Photo of Island Park Reservoir taken Friday, August 28, 2026

The ingredients for green algae blooms, the primary culprit for turbid water in Island Park, stacked up one after the other earlier this spring. Warm air temperatures. Low streamflow. Long, sunny days. And an ice-free reservoir 30 days earlier than normal. At the Henry’s Fork Foundation, where we swim in water data daily, we braced ourselves for the water quality worst-case scenario: another summer with pea-soup-poor water clarity in Island Park Reservoir and the Henry’s Fork downstream.

The August Story

But, it looks like we are in the clear. Green algae blooms have been mild and fleeting. As a result, water clarity coming out of Island Park Dam and into Box Canyon is above average. Without the growth and decay of green algae, dissolved oxygen levels are staying high—allowing the hydropower plant to remain operational and continue to move clear water from the reservoir into the Henry’s Fork downstream. Although there have been bumps in turbidity, they have been short-lived and largely the result of rain falling on exposed shoreline, as water levels in Island Park Reservoir are low (and getting lower). However, toxic cyanobacteria (“blue-green algae”) responsible for harmful algal blooms (HABs) remain present and have re-energized, creeping from Lakeside down towards the dam. For HAB recreation advisories, please visit the Idaho Dept. of Environmental Quality website: https://www.deq.idaho.gov/water-quality/surface-water/cyanobacteria-harmful-algal-blooms/.

Unfortunately, where water clarity overachieved, water temperatures underperformed. Outflow from Island Park Reservoir hit record-high temperatures this summer and limited angling opportunities on the Henry’s Fork downstream. Thankfully, river temperatures have cooled in many places over the last week.

Water Quality Forecast for September

Here is what to look for in September: By Labor Day, the reservoir will likely be too low for the hydropower plant to operate and all outflow will move to the gates (on the dam’s west side). Once that occurs, outflow from Island Park Dam will be turbid and off-color. Water clarity will be at its worst for a day or two before leveling out to something still not great, but not absolutely terrible. With the gates open at these low levels, plant chunks, algae mats, suspended gunk, and even fish will flow out of the reservoir and into the river. These conditions will persist until late September, when outflow will be drastically reduced to begin filling Island Park Reservoir for the 2027 irrigation season.

Resources

For water quality resources for the river and reservoir, please visit henrysfork.org/river-conditions and send questions to hff@henrysfork.org.

The Henry’s Fork Foundation envisions a better future for water quality in Island Park. Learn more about HFF’s plan to mitigate algae cycles and preserve cool water in Island Park Reservoir by visiting henrysfork.org/dirtt-plan.