South Fork Water and Weather Outlook
Photo Credit: Michelle Bliss Photography
The South Fork watershed has been blessed with average to above-average water supply the last four years – blessed in the sense that eastern Idaho finally emerged from the 2013-2016 drought – which has likely played a significant role in improvements in aquatic insect compositions and trout abundances over the last three years. While the South Fork has fared marginally better than the Henry’s Fork this year (slightly higher peak snow pack and natural flow, but still just 46 percent of median SWE as of May 21), nearly all of the South Fork headwaters have been designated in “moderate” to “severe” drought, and long-term weather forecasts show no signs of improvement.
Ideally, the South Fork Initiative (SFI) would have the capacity to provide extensive water supply predictions and analysis of impacts on water quality, similar to those for the Henry’s Fork. However, water-supply modeling and water-quality data collection programs on the South Fork are only one to two years old (the Henry’s Fork programs were established 5-7 years ago). That being said, 2021 will provide a great opportunity to collect the data necessary to understand and quantify the dynamic relationship between Palisades Reservoir management and downstream river conditions, thanks to the South Fork water quality monitoring instruments (sondes). The data collected this year will be key for future South Fork predictions and forecasts. To support the SFI’s capacity to conduct this kind of work in the future, please consider donating at henrysfork.org/donate.
With the SFI’s current capacity and data, there are several aspects of summer conditions on the South Fork that can be described with some certainty. First, it is extremely unlikely there will be increases in flows due to flood control the rest of the spring. Second, summer and fall flows will be driven by irrigation demand, which will require outflow from Palisades in the range of 13,000-15,000 cfs until irrigation demand begins to recede in late August. Third, Palisades Reservoir is likely to be drawn to very low levels by the end of September. If water year 2016 is any indication of what is to come, Palisades Reservoir is likely to drop below 20 percent capacity by the end of September. Last, as Palisades Reservoir gets lower later in the summer, it is likely turbidity in the river will start to increase as sediment gets mobilized from Palisades Reservoir and transported downstream.
One silver lining to the situation is that high abundances of aquatic macroinvertebrates and high abundances of trout will likely provide good to excellent fishing opportunities this summer, similar to the last few years on the South Fork. The bad news is the impacts of the forecasted drought this summer will still be realized in upcoming years, particularly if the drought continues into next winter and spring.
To view real-time water quality data on the South Fork, click here.