FAQ: Is Outflow from Island Park Dam Higher Than it Used to Be?

The short answer is no.

Summer outflow from Island Park Reservoir has been lower since 2001 than it was between 1970 and 2000.

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.

OK, then why does it seem that way?

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.

Why is summertime outflow LOWER than it used to be?

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.