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.