Henry’s Lake and its not-so-secret problem | Hatch magazine


News of the great capture spread like wildfire throughout the West. Lake Henry, a shallow impoundment in eastern Idaho just across the Montana border, gave up another legal trophy – a 36-inch rainbow-cutthroat trout hybrid.

On Oct. 4, Rigby, Idaho angler Hailey Thomas snagged the monster trout while fishing the popular trophy-producing lake with her husband and two children. According to local news reports, the water was slightly colored and fishing was slow. Just as her husband, Shane Thomas, raised anchor to move to a new location, the monster trout came calling. The fish surpassed the Idaho hybrid trout record by a full six inches, and the fish measured more than 21 inches in circumference.

A safe and good beast for Hailey – it’s a capture of life that few others will experience.

Unfortunately, Lake Henry may not be the place to enjoy fishing years from now. Just three days after Hailey boated her trophy, the Idaho Department of Health and Welfare issued a familiar warning to anglers and boaters who use Henry Lake for recreation — the lake was in the midst of a toxic blue-green algae bloom. and state health officials also warned not to touch the waters of the high lake.

According to a release from the state DEQ, water samples collected from the shore of Lake Henry showed the presence of cyanobacterial species. Microcystis, Dolichospermum, AND Aphanizomenon. All three bacteria can produce dangerous toxins that can be harmful to people, pets and livestock, and the DEQ warned that those with liver or kidney damage are at an increased risk of the disease.

Idaho being a politically red-blooded state where everything from science to election results is regularly called into question was not cited by the state’s DEQ as the cause of the algae bloom, except for these types of blue-green algae. “are a natural part of Idaho’s water bodies” that can thrive to dangerous levels when the weather and water temperatures warm. The cause of the warming? No mention in the state’s health warning, other than the seemingly innocuous rise in water temperatures.

Unfortunately, this is nothing new to Henry Lake. It is a shallow cover and has weed beds that provide cover for giant trout and great habitat for the extremely abundant aquatic insects. Warms up under the summer sun – no doubt about it. The lake has been subject to documented outbreaks of blue-green algae every summer and fall since at least 2017. And it’s not alone. Many of Idaho’s famous reservoirs have endured such algal blooms, including Cascade Reservoir, a popular fishery near McCall in western Idaho.


Hailey Thomas and her record trout

Hailey Thomas and her record trout (photo: Shane Thomas / Idaho Fish and Game).

But for the past two years, the bloom has spread to Island Park Reservoir, an irrigation impoundment on Henry’s Fork near the head of the river’s drainage (Lake Henry is not directly in Henry’s Fork). This is remarkable because Island Park Reservoir is huge—at full pool, it covers 8,400 acres of high-elevation real estate.

But it’s not exactly a surprise, at least to scientists who are predicting more as the climate continues to warm and water temperatures, even in reservoirs located at 6,500 meters, continue to rise. It may also be one of the first signs that climate change is affecting the waters of the West to the point where even diving may be bad for human health. Although climate change is not mentioned in Idaho DEQ’s health advisory, it doesn’t take much to make the connection between algal blooms and a warming climate.

Over the past two decades, water temperature advisories have become the norm on trout streams throughout the West. Documented die-offs of trout and whitefish due to warm water or bacterial and algal blooms as a result of warm water have continued for years.

But now we are talking about human health. And we’re talking about lakes that have serious economic benefits for communities, like Island Park Reservoir, which is very popular among boaters, jet-skiers, swimmers and fishermen. Blue-green algal blooms are no longer isolated incidents on reefs—they are, predictably, concomitant to increases in water temperatures fueled by climate change and resulting droughts.

As of this writing, Island Park Reservoir is less than half full and was a full 4,000 acre-feet lower a week ago. Other reservoirs in the area are also dry. Palisades Reservoir on the nearby South Fork is only 6 percent full as of this writing – actually dropping 1,000 acre-feet in the last week. The upper end of the reservoir is completely dry, leaving important migratory tributaries for brown trout exposed to predators such as pelicans and raptors. This lake, too, is important to the recreational community in eastern Idaho and western Wyoming. Thankfully, there have been no DEQ water quality warnings for the Palisades. Still.

Sometimes, in order for problems to be fully understood, the impact must be felt by more than a single group of users. As fishermen, we understand climate change and what is happening to our rivers, lakes and streams because we are directly affected. Now that blue-green algae blooms are beginning to spread in lakes and reservoirs where more and more people come into direct contact with the water, others may begin to understand the serious challenge presented to the West by a warming climate.

In the case of Lake Henry Reservoir and Island Park, impacts can be widespread. There are thousands of lakeside cabins and second homes along the shores of both lakes – they are tourist destinations for visitors to the area – it’s close to Yellowstone National Park and the fishing, ATV riding, Nordic skiing and snowmobiling are legendary. But if you’re a summer visitor to Lake Henry and aren’t sure if you can even touch the water, does that change the attraction? You bet.

It also changes where you spend your money. Local gas stations, lodging facilities, flight shops, restaurants, and campgrounds could be in for a rude awakening if the water-motivated visitor takes a dip because … well, because the dip can make people sick. Who would want to cast a line and chase giant trout if touching the water could be dangerous?

This isn’t just an Idaho thing. Blue-green algae is a viscous bloom across the country. As it blooms, it absorbs more sunlight, making the waters around it even warmer and more toxic. Idaho is just the latest on the list of victims.

But what is alarming is that it is not the culprit. It is only a symptom of a much bigger problem. And until state health departments and environmental safety departments have the stones to let the citizens they’re trying to protect know the full story, it likely won’t get any better.



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