Christina Morrisett, Author at Henry's Fork Foundation

The blue water of Island Park Reservoir under a blue sky with a single white cloud. Sawtelle sits in the background and you can see rocky edges of the reservoir as the water levels have dropped.

Summer 2026: Water Quality Reflection and Forecast for Island Park

Photo of Island Park Reservoir, taken July 31 2026 Summer 2026 is the Henry’s Fork Foundation’s 11th summer monitoring conditions in Island Park Reservoir, and we continue to learn something new even a decade later. This summer’s lesson? How rain and high outflow disrupt algae blooms—maintaining clear water, for longer. The Weather Event At the end of June, data from HFF’s weekly monitoring on Island Park Reservoir showed that algae growth was creeping up and water clarity was crawling down. Without some wet weather, we expected warm water and low water clarity flowing out of the reservoir as early as July 4th. But then it rained. June 25–July 2 brought heavy rain and cool temperatures. Rain falling directly on the reservoir created a thick surface layer of cold water. This new cold layer sunk down and caused the reservoir to mix and turnover. In fact, the late June rain event cooled the entire reservoir by a whopping 6°F. After a cold-cycle tumble wash, the reservoir’s summer slate was wiped clean. The July Story We started over with a cool, clear reservoir on July 3. Then, only nine days later, irrigation demand peaked. Outflow from Island Park Reservoir skyrocketed to 1,500

July 2026 Water Quality Forecast for Island Park

In early April, when ice melted off Island Park Reservoir one month earlier than usual, scientists at the Henry’s Fork Foundation (HFF) predicted an early onset of poor water quality conditions in Island Park Reservoir and the Henry’s Fork downstream. Warm air temperatures, low streamflow into the reservoir, and longer days with more sunlight are a recipe for algae blooms, the primary culprit of turbid water in Island Park. But we arrived at the end of June pleasantly surprised. The algae blooms and low water clarity that seemed imminent just one month ago have been delayed by weekly cycles of cooler, wetter weather that regularly disrupt algae growth. When asked about the water quality forecast for July, HFF scientist Dr. Jack McLaren said, “We’ve gotten lucky. It is going to get bad, but how bad things get depends on the weather at this point.” Water Quality Forecast Jack and his field team have been going out on Island Park Reservoir every Wednesday since the ice melted to monitor water quality conditions from the reservoir surface to its depths. Data from these efforts show that algae growth in the reservoir is creeping up and water clarity at Island Park Dam is

An aerial view of Island Park Dam. The reservoir sits behind it and the river flows below it.

Summer 2026 Water Quality Forecast for Island Park

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. Water Quality Forecast 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

Aerial photo of Island Park Reservoir. The dam is in the bottom left corner. The Centennial Mountain Range is in the top.

History of Water Quality in Island Park Reservoir

Warmer and drier conditions have slowly degraded water quality in Island Park Reservoir over the last 50 years. In the 1970s and 1980s, federal reports and university research indicate that the reservoir had relatively good water quality. Island Park Reservoir was mesotrophic—with mostly clear water and occasionally low dissolved oxygen at the reservoir’s deepest depths during the summertime. But, as time went on, water quality began to decline. Historical satellite imagery analyzed by the U.S. Geological Survey (USGS) recently classified Island Park Reservoir as eutrophic in the 1990s—warmer water, more algal scum, poor visibility, and low deepwater oxygen levels occurring annually each summer. Unfortunately, the decline has continued. Using HFF monitoring data from 2015–2025 and funding from Friends of Reservoirs, consultants Hazen and Sawyer determined that Island Park Reservoir is now hyper-eutrophic—warm, nutrient dense, dominated by algae, creating exceptionally high turbidity and persistently low deepwater oxygen levels. With warmer air temperatures and less streamflow into the lake, Island Park Reservoir has become a metaphorical stewpot in the heat of summer. After 50 years of decline, summertime water quality in Island Park Reservoir is now a major limiting factor in the Henry’s Fork watershed. Poor summertime water quality minimizes fish habitat

A photo taken of Island Park Reservoir while standing on the dam and looking north. The sky is cloudy, Mt Sawtelle sits with little snow in the background, and the reservoir is open water (save for a few small sheets of ice in the distance).

Reservoir Ices Off, Growing Season Turns On

Photo of an ice-free Island Park Reservoir taken on March 31, 2026 On March 31, 2026, with no accompanying commentary, HFF scientist Christina Morrisett stood on top of Island Park Dam and texted colleagues Rob Van Kirk and Jack McLaren photos of open water on Island Park Reservoir. Rob, eager to see that the reservoir reality matched what he was seeing in the real-time river data downstream, texted back, “Yep! That’s close enough to ice free for the purposes of calculating reservoir evaporation. Water quality signature shows substantial atmospheric influence.” Jack, the foremost expert on all things Island Park Reservoir, responded with a single word, “Brutal.” The Henry’s Fork Foundation has been monitoring water quality in Island Park Reservoir since 2017. During the open water season, typically May through October, HFF staff navigate to five reservoir locations to collect data like water temperature, dissolved oxygen, and turbidity from the reservoir surface to the reservoir bottom. This monitoring effort allows HFF to “see underwater” to map in-reservoir fish habitat, track lake dynamics, and forecast the temperature and turbidity of dam outflow to the Henry’s Fork downstream. The project has become routine. We know when to prep the boat, when to calibrate

