The short answer is no.
The mean outflow during the month of July was 1,227 cfs from 1970-2000 and 1,173 cfs from 2001-2023. Mean outflow during August was 1,173 cfs in the 1970-2000 period and 920 cfs in the 2001-2023 period.
Outflow may seem higher for two reasons:
1) Aquatic plant abundance is much higher now than it was decades ago, which makes the water deeper at a given outflow.
2) Turbidity is higher now than it was decades ago, so the water appears deeper than it really is because it’s harder to see the bottom of the river.
The reasons for higher macrophyte growth and higher turbidity are somewhat complex but can be roughly boiled down to changing climate and increased nutrient concentrations, the latter most likely tied to development.
1) Irrigation diversion is around 20% lower than it was in the 1970s-1990s as a result of increased irrigation efficiency, primarily conversion from flood irrigation to sprinkler irrigation. HFF recently published a groundbreaking paper on this subject.
2) Water management has become much more precise over the past 10 years due to remote-controlled irrigation technology and a lot more stream and canal gaging. In the Henry’s Fork watershed, we led the technological advances and funding acquisition that made this possible.
Since the start of Precision Management efforts in 2018, the amount of water left in Island Park Reservoir at the end of the irrigation season has exceeded expectations based on water supply by 26,000 ac-ft/year (over 10,000 Olympic swimming pools worth of water). This has been accompanied by a 100-cfs decrease in summertime outflow, despite the fact that water supply is 15% lower than it was in the 1970s-1990s. If we had the same water supply we had in the 70s-90s, it’s fathomable that we would hardly use any reservoir storage at all these days.
Our Aquatic Ecology Program Manager, Dr. Jack McLaren, joined Idaho Falls city and FIPS-Mouche leadership at the opening ceremony parade along the Snake River Greenbelt to welcome competitors to the watershed and to make sure conservation was front and center.
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
This story was written by Audrey Hunter (2026 intern) and was originally published in the Fall 2026 Newsletter. It’s no secret that late summers on Island Park Reservoir haven’t been in perfect conditions in recent years. Warm, turbid, algae-filled water plague reservoir recreationists while diminishing water quality downstream. Cyclical algal blooms decrease water clarity and create less-than-ideal circumstances for the reservoir’s flora and fauna. What’s the solution? HFF’s DIRTT Plan. In-Reservoir Oxygenation Nutrients released from
Drone photo taken on September 9, 2026 The Henry’s Fork Drought Management Planning Committee met on Wednesday, September 16 2026 to develop an updated reservoir management strategy. Downstream of the Henry’s Fork watershed, lower-valley irrigation demand over the rest of the season is anticipated to be slightly lower than projected last week, so Island Park Reservoir and Grassy Lake (in the Fall River drainage) do not need to deliver any more water out of the
