April 17th, 2021

Fun Farm Bridge

Currently the best stream fishing in the area is on the  lower Henry’s Fork. From the Ora Bridge down to Chester, BWO, midge and a scattering of March Browns bring top water action. Drifting big stonefly nymphs through riffles will is become more effective as we progress toward mid-May. Streamers under low light conditions continue to produce for browns and post-spawning rainbows.  These comments also apply to the river from Chester Dam to below St. Anthony. Drift boat fishers cause “traffic jams” at all launch sites from the Ora Bridge to Chester Dam, but there is room for walk-in wade fishing. From Warm River to Ashton Reservoir there is less crowding because distance spreads anglers around. The same comments apply with respect to fishing strategy but with drift boat fishing obviously the best way to cover water.

Henry’s Fork, April 13th, 2021

Expect to have plenty of company if you visit the river from the Ora Bridge launch down to the Chester launch. BWO and midge life cycle and stonefly nymph patterns along with streamers under low light conditions are best bets for action. With more river to visit from Stone Bridge to Ashton, visiting anglers  are more dispersed than in the river below Ashton Dam.

Here is Rob VanKirk’s latest summary of snow pack and water conditions in the Henry’s Fork drainage.  Notice that snow pack conditions appear to be a bit below normal at the time of his report.

Headlines:

• Modest snowmelt continued yesterday at all elevations, despite cool temperatures.
• Natural streamflow continued its recession from last week’s small peak and was 84% of average yesterday.
• At a mean outflow of 418 cfs, Island Park Reservoir gained 22 ac-ft yesterday. Current content is 120,795 ac-ft (89% full), compared with an average of 115,907 ac-ft (86% full).

Details:

Yesterday was dry yet again, but at least temperatures were cooler. Mean temperature was 9 degrees F below average and the coolest since March 31. Water-year precipitation to date stayed at 81% of average. Despite cool temperatures, all but one SnoTel station lost snow water equivalent (SWE) yesterday. Island Park lost the most, at 0.5 inch of melt, and the watershed total was 0.2 inch. Melt rate has averaged 0.13 inch/day over the first 12 days of April so far. SWE is 78% of average and lower than it was on this date in 2016, the last very dry year experienced in the watershed.

Light precipitation—in the form of snow at all elevations—is expected tomorrow and Thursday. Forecast amounts range from around 0.1 inch in the northeastern corner of the watershed to 0.5 inch in the southern part of the Teton Range. Valley areas and the Teton subwatershed are expected to receive the highest amounts. Cool temperatures will stick around through the end of the week. Warm, dry conditions are expected after that.

Natural streamflow continued to drop yesterday, despite ongoing snowmelt. Natural flow was 84% of average yesterday: 81% in upper Henry’s Fork, 83% in Fall River, and 91% in Teton River. Diversion continues to increase slowly, although data are not yet being reported for all canals. Moisture availability in the agricultural areas—defined by my crude difference between precipitation and evapotranspiration—continues to drop and is now almost 7 inches below average. I have already had to extend the vertical axis on the chart twice since last summer and may need to do so again in a week or so.

At an average outflow of 418 cfs, Island Park Reservoir gained 22 ac-ft yesterday. Current content is 120,795 ac-ft (89% full), compared with an average of 115,907 ac-ft (86% full). Although ice is starting to melt at the edges at some locations around the reservoir, Fall River Electric personnel report solid ice cover around the dam.

Graphics:

Watershed SWE

Natural Stream Flow

Henry’s Fork, March 23rd, 2021

The Henry’s Fork below St. Anthony is a productive fishery with an increasing brown trout population. Recently IDF&G fisheries personnel have completed a survey of its salmonid population. The results of this survey are given below.

Trout Populations are Strong in the Henry’s Fork near St. Anthony

By John Heckel

Regional Fisheries Biologist

In early October 2020, we conducted our 6^th population estimate on the St. Anthony reach of the Henry’s Fork of the Snake River. This estimate is conducted in the fall every three years and extends from the Railroad Bridge downstream to the Red Road Bridge. Based on our sampling data since 2010, we can see an increasing trend in Brown Trout abundance in this reach.

We can see in the figure above that the number of Brown Trout per mile has been increasing since we started surveying the reach in 2004 while Rainbow Trout abundance has remained low and relatively constant. Currently, Brown Trout make up 96% of the trout composition in this reach where we caught 1,468 trout in total. As can be seen in the images, there are some very large Brown Trout with fish up to 25” in this reach!

The trout population in this portion of the Henry’s Fork is managed as a wild trout fishery, so these large trout captured in this reach are in fact wild fish. Therefore, referring to the length distribution of fish captured here, this section of river grows some very large trout.

Although water temperature in this section of the Henry’s Fork can become warmer than 70°F during portions of the summer, trout can find thermal refugia from groundwater and spring inputs that help coldwater fish remain cool. In addition to Brown and Rainbow trout, we also estimated Mountain Whitefish abundance in the reach. There were 685 whitefish per mile. Based on our length frequency data from Mountain Whitefish we observed several age classes present, which is evident by the multiple peaks in different length groups.

Survey results indicate sport fish populations in the St. Anthony reach are healthy and in good numbers. Managing this section of the river as a wild trout fishery with a 2 trout limit appears to be conducive for growing large fish and allowing population numbers to increase.