Drift boat with anglers rowing downriver surrounded by tall trees

The River Rundown: Your FAQs from June 2025

“It feels like July!” was a common sentiment shared around the watershed last month. Anglers enjoyed Brown Drakes on the Ranch three weeks earlier than usual, basked in 70°F more days than not, and endured high outflows from Island Park Dam that are more typical for mid-July. Some of these changes were welcome… others, not so much. Below, we answer your most Frequently Asked Questions from June. FAQ: Outflow from Island Park Dam peaked around 1,600 cfs. Why so high, so early? Warm and dry conditions increased irrigation demand in the lower watershed 10% higher than average. To meet irrigation demand and maintain fish habitat around St. Anthony, water managers rely on water delivered from Island Park Reservoir and on natural streamflow Natural streamflow comes from mountain snow and groundwater springs—but is 38% below average. Therefore, Island Park Reservoir has to work that much harder to make up the difference. With so little water in the system, high outflow in Island Park is needed just to keep the river wet 60 miles downstream. FAQ: What’s up with the constant flow changes at the dam? In wetter years prior to the year 2000, there was enough water to go around—so managers

Meeting notes for the 2025 HFF membership meeting collected in doodle form

2025 Membership Meeting Recap

Visual abstract for the 2025 HFF Membership Meeting created by Teddy Montalvo, 2025 Baucus Climate Scholar On June 19, we gathered at the Boys House in Harriman State Park for the 2025 HFF Membership Meeting. In addition to meeting in a new place, we also tried a new structure: a 30-minute presentation by our Communications Team followed by a 30-minute Panel Q&A with members of HFF’s Science & Technology Team. Thank you to all who joined us! Missed the meeting? You can watch the meeting recording here. Want the highlights? Read below or check out the visual abstract above! Programmatic Highlights Community Connections Education: HFF expanded our Trout in the Classroom program to five classrooms, now reaching 130 students. High school volunteer opportunities for the South Fork Initiative helped restore Rainey Creek and improved local intern recruitment. Our college internship program continues, now in its 37th year. Outreach: HFF’s executed 44 Morning Bites events at local river access sites, gave 40+ scientific presentations, and hosted booths at 12 community events. In total, HFF reached an estimated 5,000 community members. Voice of the River: HFF was an active participant at 10 Planning & Zoning meetings. We hosted three river clean ups and want to

A salmonfly nymph and newly-hatched adult sit on a blade of grass alongside the river. Text reads "What's Hatching? (Or Not)"

What’s Hatching on the Henry’s Fork?

In this blogpost, learn about what we know, what we don’t know, and how you can help by reporting the hatches you see (or don’t see) every time you visit the river. Check out the Henry’s Fork Hatch Survey today at tinyurl.com/hfhatchreport. What We Know The Henry’s Fork Foundation has been monitoring aquatic macroinvertebrates (insects) at multiple locations since 2015. Each March (normally on St. Patrick’s Day), HFF and a team of entomologists from River Continuum Concepts head out to the Henry’s Fork to scrub rocks and collect bugs from the river-bottom for expert identification and robust analysis to answer questions like: How many bugs are there? (Abundance) What kind of bugs are there? (Species Diversity, Richness, Composition) How have the bugs changed through time? As scientific river detectives, we pair aquatic insect numbers with water quality and streamflow data and run it through rigorous statistical analysis to better understand changes to the macroinvertebrate community. What have we learned so far? A decade of data show that aquatic invertebrates on the Henry’s Fork are largely happy and healthy. When it comes to Pale Morning Duns, we are seeing declines in abundance at Flat Rock and St. Anthony, but no trends one way

Snow to Streamflow: A Story of Decline [Part 2]

Snow to Streamflow: A Story of Decline [Part 2] In snowy regions like the Henry’s Fork, the amount of water held in the snowpack on April 1st can help predict summer streamflow. But over the past two decades, the relationship between the water stored in spring snowpack and summer streamflow has changed, making summer streamflow less predictable. New research by HFF and Idaho State University scientists show how we can improve our summer streamflow predictions by better understanding when snowmelt and rain enter the ground relative to the forest growing season. Key Takeaways: The amount of snow that becomes streamflow depends on both past and current conditions. Predicting streamflow requires accounting for changing forest dynamics. Less snowmelt becomes streamflow after years when more water enters the soil during the forest growing season. Why is it important to be able to accurately predict water supply? Part 1 discussed how warmer temperatures and changing forest dynamics are contributing to water supply decline in the Henry’s Fork. At the Henry’s Fork Foundation, if we want to reduce drawdown of Island Park Reservoir and increase winter flows to grow more trout, we need to be able to accurately predict water supply. Understanding watershed processes are

Snow to Streamflow: A Story of Decline [Part 1]

Henry’s Lake mountains, photo credit Sarah Newcomb Due to warmer growing seasons and denser forests, Henry’s Fork natural streamflow is declining and becoming less predictable. Over the past 20 years, the Henry’s Fork has seen a decline in natural flow, or the amount of water that would flow down the river in the absence of any reservoirs or irrigation diversions (aka the water Mother Nature gives us). At the same time, we are starting to see rainier, less snowy years , meaning the total amount of water entering the watershed each year hasn’t changed, but less is falling as snow. So, how do we have the same amount of water coming in… but less becoming streamflow? Where is the water going? With funding from the National Science Foundation in 2023, Dr. Sarah Newcomb set out to answer this question as part of her dissertation research at Idaho State University. Sarah worked alongside the HFF Science and Technology Team to investigate how snowmelt and rain becomes streamflow in the Henry’s Fork. As a mountain hydrologist, Sarah studies how water moves through mountain systems. Water in the Henry’s Fork primarily starts as snow that falls high in three major tributaries: the upper