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How Drought Affects the Henry's Fork: A unique case among western rivers

What is the state of the drought in the Henry’s Fork Watershed?

As of early April, water year precipitation sat at 79% of average (21% below average), and snow water equivalent (SWE) was a mere 66% of average (34% below average). Possibly the most concerning was Henrys Lake to Island Park Reservoir inflow, 323 cfs compared to an average 431 cfs, the second lowest base flow on record, behind only 1935. The Henry’s Fork Watershed is in “extreme” drought as categorized by the U.S. Drought Monitor, and nearly two-thirds of the country is now in some form of drought. More information can be found at

What river and fishing conditions can we expect this season?

Island Park Reservoir will fill, likely in late May. Between now and then, we can expect flows in the range of 200 cfs – 500 cfs. Draft, or outflow to meet irrigation need once natural flow isn’t enough, will likely be needed shortly after that. This means that flows will increase from that nearly 500 cfs range to more of a 1,200 -1,500 cfs range in June, and higher than 1,500 cfs during the period of peak demand in July.

After that, so much is still yet to be determined. Spring and summer rains could make a positive difference, and various water-rights technicalities and management decisions yet to be determined will have an impact. For now, we know that there is a 45-50% chance that maximum outflow from Island Park Reservoir will exceed last year’s maximum of 1,500 cfs, and the upper Henry’s Fork (above Ashton) is currently headed for the 2nd driest year on record behind 1934.

As of this writing, we anticipate June and July to be fairly similar to last year. It’s primarily the end of the summer that we expect to see the greatest differences from “average” conditions. Also, late May is not anticipated to have the beneficial, high, spring freshet as outflows will need to be low to finish filling the reservoir after ice-off. Outflow should be just about equal to inflow or the natural baseflow between Big Springs and Island Park Reservoir.

How will water temperatures play into all of this?

Here’s a place where the Henry’s Fork is unique. On the Henry’s Fork, water temperatures are largely independent of stream flows. Instead, water temperatures on the Henry’s Fork are primarily affected by air temperatures and solar radiation, because the Henry’s Fork is naturally wide, shallow and clear, with dark volcanic sand and rocks on the stream bottom that absorb heat. Streamflow is a secondary factor until flows are so low that water hardly moves (e.g., 100 cfs through the Ranch in June and July without macrophytes to increase water depth). Irrigation need ensures that will never happen in during the heat of the summer, when flows average closer to 1,000 cfs.

In Island Park Reservoir, research shows that the number one factor impacting water temperatures is reservoir draft, specifically how fast the reservoir drops and causes mixing of warm and cool layers and loss of a cool pool of water at the reservoir bottom. The faster the reservoir drops, the warmer the water will be, both in the reservoir and in the reservoir’s outflow into Box Canyon. In this case, air temperature and sunlight don’t make as much of a difference compared to reservoir draft, until the river reaches Last Chance, where temperature and sunlight take over as the primary factors. By the time the river reaches Pinehaven, the effects of Island Park Reservoir on temperature have disappeared.

Dissolved oxygen also plays an important role for fish. The hydrology, geology and ecology of the Henry’s Fork maintain high dissolved oxygen throughout all sections of the river. In reaches such as the upper Henry’s Fork, Harriman State Park, and Ashton to St. Anthony, photosynthesis by aquatic plants injects substantial amounts of oxygen, regardless of water temperature. For example, on a typical July afternoon, dissolved oxygen at Pinehaven is 12 mg/L (anything over 7 mg/L is considered optimal for trout), even while water temperatures are 70 degrees. In canyon reaches, high velocities, cascades, and waterfalls mix oxygen from the atmosphere into the water. Even in reaches downstream of St. Anthony, a combination of aquatic plant photosynthesis and atmospheric mixing as the river falls over lava shelves and diversion structures maintains high dissolved oxygen. In nearly 8 years of continuous monitoring of dissolved oxygen throughout the watershed, HFF’s sonde network has never detected any dissolved oxygen issues, the only exception being temporary decreases at Island Park Dam due to equipment issues. However, the hydropower company works diligently to get issues resolved quickly. View historic and live dissolved oxygen data for the Henry’s Fork at under “HFF Water Quality Data”.

Yes, we are anticipating a warm summer, and we can expect more and more “hot” summers in future. Extreme drought and an anticipated warm summer are very serious issues that we do not want to diminish. Fully understanding our system and its unique characteristics, including those that work in our favor, will put us in the best possible position to face these challenges and ensure a healthy future for the fisheries we love.

What is HFF doing?

Our primary focus this year is avoiding a sediment event out of Island Park Reservoir similar to the detrimental 1992 event. Note that some sediment from the ’92 event still sits in the Ranch, being cleared out during spring freshets (high flows) before aquatic plants have time to grow and hold it in place or capture new sediment. A secondary focus is maintaining winter flow next year—to benefit survival of this year’s cohort of juvenile trout--but both of these can be addressed by keeping as much water in Island Park Reservoir as possible. That will just be more difficult to achieve this year than ever before.

Historical water data shows that the year 2022 is most similar to is 2001, the last time the Henry’s Fork Watershed experienced drought of this magnitude. In 2001, the minimum Island Park Reservoir reached was 13% full. Including 2001, the five driest years since 2000, were 2002 (reservoir minimum = 12% full), 2003 (9% full), 2007 (12% full), and 2016 (15% full). The average of these five is 12% full. HFF’s goal is to see the reservoir remain higher than this. If we are successful, we will have beat the odds for reservoir carryover relative to water availability for five consecutive years.

The challenges we face include additional demands on the upper Snake River system that did not exist in 2001, let alone in 1984 when HFF was founded. These include legal requirements to send 200,000 ac-ft of upper Snake River storage to the lower Snake River for the benefit of out-migrating juvenile salmon and for groundwater users to provide 162,600 ac-ft of storage to surface-water users as mitigation for groundwater pumping. Although low water supply this year will limit the amount that could actually be used for either of these needs, it is possible that around 216,000 ac-ft of water could come out of the Upper Snake Reservoir system that was not needed in 2001. With Island Park Reservoir accounting for 3% of the total reservoir system capacity, its share of that 216,000 ac-ft is 6,000 ac-ft. This additional demand is like starting out in a 6,000 ac-ft hole. Or it is the difference between 14,000 ac-ft remaining in the reservoir at the end of the irrigation season or 20,000 ac-ft, which a big difference for our watershed.

Infrastructure and technology installed, experience gained, and the past four years of work have set us up to make the most of the water available. We are headed into this year with strong fish populations and healthy river conditions, thanks to maximizing winter flows and taking advantage of strategic spring freshets from 2018 to 2020. HFF’s programs will continue to find ways to conserve water, keep water in Island Park Reservoir and make a positive difference for our fisheries through data and information sharing. These include programs like precision management, Farms and Fish, partnerships and collaboration, and the South Fork Initiative, among others.

We will also continue to share information about the dynamics of Henry’s Fork river hydrology, and what to expect as the summer goes on, but please share HFF newsletters, social media, and blogs with other anglers, friends and family and help us spread the word about why this beautiful place is worth conserving. Thank you to our staff, partners, and especially you, our members for being part of such a passionate, generous, and capable community.

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