This is the time of year when many of us start to shake off the winter blues to bask under blue spring skies. The birds are starting to return, and fieldwork is filled with birdsong. Rob, Amber, and I joyfully listened to a sandhill crane while doing our first streamflow measurement of the year at Parker this week. Here at the Henry's Fork Foundation, we have been planning for spring since the doldrums of winter. Over the last three months, Rob and I have updated historic temperature data and dug deeper into Henry's Fork hydrology—asking questions about what determines water supply and runoff timing for river reaches from Henry's Lake to the Teton River. We ask these questions and re-run our analysis ever year. We include new data from the latest water year because more data means more information to learn from.
In the weekend leading up to Monday, April 1st, Rob put in extra hours to update his computer code in preparation for his much-anticipated April 1st Prediction—a fan favorite among Henry's Fork anglers and an important piece in HFF's science-based collaboration with water managers since 2017. Rob ran his model on April 1st and, after five hours of his laptop chugging its way through 5,000 simulations of possible Henry's Fork summers, HFF's data-driven water prediction for 2024 emerged.
So, what did we learn? What do we anticipate for the spring and summer ahead? Note: all information is based on data available on April 1st.
Just here for the one-page highlights? Access them here. Or would you rather read this information in PDF form? The information is available here:
Natural Streamflow Statistics (click here)
Reservoir and Irrigation System Management (click here)
Water Quality (click here)
Quick Basics
The Henry’s Fork is a snow-dominated, groundwater-spring fed river system. This means that our mountain snowpack acts like a winter-season reservoir: temporarily holding water in a frozen/semi-frozen state and then releasing it as air temperatures warm and melt snow. Some of this water evaporates, is captured by plants, or seeps down into groundwater to re-emerge in springs sometime in the future. Some of this water makes it way to rivers where it becomes “natural streamflow” and might be captured in reservoirs to ensure they are full in time for agricultural irrigation season. Whatever isn’t needed for reservoir storage is passed downstream.
Slides from Sarah Newcomb's presentation to the Henry's Fork Watershed Council in December 2023.
Winter 2024: The set up for spring and summer
April–June water supply is largely predicted by three things:
April-1 snow water equivalent (SWE)
October–March streamflow ("baseflow")
One-year average soil moisture*
For the watershed as a whole, April-1 SWE was 96% of average (4% below avg) and baseflow was 95% of average (5% below average). Soil moisture is 7 inches above average (compared with 2.5 inches above average at this time in 2023). *Okay, I'm using soil-moisture here as shorthand. Rob defines it as the one-year average difference between precipitation and evapotranspiration, which he calls “moisture availability” because it is a surrogate for overall soil moisture and shallow aquifer conditions that can be easily calculated from available data.
Streamflow and Water Supply: Average snow, average flow
Spring and summer 2024 are not expected to stand out, but instead be solidly average. In fact, April–June natural streamflow is slated to be 100% of average (aka average) for the watershed.
By subwatershed, we’re looking at 97% of average in the upper Henry’s Fork, 104% in Fall River, and 97% below average in the Teton River.
Compared to 2023, streamflow is predicted to be lower at Henry’s Lake and in the Teton River because these reaches depend largely on snowmelt (instead of groundwater) and snowpack is lower this year.
If you’re looking for a reference year, look back to years like 1980, 1989, 2012, 2018, and 2019 for a similar spring and summer experience relative to natural flow.
Let’s chat worse-case scenarios. It is possible, though unlikely, that we could see streamflow 14% below average. More specifically, 15% below average in the upper Henry’s Fork, 13% below average in the Fall River, and 23% below average in the Teton River. These numbers are far better than what we experienced in 2021 and 2022, when streamflow was ~30% below average across the watershed.
Runoff Timing: Another average prediction
Runoff timing for the upcoming spring are very close to long-term averages: June 21 in the upper Henry’s Fork, June 16 in the Fall River, and June 17 in the Teton River. These are ~2 days earlier than last year at the watershed scale. This year’s runoff timing is predicted to be similar to that of 2005, 2018, and 2019.
Runoff timing depends primarily on the amount of snow to be melted. It takes longer to melt a higher snowpack, so runoff happens later in the summer. But other factors like late-winter temperature, winter evapotranspiration (loss of water from the soil and plants), and springtime temperatures are important too. A warmer spring can melt snow faster and cause an earlier runoff. Since 1989, mid-April to mid-July temperatures have increased by 0.8°F per decade in the upper Henry’s Fork watershed. We aren’t seeing the same trend in the Fall River or Teton River subwatersheds.
Irrigation Demand: How many times can we say average?
