Fishing Reports

Henry’s Fork, September 14th, 2021

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.

Headlines:  

• Dry conditions continue, with only light precipitation forecast over the week.
• Diversion decreased yesterday on the lower Henry’s Fork, increasing stream flow there by around 20 cfs.
• At an average outflow of around 500 cfs, Island Park Reservoir dropped 205 ac-ft yesterday and is 41% full, compared with 46% full on average and 15% full in 2016.

Details:

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 4^th-7^th 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

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX

Henry’s Fork, September 11th, 2021

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.

Henry’s Fork, September 7th, 2021

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.

Headlines:  

• Dry weather over the past two weeks has dropped water-year precipitation to 81% of average, down from 82% after the mid-August rains.
• Diversion has decreased by 140 cfs over the past four days, while natural flow has stayed constant, increasing streamflow in the lower Henry’s Fork to around 200 cfs above its target flow.
• At an average outflow of 490 cfs, Island Park Reservoir has dropped 200 ac-ft/day over the past three days and is 42% full, compared with 47% full on average and 15% full in 2016.

Details:

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

Henry’s Fork Foundation

P.O. Box 550

Ashton, ID 83420

208-652-3567 OFFICE

208-881-3407 CELL

208-652-3568 FAX