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Henry's Fork Mid-Summer River Update: 2023

Water supply, fishing conditions, and river health as of July 2023.

July 2023:

  • Was warm and dry, with natural streamflow 17% below average (83% of avg) despite above-average snowpack this year.

  • Outflow from Island Park Dam was 6% above average and the highest since 2015.

  • High outflow is due to: 1) good basin-wide water rights priorities, 2) low natural flow in the Henry’s Fork, and 3) high crop water needs.

  • Current reservoir volume is 12,000 ac-ft higher than in past years with similar water supply and 68.1% full, right at average for the date.

  • Mean daily water temperatures were near average, and turbidity was well below average between Island Park Dam and Pinehaven.

  • Water quality was good because of late reservoir ice-off, late start to draft, and good snowmelt input during May and June.

Natural Water Supply

Unfortunately, despite this year's solid snowpack, we are still in an extended period of below-average water supply. The Henry's Fork's solidly above-average snowpack [link] won't be able to produce even an average water supply this year because baseflow (the amount of water supplied by soils and aquifers) was at record lows last fall and winter after 30 months of severe drought. One solid snow year is not quite enough to overcome the long-term effects of a drought of that duration and magnitude. The U.S. Drought Monitor currently shows most of the Henry’s Fork watershed as abnormally dry or in moderate drought.

Diversion (irrigation water for crops)

Irrigation diversion (water diverted from the river for crops) was 6.7% above average in the watershed for the month of July, relative to the 2001-2023 period of modern irrigation methods, ranking 6th highest in that record.


  1. Water supply over the entire Snake River basin (much of eastern Idaho) was just good enough this year to fill all storage rights and keep natural-flow water-rights in priority. Those storage rights did not fill in either 2021 or 2022, and this year irrigators have a full allocation of water available to use, after two consecutive years without.

  2. Warm temperatures in May followed by cool, wet weather in June provided good conditions for early-stage crop development, but delayed mid-stage development. These timing issues led to a situation in which high acreages of crops all across the watershed required maximum irrigation at the same time over the past three weeks. Usually, peak crop needs are staggered a little across the different elevations of the watershed.

The combination of good basin-wide water-rights, high acreage of high-value and high-quality crops, but relatively low water supply here in the Henry’s Fork watershed and especially on the Teton River, have necessitated high outflow from Island Park Reservoir to meet irrigation demand and lower-watershed streamflow targets. The last two years, the situation has been reversed. Irrigators in the Henry’s Fork had low storage allocation because their storage rights are junior in the system. As a result, irrigation demand was low, even relative to very low supply in the watershed, and outflow from Island Park was relatively modest.

Outflow from Island Park Reservoir

Outflow from Island Park Reservoir was around 640 cfs on July 1, matching reservoir inflow to keep the reservoir full until reservoir draft was needed to meet irrigation demand. Outflow was increased 6 times between July 1 and July 20 to a maximum daily outflow of around 1,620 cfs (plus or minus 100 cfs or so due to stream gage shift) on July 21. Both peak and mean outflow from Island Park Reservoir was the highest this July than we have seen in any irrigation year since 2015. However, outflow in excess of inflow was not needed until July 2, compared with June 23 on average.

Also, current reservoir volume is 12,000 ac-ft higher than the August 1 average of the four other years in the 1978-2022 record of most comparable water supply, namely 1979, 1981, 2001, and 2020. We are already well on our way to beating expectations for a year with 85-90% of average water supply.

Water Temperatures

Both mean and maximum daily water temperatures in the lower watershed (downstream of Ashton Dam) were below average for most of the month of July, peaking for the season July 21-23.