Last year we had heavy rain late in the summer that diminished irrigation demand. Barring a similar rain outcome this year, we expect diversion to be similar to years 2014, 2017, and 2020. We expect total irrigation demand to be near the 2001–2023 average. Delivery through the Crosscut Canal is expected to peak at a little over 300 cfs in late July, a little higher than average but a little lower than last year, and remain above 100 cfs until mid-September. Expected exchange well pumping this year is a total of 1,550 acre-ft during the month of July.
Ice-Off at Island Park Reservoir: Average timing
With March temperatures coming in right at average, expected date of ice-off from Island Park Reservoir is April 28 (right at average). BUT ice-off dates could occur about two weeks on either side of that, depending on weather from here on out. Given another round of cold temperatures and snow coming up this weekend (April 6–7), it is unlikely that ice-off will occur at the earlier end of the range.
Spring Reservoir Fill and Outflow Status
We expect the reservoir to fill by June 9. Current outflow from the dam is 440 cfs. This will likely increase to 1,200 cfs in mid-May. Once the reservoir is full, average outflow will likely decrease to ~900 cfs by mid-June and 770 on July 1.
No managed freshet is planned this year, BUT high spring and early summer flows are still possible. Our number one priority is to keep Island Park as full as possible. This benefits everything from water quality to winter flow. With a full or near-full reservoir, natural inflow will be passed through and could reach as high as 1,750 cfs in mid-May (about where the 2023 “natural” freshet was). Recall that last year’s managed freshet was around 2,000 cfs for a few days at the end of April. What's a freshet? Learn more here: https://www.henrysfork.org/post/faq-what-is-a-spring-freshet
Summer Outflow for Irrigation Delivery
In most years, the need for draft (drawdown) of Island Park Reservoir is driven by the need to deliver water to the Teton River through the Crosscut Canal. We expect to draft Island Park Reservoir to meet irrigation demand July 5, compared with July 2 last year and June 29 on average. However, draft could start sooner depending on whether we have a hot + dry or cool + wet spring/summer.
Outflow from Island Park Reservoir is expected to be ~1,250 cfs in mid-July (very close to the long-term average). However, in the most extreme case, outflow could reach 1,750 cfs for a week or two in July. That’s a little higher than last year. Outflow is expected to decrease gradually from late July through the end of September—eventually reaching about 500 cfs by then. As we saw in August 2023, wet periods during the summer can result in little to no reservoir draft producing outflows that are much lower than average in the regulated system. It’s possible that could happen this year too.
Island Park Reservoir Carryover
We expect Island Park Reservoir content on September 30 (“carryover”) to be 98,618 acre-ft (73% full and very close to what happened in 2019). For comparison, mean carryover is roughly 60,000 acre-ft (44% full). The middle 50% of simulations predicted a range of carryover from 62% full to 90% full. Carryover is important for determining winter flow management. High carryover is good news for trout!
Water Quality Below Island Park Reservoir (June 15–September 15)
Broadly, there is a large range of possibilities. But overall, it is unlikely to be as bad as the worst years on record–2015 and 2016–and unlikely to be as good as it was during years of good water supply in the 1990s.
Turbidity and suspended sediment load are related to dam outflow. Turbidity is a measure of how clear the water appears. Cloudier water can be caused by organic (ex. algae) or inorganic (ex. sediment) substances. This year, average summertime turbidity is predicted to be 3.8 NTU (near average). For context: We tend to hear concerns from anglers once turbidity is above 5 NTU. However, with only 10 years of data, our range of possibilities is rather wide. The good news is that we can predict turbidity events several days to a week in advance based on weather forecasts and have successfully done so for the past several years. We’ll continue to do so this summer and fall. We predict summertime suspended sediment load will be 1,132 tons (near the 10-year average), compared with 966 tons last year. Again, average outflow is expected to produce average sediment load. Learn more about turbidity and sediment here:
Summertime water temperature of dam outflow are related to reservoir drawdown and summertime reservoir inflow. This summer, we predict average outflow temperature to be 59.7°F (lower than the 10-year average of 61.3°F). Although our simulations had a wide range of results, it is unlikely that outflow temperatures will be as warm as they were in 2021 and 2022.
Conclusion
It is getting drier in the Henry's Fork. Considering the last 23 years have been much drier than the 23 previous years (see below), an "average" spring/summer 2024 Henry's Fork water supply just might be something to celebrate! To keep up to date with all things river and reservoir, consider signing up for the daily water report (technical email) or weekly river update (summary highlights). Sign up here: https://www.henrysfork.org/science-and-technology (scroll down). You can also read the daily water report without an email subscription at https://henrysforkdata.shinyapps.io/DailyWaterReport/. Track real-time water quality at https://henrysforkdata.shinyapps.io/scientific_website/ and real-time streamflow at https://henrysforkdata.shinyapps.io/WaterQuantity/.
