Percent mayflies, stoneflies, and caddisflies in invertebrate samples at Osborne Bridge averaged 57% in 2019, compared with 41% during and after the recent drought and 17% in 1993.
A high springtime freshet and low irrigation-season outflow from Island Park Reservoir, as occurred in 2018-2020, accelerates transport of sediment out of the Harriman reach, providing an explanation for improvement in hatches since 2017.
In the long run, net sediment transport out of the river between Island Park Dam and Pinehaven is around 1,600 tons per year. At that rate, 45-90% of the sediment deposited during the 1992 event has been removed to date, providing an explanation for substantial improvement since 1993.
HFF will continue to work with the Henry’s Fork Drought Management Planning Committee to implement the high springtime/low summertime flow regime that appears to be beneficial for aquatic insects.
Weather during the April-June spring period is a major factor determining summer water supply.
Although April-June temperature was average and 1.6 degrees below expectation, high variability resulted in three very warm periods that melted this year's average snowpack 10 days earlier than average.
The second half of June was very cold and wet, compensating for the early snowmelt and delaying need for Island Park Reservoir draft by one week relative to average.
As of July 14, 10 days into the draft period, Island Park Reservoir is still 91% full, compared with 83% full on average, setting up a fourth consecutive year of above-average carryover and high winter flows.
A cold, wet March brought snow water equivalent up to 97% of average, after it spent most of the winter down around 90% of average.
Watershed-wide natural streamflow for April through September is forecast to be 97% of average, compared with 99% of average in 2019.
Delivery of water from Island Park Reservoir is predicted to begin around July 1, 12 days earlier than in 2019, and stay in the range of 800-1100 cfs during July and August.
The reservoir is expected to end the irrigation season around 77% full, compared with 73% full in 2019.
Higher water levels in Island Park Reservoir have numerous, scientifically documented benefits to the Henry's Fork fishery, prompting HFF to continue to expand the number of methods we use to limit delivery of water from the reservoir.
In January of 2015, I started a new run of "fish of the month," a tradition I started years ago with long-time friend Tom Grimes, who is a guide at Henry's Fork Anglers. The idea is to catch a wild trout or whitefish every month of the year in our local waters, the streams and lakes of Montana, Wyoming, and Idaho in the Yellowstone region. My previous record was 55 months, from July 2004 to January 2009. Five academic years spent in California broke that streak, but I set a new record at 56 months in August of this year and finished 2019 with four good outings in December.
Climate: Record-breaking February snow, springtime temperature 2 degrees below average, and heavy precipitation in September. Mean temperature for water year 2019 was equal to the 1989-2018 average, but mean April-June temperature was 2 degrees F below average.
Natural streamflow: 98% of average: 97% in upper Henry’s, 103% in Fall River, and 96% in Teton River. Snowmelt in Fall and Teton rivers lasted into July. Upper Henry’s Fork water supply was above the 1930-2019 average for the first time since 2012.
Irrigation Management: Draft of Island Park Reservoir started 9 days later than average and ended 4 days earlier than average. Over the period of draft, streamflow in the Henry’s Fork at St. Anthony averaged 1,070 cfs, compared with this year’s target of 1,000 cfs.
Island Park Reservoir: Ended the water year 73% full, compared with 46% full on average. Outflow during the upcoming winter is expected to be 500-550 cfs for the third consecutive year.
Predictive Model Performance: Based only on April 1 conditions, my predictions for most key hydrologic parameters were within 10% of actual values except those directly affected by spring and summer weather. The model over-predicted natural flow by 8.1% and Island Park Reservoir carryover by 6.8%.
Record precipitation in February moved snowpack from 80% of average on February 1 to 110% of average on April 1.
Watershed-wide natural streamflow for April through September is forecast to be 105% of average, compared with 104% of average in 2018.
Delivery of water from Island Park Reservoir is predicted to begin around July 1 and remain in the range of 800-1200 cfs during July and August.
Because of very good inflow, the reservoir is expected to end the irrigation season around 80% full, compared with 72% full in 2018.
Higher water levels in Island Park Reservoir have numerous, scientifically documented benefits to the Henry's Fork fishery, prompting HFF to continue to expand the number of methods we use to limit delivery of water from the reservoir.
January climate stats: 1 degree F above average temperature, 89% of average precipitation, and 83% of average snow water equivalent (SWE) accumulation.
February 1 water-year totals: 85% of average precipitation and 77% of average SWE.
February has better than even odds of being colder than average, and has started out very wet.
Island Park Reservoir has been steady at 88-89% full for the past two months. Outflow has averaged 520 cfs.
In January of 2015, I started a new run of "fish of the month," a tradition I started years ago with long-time friend Tom Grimes, who is a guide at Henry's Fork Anglers. The idea is to catch a wild trout or whitefish every month of the year in our local waters, the streams and lakes of Montana, Wyoming, and Idaho in the Yellowstone region. My previous record was 55 months, from July 2004 to January 2009. Five academic years spent in California broke that streak, but I'm four years into the current one. How did 2018 turn out?
Diversion from the Henry's Fork has decreased 30% over the past 40 years, yet consumptive use of water by crops has increased, there is less water in the river during the middle of irrigation season, and use of storage water has increased, all other things being equal. How could this be?
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