This article was originally published in Island Park News on May 29, 2026
Ice melted on Island Park Reservoir on March 31, 2026—four weeks earlier than normal and the earliest the Henry’s Fork Foundation (HFF) has observed in 12 years of monitoring. Such an early ice melt was not unexpected given the region’s near-record warm and dry winter. But ice melting a month early has implications for water quality in Island Park Reservoir and the Henry’s Fork downstream. Reservoir recreationists and river users can expect algae blooms, low water clarity, and warm water earlier in the season.
Algae and cyanobacteria create a green-brown growth-death cycle in Island Park Reservoir every year, typically in late July and early August. But an early melt brings an extra month of sunshine and growth. As these organisms grow, die, and sink, brown-tinged water is exported out of Island Park Dam to the Henry’s Fork downstream. Early ice off, low flow into the reservoir from sources like Big Springs, and warm air temperatures will extend the growth-death cycle earlier, and for longer. An extended cycle will likely increase the risk of harmful algal blooms (HABs) on Island Park Reservoir and result in lower water clarity at Island Park Dam—possibly as early as mid-June.
The small, deep pool of cool water near the dam is already warming up and will be rapidly depleted as outflow at Island Park Dam increases. As a result, HFF expects warmer water temperatures—and potentially record-high water temperatures—coming out of Island Park Dam this summer. Anglers on the Henry’s Fork downstream may want to monitor river temperatures and dissolved oxygen levels to minimize impact on fish.
Overall: April looked like May, so July will likely look like August.
In addition, Island Park Reservoir is fed by snowmelt and groundwater springs like Big Springs, which are also ultimately fed by snowmelt. But with a meager snowpack that melted early and less snowmelt replenishing Big Springs year after year, streamflow into Island Park Reservoir is currently 70% below average and the lowest observed in almost 95 years. As a result, reservoir recreationists should expect low reservoir levels, more boating hazards, and an early onset of harmful algal blooms.
With federal funding, HFF is working with engineering consultants to investigate ways to mitigate algae cycles and preserve cool water in Island Park Reservoir. Visit henrysfork.org/dirtt-plan to learn more.
For HAB recreation advisories, please visit the Idaho Dept. of Environmental Quality website. For water quality resources for the river and reservoir, please visit henrysfork.org/river-conditions and send questions to hff@henrysfork.org.
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