Yes, anglers reported sightings of dead fish at the Last Chance boat ramp August 1-2. HFF's Science and Technology Program Manager, Dr. Van Kirk, immediately and his team went out to the river to investigate. All of the fish that we discovered were at Last Chance and all of the fish were juvenile Whitefish (3-5 inches in length). The reports we received were consistent with these findings.
What was the cause of the dead fish?
We believe that the root cause stems from the Buffalo River at the hydroelectric power dam. There has been so much aquatic vegetation growth this summer that the power plant has been unable to operate. Fall River Rural Electric has been periodically drawing down the forebay for an hour or two and raking this vegetation in order to remove it from the power plant intake. The last time this occurred was the Wednesday morning of July 31. We know from our operation of the Buffalo River fish ladder that the Buffalo River supports large numbers of juvenile Whitefish, and it is likely that Wednesday's operation displaced the juvenile fish from the forebay, especially if they were using beds of aquatic vegetation as cover.
Are the reports of dead fish connected to turbidity (water clarity)?
The short answer is that turbidity, while not aesthetically desirable, impacts our fishing day more than it impacts fish and other aquatic life. Turbidity alone is not the cause of the dead fish.
On the Henry's Fork, water clarity is affected by the presence of two things:
Inorganic matter (nonliving material, such as mineral sediment)
Organic matter (material that comes from a living organism, such as blue-green algae or decaying plants).
HFF monitors turbidity by manually taking water samples and analyzing them in our lab, and through our water quality monitoring network of sondes (instruments). Recently, our equipment registered the highest turbidity in Island Park Dam outflow since the September 2020 turbidity event. However, this event is different from other turbidity events because it was caused from high algae growth in the reservoir which is due to the hot and sunny weather conditions throughout July (providing the perfect environment for the algal bloom to occur).
Photo showing turbidity event - dated 08/05/2024
Looking ahead, we still do not anticipate any destabilizing events that could transport bottom mineral sediment to the dam, but it is likely that the current high algae production in the reservoir will continue for several more weeks.
What about water temperatures?
The warm water temperatures we have experienced this summer are certainly stressful for fish. Despite this, if trout continue to have enough places of refuge, dissolved oxygen, and food available, they can still thrive in warmer water temperatures. The Henry's Fork is special in that it has abundant food and high levels of dissolved oxygen, which offsets the stress that trout might experience at these higher temperatures.
Dissolved Oxygen is crucial!
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Dissolved oxygen is the amount of oxygen available in the water and available to aquatic life, and it plays a critical role for fish survival. The Henry's Fork is able to maintain high dissolved oxygen levels, regardless of water temperatures, throughout all sections of the river as a result of the photosynthesis of rooted aquatic plants.
HFF’s water quality network is not showing stressfully low levels of dissolved oxygen in the river downstream. This tells us that the dead fish sightings are a result of critically low dissolved oxygen in Island Park Reservoir due to the algal bloom consuming massive amounts of oxygen from the water. Small fish can easily fit through the screen at the dam and wash into the river downstream.
Rooted aquatic plants in IP Reservoir - Photo dated 08/05/2024
What is HFF doing to prevent this from happening in the future?
Water Quality Monitoring on the Reservoir: HFF's field team will continue to go out on the reservoir weekly to take water samples and monitor what is happening. This data is being used to determine how to predict these conditions in the future.
DIRTT: HFF's plan to Develop Infrastructure to Reduce Temperature and Turbidity is working to create infrastructure at the dam that will minimize the outflow of unclear water into the river, which would improve water clarity on the Henry's Fork long-term.
Precision Water Management: HFF's Precisions Water Management program helps to reduce outflow, sending water through the power plant rather than the gates. This is a good thing, because water that goes through the gates tends to be less clear than water that goes through the power plant. This minimizes the amount of suspended material that is exported outside of the reservoir.
What can you do?
Thank you to the anglers who reported this issue to us. If you are out on the river and discover dead fish, please report this to us! Please send us the location, time, any close-up photos, and an ID of the species. Thank you for your help in protecting this incredible fishery.
Also, please remember that on the Henry's Fork, peak dissolved oxygen occurs when the sun is at its peak (aquatic plant growth) around 3 or 3:30 pm. Water temperatures will still continue to rise until 7 or 8pm. To limit any negative impact on fish, anglers could considers limiting their fishing until after 4pm, as that is the most stressful time for fish on the Henry's Fork.
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