Mean July water temperature at Island Park Dam was 62.5 degrees F, right at the 10-year average, and ranking 4th warmest over those 10 years. The coolest monthly mean in our record was 60.1 degrees in 2019, and the warmest was 66.4 degrees in 2021. Based on Jack McLaren’s master’s thesis research, water temperature at Island Park Dam is first and foremost determined by reservoir volume and second by air temperature. Given only slightly above average air temperatures during July, these new daily record high water temperatures were set because of very rapid draft of the reservoir, which quickly eliminates any cold water on the bottom of the reservoir and replaces it with warmer water. Temperatures would have been even higher had reservoir ice-off not occurred as late as it did (May 11, compared with an average of April 28 over the preceding 9 years) and had June not been as cool and wet as it was. Furthermore, as indicated above, reservoir volume is still higher than expected for a year with comparable water supply, due to precise water management, preventing water temperatures from getting any higher than they already are.

At Pinehaven, mean July water temperature was 66.0 degrees F, slightly higher than the 10-year average of 65.1 degrees, and ranking right in the middle of the 10 years. Water temperature at Pinehaven is determined primarily by air temperature, and secondarily by solar radiation. Streamflow is not a factor in water temperature there. Water temperatures at Pinehaven have most likely peaked for the year, given shorter day length, lower solar radiation, and a forecast for much cooler temperatures for at least the next week.

We have been recording water temperatures at numerous locations in the river between Last Chance and Pinehaven (the “Ranch” reach). Results to date are available on a devoted webapp. Data are still being collected, but so far our Pinehaven sonde location is within a degree or so of the warmest locations on the Ranch. Most other locations are cooler than Pinehaven, some 5 degrees or more cooler. There are no fish migration barriers in the Ranch, and previous HFF-sponsored research has shown that rainbow trout in the Ranch migrate freely throughout the river system from the dam to the Buffalo River to Riverside. So, fish can choose to be anywhere in the river they want to optimize cover, food intake, growth, and metabolism. In many cases, fish may choose to stay in water that is warmer than optimal a few hours during the day to take advantage of high food availability and growth potential the rest of the time. Currently the warmest temperatures occur between 2:00 p.m. and 8:00 p.m., but that window will shrink as the days get shorter. We will fully analyze data from this study this winter, especially in the context of our aquatic invertebrate data.

Hatch Timing

Hatch timing, which is determined by accumulated temperature units since the beginning of the aquatic ecosystem growing season (~April 1 around here), is still behind the 10-year average. Recent warm weather has still not been enough to offset very cold water temperatures in the spring and early summer. Hatch timing has been pretty flat at 1 day later than average at Flat Rock, 4-5 days later than average between Island Park and Pinehaven and downstream of Ashton Dam, and 8 days later than average at Marysville.


Turbidity - a measure of “how dirty” the water is due to suspended material. Mean turbidity for the month of July at Island Park Dam was 1.94 Nephelometric Turbidity Units (NTU), compared with a 10-year average of 3.35 NTU and ranking as the 3rd lowest in the 10-year record. The lowest mean July turbidity was 1.81 in 2018, and the highest was 5.16 in 2020. By far, the lowest July turbidities we have observed at the dam occurred in years of good snowpack—2017, 2018, 2019, and 2023. This year, late reservoir ice-off and a late start to reservoir draft have helped keep turbidity low.

Turbidity has increased over the past week—coincident with water temperatures—as the reservoir has continued to drop quickly. However, turbidity on both sides of the dam remains well below average for this time of year, and I do not anticipate excessively high turbidity until or unless a large weather change rapidy cools the West End of the reservoir, creating conditions for mixing and transport of suspended material to the dam. There is a small chance that could happen later this week if heavy rain and hail fall on the West End. A higher chance of one of these events will occur around Labor Day with the arrival of the first autumn-like cold front.

Turbidity has also been low at Pinehaven, averaging 2.82 NTU for the month, compared with the 10-year average of 3.80 NTU. Again, this ranks 3rd lowest, behind 2017 and 2018. By far, the highest turbidity of the past 10 years at Pinehaven occurred last year, at 6.83 NTU.

Full Technical Report

Want more information? Read Science Program Director, Dr. Rob Van Kirk's full report.

Download PDF • 5.42MB

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