Anglers are noticing low flows across the Henry's Fork. What's the culprit? Low natural flow in the upper Henry's Fork subwatershed (everything above Ashton).
Island Park Reservoir is required to be kept full until water is needed to meet irrigation demand (to water crops), so outflow from the reservoir is matching inflow.
Inflow to the reservoir comes from natural sources like Big Springs and takes 3-4 years to recover from drought.
2013-2016 = driest since Dust Bowl 2017-2019 = pretty good, got us back to ~90%
2020 = not great
2021-2022 = really dry, only thing avoiding another 2016 was water conservation efforts like Precision Water Management
We need 3-4 good water years to fully recover from recent drought, and we haven't gotten that yet. That said, our work is making a meaningful difference, avoiding repeats of 2016 despite similar low water supply.
More information:
Natural flow in the upper Henry’s Fork subwatershed—including inputs between Island Park Dam and Ashton—continues to be really low as a result of long- and short-term drought and climate trends. Loss of snowmelt and soil moisture to evapotranspiration has been steadily increasing in the upper Henry’s Fork subwatershed for decades, resulting in a steady decrease in streamflow per unit of precipitation we receive.
In addition, the deep aquifers that feed the upper Henry’s Fork, including Big Springs, Buffalo River, and Warm River, require 3–4 years to recover from drought, even without the additional uptake by drought-stressed forests in headwater areas.
The most recent drought period lasted roughly from July 2020 until November 2022, producing near-record low streamflow in the upper Henry’s Fork. Much of last year’s above-average snowpack went to replenishing soil moisture and accumulated vegetation water deficits, leaving little to contribute to long-term ground water recovery.
This year’s snowpack in the upper Henry’s Fork was below average again, and water-year precipitation to date there is 92% of average. Short- and medium-term drought indicators show descent back into drought conditions, or at least abnormally dry conditions. The best case over the next few months is that flows will stabilize at around the 25th percentile of water years since 1978—not at the bottom like in 2016 or 2022—but more like garden-variety dry years such as 1988, 2014 and many of the years in the mid-2000s.
That said, our work is making a meaningful and unprecedented difference, avoiding repeats of 2016 despite similar low water supply, and putting the Henry's Fork in a good position in the West.