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This is the third blog in a series about research activity in the Upper Henry’s Fork. Click here for parts 1 and 2, and stay tuned for the next part!

  • The lynchpin: Quality fish populations and fishing experience in the Henry’s Fork River upstream and downstream of Island Park Reservoir is dependent on water quality in Island Park Reservoir

  • Hammer and anvil: drawdown in Island Park Reservoir increases water temperatures at the surface and decreases dissolved oxygen near the bottom, putting the squeeze on available habitat for trout and salmon in Island Park Reservoir

  • Springs provide refuge: spring-fed inflows provide areas of refuge for trout and salmon in Island Park Reservoir, explaining why at least some trout and salmon survive to adulthood even when drawdown reduces reservoir volume by 90%

  • More to learn: Can we predict water quality, instead of just monitoring it? Could drawdown affect toxic cyanobacteria blooms that endanger humans, pets, and livestock? What management tools are available to conserve the fisheries, recreational, and aesthetic qualities of Island Park Reservoir? Future research and intern projects aim to address these and many other questions. We appreciate your support!

The Lynchpin

So far, we’ve found that drawdown in Island Park Reservoir is the chief bottleneck limiting kokanee salmon survival in the Island Park Reservoir/Henry’s Fork system, and likely affects other popular sportfish like Rainbow Trout, too. Other work by the Henry’s Fork Foundation has also found that keeping Island Park Reservoir full has benefits for the downstream fishery by reducing sediment movement, reducing water temperature, improving aquatic invertebrate populations, and increasing all-important winter flows, leading to better fish populations in the famed Box Canyon and Harriman reaches of the Henry’s Fork.

In other words, we’ve found that Island Park Reservoir is the lynchpin to water quality and fish populations throughout the Henry’s Fork Basin. But why? In some tailwaters, high water levels in the upstream reservoir are a bad thing for trout. This is completely opposite to what we’re experiencing in Henry’s Fork country! Could what’s going on in Island Park Reservoir’s depths reveal to us some mechanism by which drawdown affects water quality? And could we repurpose this knowledge to achieve conservation goals?

A boat on a stormy lake
The author’s research boat on a stormy day on Island Park Reservoir in late June, 2017. The surface of Island Park Reservoir is beautiful, but what’s going on underneath? Photo credit: Jack McLaren

With those questions in mind, the Henry’s Fork Foundation, the nonprofit Friends of Reservoirs, and the Reservoir Fish Habitat Partnership funded a research project to monitor water quality in Island Park Reservoir during the drought year of 2021.

As Island Park Reservoir was drawn down to only 45% of full capacity, AJ Mabaka, an undergraduate intern from Washington and Lee University, and I collected weekly water temperature and dissolved oxygen data at five habitat units in Island Park Reservoir (see map below). These five habitat units represented five different habitat units in the reservoir: the “inlet” where the Henry’s Fork meets the reservoir, the “East End” consisting of the deep and isolated Crow Creek arm, the “West End” consisting of the shallow and wind-blown western basin, the “Main Reservoir”, consisting of the majority of the volume of the reservoir between the deepest point at the dam and out past Bill’s Island, and the “Springs”, consisting of specific areas found via local knowledge where submerged springs feed into Island Park Reservoir.

At each monitoring location within each habitat unit, AJ and I measured water temperature and dissolved oxygen from the surface to the bottom using a special, waterproof automatic data collection device called a “sonde”, courtesy of the Idaho Department of Environmental Quality.

Island Park Reservoir and its five habitat units, color-coded. White diamonds represent weekly monitoring locations where we used a sonde to record water temperature and dissolved oxygen at all depths.
AJ Mabaka and Oscar-pup out for scientific business on Island Park Reservoir. In AJ’s left hand is a sonde, which measures water temperature and dissolved oxygen. Oscar stands at the bow, bravely protecting the boat from dangerous threats like ducks. Photo credit: Jack McLaren.
Hammer and Anvil

We found drawdown raised water temperature and reduced dissolved oxygen in most habitat units in Island Park Reservoir. The following graphs show “isopleth” graphs for water temperature and dissolved oxygen at all depths through 2021. The graphs are color coded to kokanee salmon water quality limits. Color-coding the graphs this way gives us a rough look at how much habitat might be available for kokanee salmon—and other salmonids like Rainbow Trout—at different depths in the reservoir through time. Red = lethal, yellow= stressful, and blue = ideal.

Isopleth temperature graphs across all dates and depths sampled for four of the five habitat units sampled in Island Park Reservoir. Temperatures and Dissolved oxygen are color-coded to reflect the preferences of Kokanee Salmon. Blue = ideal, yellow = sub-optimal, red = lethal.

As drawdown occurs, water temperatures throughout Island Park Reservoir increase to the point where there are no areas that have ideal water quality for kokanee salmon. Looking closely, we can see that good conditions exist in the pre-drawdown period on the left side of the graphs between May and late June. During the drawdown period between late June and mid August, water temperatures rapidly increase at all depths. After the drawdown period ends in August, water temperatures go back to good conditions for kokanee salmon.

Dissolved oxygen is a little more complicated. Dissolved oxygen appears to be very good at all locations in the pre-drawdown period. Once drawdown begins in late June, dissolved oxygen undergoes wild swings in the Main Reservoir and in the East End. The Inlet’s dissolved oxygen remains good, probably because of the influence of the spring-fed Henrys Fork River. Out in the West End, dissolved oxygen remains good, probably because the lake is shallow here and easily mixed up by the wind.

