Drone photo of Island Park Dam taken on September 9, 2026

Looking Back on September

September brought lows and highs to Island Park. Island Park Reservoir started September at 29% full and quickly fell to a low of 12% full on September 19. The rapid drawdown of the reservoir was the result of managers needing to move water downstream to American Falls (to irrigate farms in the Magic Valley with senior water rights). Such low reservoir levels were visually dramatic, but not unexpected given the region’s exceptional drought conditions.

The rapid reservoir drawdown also resulted in highly turbid, off-color water moving out of the dam and downstream into the Henry’s Fork at Box Canyon. Most of the turbidity came from dead algae. But once the reservoir fell to ~18% full, the river flowing into the reservoir (near McCrea’s Bridge) began to cut into old lakebed and started to move mineral sediment downstream—further exacerbating conditions. Additionally, with the reservoir too low for the powerplant to operate, all water was routed through the deep, unscreened dam gates. Gate operation, rapid drawdown, and low deepwater dissolved oxygen levels allowed large numbers of chub and perch (and possibly some trout) to escape the reservoir and move into Box Canyon.

Although high outflows, low reservoir levels, and a turbidity spike at the dam can be frustrating, concentrating rapid drawdown to a few short weeks in September helped avoid a larger and potentially longer turbidity event earlier in the year. We are grateful for such focused reservoir management.

Forecast for October

The reservoir is already slowly refilling in preparation for next irrigation season. Once outflow at the dam is decreased to its winter target of 80 cfs (the lowest setting on the dam gates), the reservoir will fill more quickly and turbidity in Box Canyon will continue to improve.

Unfortunately, water quality in Island Park Reservoir itself remains poor. On October 2, state agencies confirmed the presence of toxic cyanobacteria in Island Park Reservoir and issued a health advisory. Please visit the Idaho Dept. of Environmental Quality website for more information and take precautions accordingly. Water quality conditions in Island Park Reservoir will only be improved by cold, stable weather. Algae blooms will continue until the reservoir turns over and cools in preparation for freezing.

Some Good News

1. In September, the HFF field team observed more spawning kokanee salmon moving from Island Park Reservoir and into Henry’s Lake Outlet than expected—with kokanee spawning after flows were reduced at Henry’s Lake.
2. The Henry’s Fork Wildlife Alliance took advantage of low reservoir levels and organized a beach cleanup, removing debris from the shoreline and exposed lakebed. Thank you!

Planning for the Future

The Henry’s Fork Foundation envisions a better future for water quality in Island Park. A low reservoir does not need to lead to poor water quality. Learn more about HFF’s plan to reduce temperature and turbidity in Island Park Reservoir by visiting henrysfork.org/dirtt-plan. For water quality resources for the river and reservoir, please visit henrysfork.org/river-conditions and send questions to hff@henrysfork.org.

This blog post was published in Island Park News on Thursday, October 8 2026.