Henry’s Fork, March 13th, 2021

Midge and BWO activities are bringing fly fishers to locations from Ora Bridge downstream to the Fun Farm Bridge area.  Use life cycle patterns for each activity.  Presenting streamer and woolly bugger patterns is effective for encountering post spawning rainbows. Some rainbow spawning remains, so avoid  places where such is ongoing. Stream flow in this area is near normal.

Henry’s Fork, February 9th, 2021

Are you interested in doing a trip to fish the upper Henry’s Fork and drainage during the upcoming season? Would seeing how snow and water conditions appear to be stacking up to help in planning? The information Dr. Rob VanKirk compiles may help. Rob’s analyses, and reports on water and snow conditions is as good as it gets.   Below is Rob’s latest update.

Henry’s Fork Water Supply, Feb 09 2021

Headlines:

• After a dry day, water-year precipitation is 84% of average, and SWE is 85% of average.
• Island Park Reservoir gained 112 ac-ft yesterday, typical of fill on recent dry days.
• The reservoir is 87% full, compared with 75% full on average.
• Weather forecasts continue to lean more toward wetter, warmer conditions and away from the extreme cold forecast last week.

Details:

At 6 degrees F below average, yesterday was the coldest in 13 days. Only a trace of precipitation was recorded, leaving the water-year total at 84% of average. Snow water equivalent (SWE) dropped a percentage point to 85% of average. After declining steadily since last July, the three-year average watershed precipitation appears to have bottomed out in the past two weeks and should increase a bit over the next few weeks, if precipitation forecasts prove to be accurate. This index of long-term watershed conditions is just a hair above average right now.

Forecasts are still uncertain about the details of weather over the next week but continue to gain confidence in warmer, wetter conditions than were forecast last week. Very cold air is expected to stay just on the other side of the Continental Divide, leaving us on the warm, wet side. “Warm” is relative, as temperatures are likely to be near average for the next week, a few degrees colder than last week. Regardless of temperature, snow is certain on Thursday and Friday and likely again early next week. The 7-day forecast calls for 0.25 inch across the lower elevations and up to 3 inches at the southern end of the Teton Range. As was the case last week, precipitation will favor the southeastern corner of the watershed. If forecast amounts materialize, SWE in Fall River and Teton River subwatersheds will improve to 90-92% of average by this time next week. SWE in the upper Henry’s Fork will remain at or below 80% of average.

Island Park Reservoir gained 112 ac-ft yesterday, typical of fill rate on dry days. The reservoir is 87% full, compared with 75% full on average. The reservoir will reach the April-1 target of 120,000 ac-ft around March 1.

Graphics:

• Watershed three-year precipitation average
• Island Park Reservoir inflow/outflow: 15-minute data
• Island Park Reservoir volume: 15-minute data
• Island Park Reservoir volume: predicted vs. observed

Station guide and disclaimer

Glossary of Terms

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

[email protected]

Rob’s blog

Henry’s Fork, February 2nd, 2021

Midge hatches are beginning to appear on the lower river, but access can be a problem at many points.

Are you interested in doing a trip to fish the upper Henry’s Fork and drainage during the upcoming season? Would seeing how snow and water conditions appear to be stacking up to help in planning? The information Rob VanKirk compiles may help. Rob’s analyses, and reports on water and snow conditions is as good as it gets.   Below is Rob’s latest update.

Henry’s Fork Water Supply, Feb 02 2021

• After a dry day, water-year total precipitation and SWE are both at 80% of average.
• Island Park Reservoir gained 99 ac-ft yesterday, compared with 65 ac-ft/day needed to reach the target volume of 120,000 ac-ft by April 1.
• The reservoir is 86% full, compared with 74% full on average.

Details:

Yesterday was warm and dry. Although the daily mean temperature was similar to that on Saturday, the afternoon high was the warmest since December 9. Water-year precipitation and snow water equivalent (SWE) are both at 80% of average. Precipitation is expected again tonight through Friday. The seven-day quantitative forecast calls for around 0.25 inch in the valleys, 0.5-0.75 inch in Island Park, and up to 2 inches in the Teton Range. Precipitation over the next week is expected to favor the southeastern corner of the watershed due to northwesterly flow.

The new monthly outlook issued yesterday gives better-than-even odds for above-average precipitation during the month of February, but one- and three-month outlooks have not performed well so far this winter. As mentioned yesterday, precipitation has been below average in three of the four months in this water year so far, long-range outlooks to the contrary.

Natural flow in the upper Henry’s Fork and Teton River continue to bounce around in the range of 90-95% of average. Contrary to my expectations, warm weather last week has not resulted in new streamflow data from ice-affected stations in the lower watershed.

Island Park Reservoir gained 99 ac-ft yesterday, compared with 65 ac-ft/day needed to reach the target volume of 120,000 ac-ft by April 1. The reservoir is 86% full, compared with 74% full on average. Reservoir volume is 116,143 ac-ft, 3,481 ac-ft ahead of my November-19 projection. If the seven-day precipitation forecast proves to be correct—and these short-range forecasts have performed well this winter—the reservoir will gain 310 ac-ft from direct precipitation and a total of around 1,000 ac-ft over the next week.

Graphics:

• Watershed SWE
• Island Park Reservoir inflow/outflow: 15-minute data
• Island Park Reservoir volume: 15-minute data
• Island Park Reservoir volume: predicted vs. observed

Station guide and disclaimer

Glossary of Terms

Rob Van Kirk, Ph.D.

Senior Scientist

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

[email protected]

Rob’s blog