Isopleth dissolved oxygen graphs across all dates and depths sampled for four of the five habitat units sampled in Island Park Reservoir. Temperatures and Dissolved oxygen are color-coded to reflect the preferences of Kokanee Salmon. Blue = ideal, yellow = sub-optimal, red = lethal.

When we translate these graphs into volume of water that’s available for kokanee salmon and rainbow trout, we found that drawdown brings about a precipitous decline in the amount of habitat available for both species at almost all locations.

Stacked area plots showing the volume of water quality defined as optimal for (top) kokanee (<17 C, >5 mg/L O2) (bottom) Rainbow Trout (<20 C, >5 mg/L O2) in the five different habitat areas in Island Park during the study period.

So, what’s happening here? Island Park Reservoir, like all lakes, arranges itself in layers of water based on temperature. Warmer water is less dense, and floats over a layer of cold water. We call these layers the “epilimnion” (Greek for “upper lake”) and “hypolimnion” (Greek for “deep lake”). The epilimnion is warmed by the sun and interacts with the atmosphere, which means it is warm but well-oxygenated. The hypolimnion is cool but it’s cut off from the atmosphere; the decay of organic matter (like algae, fish, and plankton) can cause low dissolved oxygen in this layer. The “thermocline” is the goldilocks zone between the two where water temperatures are simultaneously cool and well-oxygenated. The thermocline is the area that’s important for fish like kokanee and rainbow trout.

Unfortunately, Island Park Reservoir is shallow and has a very small hypolimnion, oftentimes amounting to less than 15% of the total volume of the reservoir. When the reservoir is drawn down for irrigation, the hypolimnion and thermocline is delivered out of the bottom of the reservoir through a pipe or siphon in the dam. Thus, the cool thermocline is sent downstream. The water is nutrient-rich and well-oxygenated, which benefits the Henry’s Fork for a short time downstream. However, once this hypolimnion and the thermocline is eliminated, water temperatures rapidly rise as the deep, cool water is replaced by warm surface water. This warm surface water is also rich in organic material. The organic material rapidly decays, causing temporary “dead zones” near the bottom of the reservoir.

The end result? Water temperatures unsuitable for kokanee and trout creep down from the surface like a hammer, while low dissolved oxygen at the bottom creates an anvil. When the two meet, it results in an elimination of kokanee and trout habitat.

Springs Provide Refuge

Thankfully, there are refuges for trout and salmon in Island Park Reservoir that allow at least some fish to survive in years like 2021 when drawdown was moderate to severe. Springs are common in Island Park Reservoir. While the amount of habitat these springs preserve is small, the habitat is high-quality. Spring-fed inflows maintain excellent dissolved oxygen throughout, and maintain cool temperatures near the bottom, preserving trout and kokanee populations.

Isopleth graph showing dissolved oxygen (left) and water temperature (right) across all depths and dates of sampling in the "Springs" habitat unit in Island Park Reservoir. Temperatures and Dissolved oxygen are color-coded to reflect the preferences of Kokanee Salmon. Blue = ideal, yellow = sub-optimal, red = lethal.

Using our data, we can finally put together a picture of what’s happening in Island Park Reservoir with drawdown:

As the “hammer and anvil” strike in deep water, all that’s left for trout and kokanee are small areas of spring-fed inflows. In these refugia, many of these fish either starve, are preyed upon by other fish, or elect to leave the confines of Island Park Reservoir and emigrate downstream. The bigger the drawdown, the longer the hammer and anvil effect occurs in deep water and the longer juvenile trout and kokanee salmon are trapped in relatively small refugia.

Schematic showing what is likely happening to water quality, fish habitat, and the fish themselves in Island Park Reservoir given the data we have to date.
More to Learn!

At this point, we’ve learned that less water is bad for fish. That seems rather obvious, but in other reservoirs throughout the west, drawdown, less water, and warmer water temperatures can benefit kokanee salmon and rainbow trout by increasing growth rates. That doesn’t seem to be the case here, and these contradictory findings between what we've found in Island Park Reservoir and what other scientists have observed elsewhere highlights that no two reservoirs are alike. Our findings stress the importance of thorough monitoring and research in order to come to appropriate conclusions.

All the monitoring the HFF has done, like this project, has been supported by generous members of the Henry’s Fork Foundation. What we’ve learned at Island Park Reservoir so far wouldn’t be possible without their generous contributions.

However, monitoring and reacting to current conditions isn’t as good as being able to predict conditions ahead of time. New, advanced water quality models could allow us to predict probable water temperatures and dissolved oxygen through the year. These same models could help us research, and possibly predict and manage, the annual toxic cyanobacterial blooms in Island Park Reservoir that endanger humans, pets, and livestock.

Predictions produced by models can help us understand what might happen to fish habitat in any given year. That’s helpful for anglers: predictions could help people plan trips to adapt to difficult conditions, including high afternoon temperatures or toxic cyanobacteria blooms. Communicating predictions to management agencies could assist in experimental methods to improve water quality and fish survival even when water supply is low, thus helping trout and salmon populations adapt to a drier future. Many suggestions have been made in an attempt to fix Island Park Reservoir’s problems; with the right tools I hope to provide helpful information and work towards solutions.

I’d also like to hear from you if you live around Island Park Reservoir! What issues are pressing for you? What would you like to see out of Island Park Reservoir?

Future research and intern projects aim to address these and many other questions. We appreciate your support!

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