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Harmful Algal Blooms and Island Park Reservoir

A recent Hatch Magazine article discussed the occurrence of harmful algal blooms (HABs) or cyanobacteria in Henrys Lake primarily, but also Island Park Reservoir, with the concern that these could effect water downstream.

HFF’s Doctoral Research Associate and resident Island Park Reservoir expert, Jack McLaren, PhD, weighed in. Jack has spent the past six years studying Island Park Reservoir through his Masters and PhD work, and was able to shed some light on a few common questions.

How much data do we have on harmful algal blooms (HABs) in Island Park Reservoir?

Jack: Bad news first. Our studies of the problem to this point have been limited. Direct measurement of harmful algal bloom (HAB) species and toxins is expensive, which presents a significant hurdle for any organization or agency from implementing a long-term monitoring program. The Idaho Department of Environmental Quality (IDEQ) measures HABs, though the high cost of testing means they only monitor after public reports of blooms. See their data, information, and a link to report blooms here:

To my knowledge, IDEQ does not currently test water downstream of Island Park Reservoir. We can try to fill in some information with HFF’s network of water quality monitoring sondes, which measure “Phytoplankton” and “Cyanobacteria” concentrations in the Henry's Fork and Island Park Reservoir. Certain species of cyanobacteria are the culprits behind HABs, however our sondes don’t tell us which species of cyanobacteria are present and they don’t measure toxins. That means the data we collect can’t be used to make direct recommendations regarding health risk for humans, dogs, livestock, or fish. You can find data from the Henry’s Fork River downstream of Island Park Reservoir through the following link: Our data for Island Park Reservoir itself is not yet online, but we’re working on it.

Are HABs a risk to anglers or fish downstream of the dam in the Henry’s Fork?

Jack: Here’s the good news: the data we have indicates the risk of HABs moving through Island Park Reservoir into the Henry’s Fork is likely low. There’s some evidence that when cyanobacteria concentrations are greater than 1 Relative Fluorescence Unit (RFU) and the Cyanobacteria:Phytoplankton ratio exceeds 1:1 for five days, conditions might be right for a harmful algal bloom. Since 2015, data at the sonde immediately downstream of Island Park Reservoir has met these conditions only once: for about a week spanning August/September of 2016. We haven’t seen these conditions met at any other sonde downstream. Again, however, these are measures of live Cyanobacteria and Phytoplankton and are not direct measures of the toxins that threaten human health.

It's worth acknowledging that data from other researchers in the intermountain west have shown HABs have probably been present in places like Island Park Reservoir and Henrys Lake for decades. Whether HABs are increasing in duration or severity is less clear, and we’re still uncertain about what the future holds. Anecdotal reports of HABs in the region (“swimmer’s itch”, bad smells, algal mats, sick animals) also seem to be limited to the reservoirs themselves.

If this issue remains a concern for folks: cyanotoxins are generally absorbed through the skin or through ingestion. Waders and gloves would reduce your risk. Risk to pets and livestock is mostly from ingestion. Fish kills from HABs are rare in freshwater, the bigger problem with HABs can be a reduction in dissolved oxygen once the bloom dies—aeration infrastructure at Island Park Dam ensures this is not an issue for the river downstream.

What is HFF doing about HABs?

Jack: We’re in the process of increasing our research effort on Island Park Reservoir. I’ll be leading the project to collect data to model the mechanisms that control water temperature, nutrients, and flow in Island Park Reservoir. From this work, we should be able to better predict reservoir and downstream water quality, including HABs, and how to improve water quality. We hope to be able to use this data to assess whether HABs are becoming greater in duration and severity and what the future may hold. The high cost has limited the extent of work so far, but the recent Hatch Magazine article and folks’ questions indicate a need for more data and greater understanding of this issue.



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